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(The FASEB Journal. 1999;13:2051-2060.)
© 1999 FASEB

Markers of capacity to utilize fatty acids in human skeletal muscle: relation to insulin resistance and obesity and effects of weight loss

JEAN-AIMÉ SIMONEAU*,1, JACQUES. H. VEERKAMP{dagger}, LORRAINE P. TURCOTTE{ddagger} and DAVID E. KELLEY§2

* Division of Kinesiology, Department of Social and Preventive Medicine, Faculty of Medicine, Laval University, Ste-Foy, Quebec;
{dagger} Department of Biochemistry, Faculty of Medical Sciences, University of Nijmegen, Nijmegen, The Netherlands;
{ddagger} Department of Exercise Sciences, University of Southern California, Los Angeles, California; and Division of Endocrinology and Metabolism, and
§ Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA

2Correspondence: East-1140 Biomedical Science Tower, University of Pittsburgh, Pittsburgh, PA 15213, USA. E-mail: Kelley{at}msx.dept.-med.pitt.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
A number of biochemical defects have been identified in glucose metabolism within skeletal muscle in obesity, and positive effects of weight loss on insulin resistance are also well established. Less is known about the capacity of skeletal muscle for the metabolism of fatty acids in obesity-related insulin resistance and of the effects of weight loss, though it is evident that muscle contains increased triglyceride. The current study was therefore undertaken to profile markers of human skeletal muscle for fatty acid metabolism in relation to obesity, in relation to the phenotype of insulin-resistant glucose metabolism, and to examine the effects of weight loss. Fifty-five men and women, lean and obese, with normal glucose tolerance underwent percutaneous biopsy of vastus lateralis skeletal muscle for determination of HADH, CPT, heparin-releasable (Hr) and tissue-extractable (Ext) LPL, CS, COX, PFK, and GAPDH enzyme activities, and content of cytosolic and plasma membrane FABP. Insulin sensitivity was measured using the euglycemic clamp method. DEXA was used to measure FM and FFM. In skeletal muscle of obese individuals, CPT, CS, and COX activities were lower while, conversely, they had a higher or similar content of FABPC and FABPPM than in lean individuals. Hr and Ext LPL activities were similar in both groups. In multivariate and simple regression analyses, there were significant correlations between insulin resistance and several markers of FA metabolism, notably, CPT and FABPPM. These data suggest that in obesity-related insulin resistance, the metabolic capacity of skeletal muscle appears to be organized toward fat esterification rather than oxidation and that dietary-induced weight loss does not correct this disposition.—Simoneau, J.-A., Veerkamp, J. H., Turcotte, L. P., Kelley, D. E. Markers of capacity to utilize fatty acids in human skeletal muscle: relation to insulin resistance and obesity and effects of weight loss.


Key Words: carnitine palmitoyl transferase • fatty acid binding proteins • lipoprotein lipase • maximal aerobic power • fatty acid oxidation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SKELETAL MUSCLE HAS an important role not only for the utilization of glucose during insulin stimulated conditions, but also for the metabolism of fatty acids (FA); indeed, during fasting conditions, oxidation of FA is generally considered to be the principal substrate of skeletal muscle (1) . Numerous biochemical alterations of skeletal muscle and insulin signaling pathways have been identified in relation to insulin-resistant glucose metabolism, as well as improvements after weight loss (2 , 3) . As recently stated (4) , however, in obesity the capacity of skeletal muscle for the metabolism of FA has been less clearly delineated. Activity of heparin releasable lipoprotein lipase (Hr LPL) has been reported to be lower or equivalent in skeletal muscle of nondiabetic overweight compared to normal weight subjects (5 , 6) ; among a relatively small group of obese women, our laboratories found reduced activity of carnitine palmitoyl transferase (CPT) in muscle (7) . Skeletal muscle capacity for FA utilization is potentially important both in relation to systemic fat balance and insulin resistance. In regard to insulin resistance, fat content within muscle is a strong correlate and has been found to be increased in obesity (8 9 10 11 12) . The mechanisms that cause skeletal muscle lipid content to increase in obesity are unclear, but might be broadly postulated as an increased uptake of lipids, decreased oxidation, or a combination of both processes.

During insulin-stimulated conditions, FA oxidation in skeletal muscle is normally suppressed (13) , yet incomplete suppression of FA occurs in obesity-related insulin resistance (14 , 15) . Nevertheless, it might be incorrect to infer from this that FA oxidation is always increased within skeletal muscle in obesity. Instead, during fasting conditions, when FA oxidation normally is predominant within skeletal muscle, the rate of oxidation in skeletal muscle has been reported to be reduced in obesity and Type 2 diabetes (16 , 17) . A prospective clinical study revealed that reduced FA oxidation is a metabolic risk factor for weight gain (18) and that enzyme activities within skeletal muscle pertaining to lipid metabolism might contribute to lower FA oxidation (19 , 20) . Moreover, after weight loss, skeletal muscle in post-obese individuals may continue to be inefficient in the oxidation of fat (21 , 22) . It has also been found that aerobic exercise training can augment skeletal muscle capacity for FA oxidation and uptake (23 24 25) and that exercise training induces higher levels of activity for muscle aerobic-oxidative enzymes (26) . Reduced activity of oxidative enzymes in skeletal muscle has been found in obesity and in relation to insulin resistance (10 , 27 28 29) . Recent findings of increased skeletal muscle content of malonyl CoA and diacylglycerol in animal models of obesity and insulin resistance suggest one mechanism (30 , 31) . Therefore, a biochemical basis for impaired FA oxidation within skeletal muscle in obesity is certainly plausible, but it has not been yet clearly established.

The current study was undertaken to compare lean and obese individuals for markers of skeletal muscle capacity to utilize FA and to examine the effects of weight loss on these markers in obese subjects. We postulated that in obesity-related insulin resistance, skeletal muscle would manifest a reduced capacity for FA oxidation. Also, to examine whether patterns of muscle enzyme activity and other key proteins of fat metabolism were interrelated to the phenotype of insulin resistance, glucose metabolism was determined using the euglycemic insulin infusion method.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
Fifty-five healthy adults (28 women and 27 men) participated in this study. Forty-five subjects were white, nine were African-American, and one volunteer was Asian-American. All potential volunteers underwent a medical examination, including routine laboratory testing and a 75 g oral glucose tolerance test prior to inclusion. Those individuals with glucose intolerance, hyperlipidemia, or liver, renal, or thyroid disorders were excluded as were individuals with a history or physical findings of coronary heart disease, peripheral vascular disease, hypertension, or neuromuscular illnesses. Subjects taking medication for these disorders or taking medications known to influence carbohydrate and lipid metabolism were also excluded from participation. An additional inclusion criterion was that both lean and obese volunteers not be engaged in a program of regular physical exercise, so as to minimize differences of physical fitness as a confounding variable between the two groups. Physical activity patterns were determined by interview at the time of screening. The protocol was approved by the University of Pittsburgh Institutional Review Board and all volunteers gave written informed consent.

Body composition and VO2max
As a measure of physical fitness, maximal aerobic power (VO2max) was measured using an incremental, modified Bruce protocol. A mass spectrometer (Marquette Electronics, Milwaukee, Wis.) was used to analyze expired air for CO2 and O2 fractions during this test; hemodynamic parameters were measured prior to, during, and immediately after this test. Whole body fat mass (FM) and fat-free mass (FFM) were assessed by dual energy X-ray absorptiometry (DEXA; Lunar model DPX-L, Madison, Wis.), using transverse scans to measure fat and lean tissue mass.

Insulin sensitivity and muscle biopsy
On the evening prior to measurement of insulin sensitivity and performing the percutaneous muscle biopsy, subjects were admitted to the University of Pittsburgh General Clinical Research Center. That evening, they received a standard dinner (10 kcal/kg; 50% carbohydrate, 30% fat, 20% protein) and then fasted overnight. Subjects were instructed to consume a weight-maintaining diet containing at least 200 g carbohydrate and to avoid exercise or strenuous exertion for at least 3 preceding days; all subjects had maintained a stable weight for at least 3 months prior to the initial studies. At ~ 7:00 AM, subjects were moved to the metabolic assessment suite and a catheter was placed in a forearm vein for infusion of glucose, insulin, and a primed (20 µCi), continuous (0.20 µCi/min) infusion of high performance liquid chromatography-purified [3-3H]-glucose (New England Nuclear, Boston, Mass.); the latter was for measurement of glucose utilization during the final 40 min of the insulin infusion. Isotope was started 100 min before the insulin infusion. A percutaneous muscle biopsy of the vastus lateralis muscle was done after 60 min of bed rest and 30 min prior to starting insulin infusion (32) . Muscle samples were immediately frozen in liquid N2 and kept at -80°C for later assays. Insulin was then infused at 40 mU/min-m2 body surface area for 3 h and euglycemia was maintained using an adjustable infusion of 20% dextrose, to which 80 µCi of [3-3H]-glucose had been added to maintain stable plasma glucose specific activity (33) . Plasma glucose was determined at 5 min intervals during the clamp. Blood samples for measurement of [3-3H]-glucose specific activity were collected every 10 min during the final 40 min of insulin infusion.

Weight loss intervention
Subjects with a body mass index greater than 30 kg/m2 were invited to participate in a 4 month, out-patient weight loss program. The goal of the weight loss intervention was to produce a weight loss of ~10–15 kg. The weight loss program combined a very low-calorie diet (VLCD) with an intensive program of behavioral intervention, as described previously (34) , and was initiated immediately after baseline physiological assessments. Briefly, during the first 10 wk of this program, subjects were instructed to consume a very low-calorie diet (800 kcal/day), followed by 6 wk of gradual refeeding up to isocaloric requirements. During the VLCD, subjects consumed a combination of liquid formula (Optifast, Novartis Corporation) and lean meat, fish, and fowl. During weeks 11–13, subjects were instructed to consume 1200 kcal/day, with a gradual reintroduction of fruits, vegetables, and grains to the diet. During weeks 14–16, subjects were instructed to consume a weight-maintaining diet, with 30% of calories as fat, 15% as protein, and 55% as complex carbohydrates, in order to avoid effects of acute fasting or calorie restriction on post-weight loss physiological assessments. All subjects underwent metabolic reassessments at week 17, after 3 wk of this weight-maintenance diet. Subjects were seen weekly for 16 wk by a nutritionist/behaviorist and serum potassium was monitored at 2 wk, and then repeated with an EKG, blood chemistries, and uric acid at 4 wk intervals. Vitamin and mineral supplementation were provided during the VLCD. To avoid potential confounding effects of exercise on muscle biochemistry, subjects were carefully instructed to maintain pre-weight loss levels of physical activity. After this weight loss intervention, reassessments of body composition, VO2max, and insulin sensitivity were performed and a second percutaneous muscle biopsy of vastus lateralis, immediately adjacent to the site of the initial biopsy, was obtained.

Analysis
Plasma glucose was measured using an automated glucose oxidase reaction (YSI 2300 Glucose Analyzer, Yellow Springs, Ohio). Glucose-specific activity was determined with liquid scintillation spectrometry after the deproteinization of plasma with barium sulfate and zinc hydroxide. Serum insulin was determined using a commercially available radioimmunoassay kit (Pharmacia, Uppsala, Sweden). Rates of plasma glucose appearance (Ra) and utilization (Rd) were calculated using the Steele equations, as modified for variable rate glucose infusions containing isotope (33) .

For the determination of enzyme activity in skeletal muscle, small pieces of the muscle sample (~10 mg) were homogenized in a glass-glass Duall homogenizer with 39 vol. of ice-cold extracting medium (0.1 M Na-K-phosphate, 2 mM EDTA, pH=7.2). Homogenate was transferred into 1.5 ml polypropylene tubes; this suspension was magnetically stirred on ice for 15 min and sonicated six times for 5 s at 20 watts, on ice, with pauses of 85 s between pulses. The resulting homogenate was used for determination of activity (Vmax) of six enzymes: citrate synthase (CS; EC 4.1.3.7), cytochrome c oxidase (COX; EC 1.9.3.1), phosphofructokinase (PFK; EC 2.7.1.11), glyceraldehyde phosphate dehydrogenase (GAPDH; EC 1.2.1.12), beta-hydroxyacyl CoA dehydrogenase (HADH; EC 1.1.1.35), and CPT (EC 2.3.1.21). Spectrophotometric techniques were conducted at 30°C, according to methods previously used (7 , 16 , 35 , 36) . Enzyme activities were expressed in Units (micromoles of substrate per minute) per gram of wet weight tissue (U/g). For determination of skeletal muscle activity of lipoprotein lipase (LPL; EC 3.1.1.34), ~10 mg of muscle tissue was weighed and incubated for 45 min at 37°C in 0.5 ml of Krebs-Ringer (131 mM NaCl, 5 mM KCl, 1 mM KH2PO4, 0.7 mM CaCl2, and 1 mM MgSO4), mixed with 0.1M Tris-HCl (pH 8.6) buffer containing 1% FFA-free bovine serum albumin (Sigma, St. Louis, Mo.), and 7 U of heparin (Sigma) in order to extract the heparin-releasable fraction of muscle LPL (37) . Heparin eluate was kept on ice and the tissue was transferred to a Duall tissue grinder and homogenized with 50 volumes of 223 mM Tris-HCl buffer (pH 8.6) containing 0.25M sucrose, 5 mM sodium deoxycholate, 0.08% Triton X-100, heparin (9 U/ml), and 1% bovine serum albumin to measure the extractable fraction of muscle LPL. One hundred microliter aliquots of the Hr eluates and 75 µl aliquots of the Ext fraction were incubated in triplicate with 100 µl of 14C triolein (100 µCi/ml) and triolein (7 mM), which were sonicated in 5% gum arabic, 75 mM Tris-HCl (pH 8.6), 75 mM NaCl, 0.2% bovine serum albumin, and 10% human serum. After 2 h of incubation at 37°C under agitation, reaction was stopped by the addition of 3.25 ml of an extraction mixture containing chloroform:methanol:heptane (141:125:100) and 1.05 ml of 50 mM potassium carbonate borate hydroxide buffer (pH 10.0) (38) . After mixing, samples were centrifuged at 1500 x g for 15 min and 2.0 ml of the upper aqueous phase containing FA was added to 10 ml of a scintillation mixture (Aquasol) and counted in a beta counter (LKB-Wallac). LPL activity was expressed as nanomoles of FFA transformed/min/g of wet weight muscle.

FABPPM and FABPC content within skeletal muscle
Homogenates of muscle samples were prepared to measure cytosolic (FABPC) and plasma membrane (FABPPM) fatty acid binding protein contents. Ten micrograms of total proteins (determined according to the Bradford technique) were deposited on a 6% stacking gel and migrated through a 12% resolving gel for 120 min at 100 V. After migration, gels were electrically transferred onto PVDF membranes at 100 V for 120 min. These membranes were blocked with a 5% powdered milk solution, rinsed thrice in TTBS solution (Tris: 10 mM; NaCl: 150 mM; Tween: 0.05%), and incubated 1 h with either a polyclonal antibody developed from a rat hepatic FABPPM (10 µl/20 ml of TTBS) or a polyclonal antibody developed from a human heart FABPC (5 µl/50 ml of TTBS). After further washing with TTBS, the antibody–antigen complex was incubated with a goat anti-rabbit IgG (1 µl/50 ml of TTBS for FABPPM and 5 µl/50 ml of TTBS for FABPC) for 1 h. Membranes were washed and stained with an alkaline phosphatase staining kit (BCIP/NBT) following the recommendations of the manufacturer (Promega, Madison, Wis.). The single band detected on the blots corresponded to the expected molecular mass for FABPC (14 kDa) and FABPPM (40 kDa). Although mitochondrial aspartate aminotransferase protein is recognized with the FABPPM antibody, the effect of this detection is minimal in the present study, since almost 90 to 95% of the FABPPM contained in the total homogenate has been shown to be from the skeletal muscle plasma membrane (39) . Standard amounts of a human latissimus dorsi muscle homogenate or pure FABPC served as internal control on each membrane and the integrated density of stained bands was measured twice using the NIH Image analysis software.

Statistical analysis
Data are presented as mean ± SE, unless otherwise indicated. Two-way analysis of variance (ANOVA) was used to examine for effects of group (obese vs. lean) and gender with respect to muscle markers, body composition, and insulin sensitivity. Repeated measures ANOVA was used to examine for the effect of weight loss on these measures. Linear regression and stepwise multiple regression were used to detect a correlation among adiposity, VO2max, insulin sensitivity, and muscle markers. Statistics were performed using Sigma Stat 2.0 (SPSS, Chicago, Ill.).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Body composition and insulin sensitivity
Clinical characteristics of body composition and values for insulin-stimulated glucose metabolism and VO2max are shown in Table 1 (changes in these parameters among the cohort who completed the weight loss intervention are subsequently shown in Table 3 ). Compared with lean subjects, those who were obese had reduced insulin sensitivity, lower VO2max, and greater FM. There were some gender-related differences. Though matched for BMI, women had a higher percentage of FM than did men and had lower mean VO2max. As previously reported in this cohort of subjects (11) , there were significant negative correlations between insulin sensitivity and obesity (BMI and FM).


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  		Table 1. Clinical characteristics of lean and obese men and womena


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  		Table 3. Effects of weight loss on body composition, insulin sensitivity, and maximal aerobic powera

Influence of obesity on skeletal muscle markers of glycolysis and oxidative and fatty acid metabolic pathways
Within vastus lateralis skeletal muscle, obese subjects had higher activity for the two marker enzymes of glycolysis (PFK and GAPDH) and somewhat lower activities for marker enzymes of oxidative capacity (CS, COX, and HADH), as shown in Table 2 . The glycolytic to oxidative ratio (PFK/CS), an integrative parameter of skeletal muscle metabolic differentiation (40) , was higher in obesity (4.1±0.9 vs. 6.5±0.5 for lean and obese women, and 6.0±0.8 vs. 7.0±0.5 for lean and obese men).


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  		Table 2. Skeletal muscle markers in lean and obese men and womena

Levels of muscle FABPs were either similar or increased in obese compared to lean subjects. The FABPPM content of muscle was ~30% higher in obese compared to lean men, whereas it was almost 60% higher in obese women compared to their lean counterpart. Obese men also had a greater content within muscle for FABPC than did lean men, but there was a significant gender difference: women had significantly greater content of FABPC in skeletal muscle compared to men; among women, no difference for FABPC was observed in regard to obesity. Another marker related to capacity of skeletal muscle for the uptake of FA is activity of Hr LPL, which did not differ in lean and obese subjects, nor did activity of Ext LPL.

Although the levels of FABPs and LPL activity indicate that capacity of skeletal muscle for taking up FA or for cytosolic binding is not impaired in obesity, the activity of the CPT complex, regarded as rate-limiting for entry of long-chain fatty acylCoA esters into mitochondria, was lower in obesity (P<0.01). Across the cohort of subjects, CPT activity in skeletal muscle was positively correlated with activity of CS (r=0.55, P<0.001), COX (r=0.60, P<0.001) and with HADH (r=0.51, P<0.001), indicating that its activity was correlated to markers of oxidative enzyme capacity within skeletal muscle. To discern whether the reduced activity of CPT in obesity was more pronounced than the general pattern of reduced oxidative capacity, the CPT:COX ratio was calculated. There was no difference for the CPT:COX ratio between lean and obese subjects (nor when other oxidative marker enzymes were used in the denominator), suggesting that decreased skeletal muscle activity of CPT in obesity appears to be proportionate to reduced activity of oxidative enzymes and, indirectly, to the mitochondrial content of muscle.

To explore further the concept that the muscle capacity for FA uptake might be differentially regulated from its capacity for FA oxidation in obesity, the ratio of CPT:FABPPM was calculated. The CPT:FABPPM ratio was significantly lower in obese compared to lean individuals (P<0.001), as shown in Fig. 1 .



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  		Figure 1. The ratio of CPT activity within vastus lateralis skeletal muscle to that of FABPPM was determined for 15 lean subjects, 40 obese subjects, and for 32 of the obese subjects who were studied after weight loss. There was a highly significant difference between lean and obese subjects (P<0.001) and no significant effect of weight loss to change the value of this ratio.

Correlation of muscle markers with obesity and insulin resistance
In addition to group differences based on categorical definition of obese vs. non-obese status, data on markers of lipid metabolism were examined for correlation with phenotypes of insulin sensitivity and adiposity. There was a modest negative correlation between PFK:CS ratio and insulin sensitivity (r =-0.37, P<0.01), indicating the skeletal muscle with heightened glycolytic capacity relative to its oxidative enzyme capacity manifests insulin resistance. Several markers of FA metabolism also correlated with insulin sensitivity. Muscle CPT activity was positively correlated with insulin sensitivity (r=0.34, P<0.01) whereas muscle FABPPM was negatively related to insulin sensitivity (r =-0.33, P<0.05), as was activity of Ext LPL (r=-0.39, P<0.01). FABPC content within muscle did not correlate significantly with insulin sensitivity. Because the ratio of CPT:FABPPM was lower in obesity, its relation to insulin sensitivity was examined and a significant positive correlation was observed (r=0.43, P<0.01). Moreover, the CPT:FABPPM ratio was significantly and negatively correlated with the PFK:CS ratio (r =-0.46, P<0.001), suggesting that the perturbation between the glycolytic and oxidative capacity is interrelated to an imbalance between uptake and mitochondrial oxidative capacities for FA metabolism.

Effects of weight loss on FA, glycolytic, and oxidative metabolic capacities of skeletal muscle
Thirty-two obese subjects completed the 4 month weight loss program and a post-weight loss (post-WL) metabolic assessment. Data on body composition and insulin sensitivity are shown in Table 3 . After weight loss, there were significant decreases in adiposity and a significant improvement in insulin sensitivity. One of the goals of our intervention was to isolate effects of dietary-induced weight loss and not introduce effects of exercise training, at least in an uncontrolled manner. This goal was met as values for VO2max were unchanged after weight loss and were within a range associated with sedentary lifestyles.

The metabolic data from pre- and post-WL muscle biopsies are shown in Table 4 .Only very few gender differences appeared in the response to body weight loss intervention. Glycolytic enzyme activities in skeletal muscle (GAPDH and PFK) did decrease after weight loss. The activity of CS in skeletal muscle did not change after weight loss, whereas COX and HADH activities decreased significantly in women, but not in men. Activities of muscle CPT as well as of Hr and Ext LPL did not change in men or women. Muscle content of FABPC and of FABPPM were not altered by weight loss. There were no changes in the ratio of glycolytic to oxidative enzyme capacity (PFK/CS) or in the ratio of CPT:FABPPM (Fig. 1) in men and women after weight loss.


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  		Table 4. Skeletal muscle markers in lean and obese men and women before and after weight loss


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Obesity can cause insulin resistance of skeletal muscle, whereas even moderate amounts of weight loss can improve insulin sensitivity. The current study, which entailed both a cross-sectional comparison of lean and obese individuals and weight loss intervention, reaffirms these principles while addressing a more novel concept, that the levels of metabolic markers of skeletal muscle for the utilization of FA are altered in obesity in a manner that favors lipid accumulation. Skeletal muscle, owing to its size and importance in the regulation of systemic energy expenditure (41) , has a key role in the utilization of both carbohydrate and lipid substrates (13) . Several metabolic steps might contribute to an impaired reliance on FA oxidation within skeletal muscle during fasting conditions. In a separate report of the postabsorptive rates of FFA uptake and oxidation from the participants in this study based on values measured with the use of the arteriovenous leg balance technique, key findings in obesity were that fasting patterns of fatty acid metabolism within skeletal muscle favor lipid esterification rather than oxidation (42) . The goal of the current investigation was to assess whether there were obesity-related perturbations in the activity or content of metabolic markers that play a role either in the entrance of FA through the membrane of the muscle cell, in intracellular transport, in entrance of FA into mitochondria, and in the capacity for FA oxidation by mitochondria. Novel findings reported in the current study demonstrate that a number of biochemical markers of the capacity of skeletal muscle to metabolize FA are altered in obesity.

Numerous FABPs have been described in recent years (24 , 43 , 44) . Some are thought to be involved in the transmembrane movement of FA, but so far there have been relatively few studies in human skeletal muscle. Although the exact role of the plasma membrane fatty acid binding protein has not been completely elucidated, recent evidence has shown that it may be an important component of a fatty acid transport system in muscle. It was recently shown that fatty acid binding to plasma membrane proteins and fatty acid uptake by giant sarcolemmal vesicles in muscle saturate with an increase in unbound fatty acid concentration and are substantially inhibited by the presence of antibodies to FABPPM (45 , 46) . The uptake data are an important piece of evidence, because when using giant sarcolemmal vesicles, uptake reflects only cellular transport rather than cellular transport and metabolism. Thus, the presence of saturation for fatty acid uptake and of inhibition by the antibodies to FABPPM suggests that a fatty acid transport system exists in muscle and that FABPPM plays a role in this transport system (24 , 45 , 46) . In the present study, FABPPM content was increased in obesity in both men and women and also correlated with the phenotype of insulin-resistant glucose metabolism. These data on FABPPM suggest that skeletal muscle in obese individuals maintains at least an equivalent and perhaps an enhanced capacity for the entrance of FA within the muscle cell.

Lipoprotein lipase, implicated in the processes of FA entrance through its action as a triglyceride hydrolase within the muscle cell (47 48 49) , has received particular attention in obesity studies. This enzyme within muscle exists as two metabolically active fractions; each represents approximately half of total LPL activity in human muscle (50) , proportions that were confirmed by the Hr and Ext LPL activities assayed in the current study. The Hr LPL is considered to be present at the capillary endothelium and active in hydrolyzing triglycerides within lipoproteins (48) . Although the cellular control of intramuscular (i.m.) triglyceride metabolism presumably involves two major identified lipases—hormone sensitive lipase and LPL—the Ext LPL was proposed to serve to replenish enzyme at the capillary surface or to be involved in the hydrolysis of intracellular triglyceride in skeletal muscle (51) . In the current study, Hr LPL activity was not different between lean and obese subjects and was not correlated with measures of body composition or insulin sensitivity. These results agree with prior observations in nondiabetic obese and non-obese individuals (6) , though some conflicting data have also been reported (5 , 52 , 53) . However, Ext LPL activity was increased in relation to insulin resistance. If the concept is correct that Ext LPL is involved in i.m. triglyceride lipolysis, then its content within muscle might contribute to insulin resistance in a causal manner by elevating the intracellular content of triglycerides within the muscle cell. Further research is needed to specifically examine this concept, particularly in view of the increased fat content that has been described in the myocytes of obese individuals (12) .

Other novel information reported here is the determination of the muscle cytosolic 14 kDa FABP. FABPC is a relatively abundant cytosolic protein, and although our current understanding of its precise physiological role(s) has not advanced as far as knowledge of structure, it can viewed as a molecule that provides niches for hydrophobic binding of fatty acids and long-chain acyl CoA esters within cells (43) . Accordingly, it may serve to protect the internal milieu from adverse effects of "free' FA and perhaps subserve some directional trafficking of lipid substrates. Absolute values of FABPC obtained in the present study are similar to those previously reported by Van Nieuwenhoven et al. (54) . Although a comparison is not always possible, cytosolic FABP data have also been published for rat muscles; the values reported have ranged from 13 ± 1 (superficial and white portion of quadriceps) to 303 ± 24 (soleus) µg/g of wet weight muscle (55) . The current study demonstrates that the content of FABPC within skeletal muscle is normal in obesity. Among our cohort of obese and non-obese subjects, no correlation between insulin sensitivity and FABPC was observed, though women had a significantly greater content of FABPC within muscle. Although FABPC content did not differ between lean and obese women, there was a significantly higher content in obese compared to lean men. This increased amount of that protein in muscle may be without major consequence on the processes of FA oxidation, however, since no effect on rates of FA oxidation capacity was observed in a diabetic animal model with overexpression of FABPC in skeletal muscle (56) .

To test whether there was an imbalance between the capacity for the uptake and binding of FA within the cytosol and the mitochondrial enzyme capacity for FA oxidation, the activity of CPT, the enzyme complex regarded as a rate-limiting step for the entrance and oxidation of long-chain fatty acid CoA esters (57 , 58) , and two markers of the mitochondrial oxidative capacity (CS and COX) were measured. Confirming previous findings obtained in a smaller number of women (7) , there was a reduction in muscle CPT activity. This lower CPT activity in obesity was proportionate to a reduction in CS, an enzyme of the Krebs cycle, and COX, one of the regulatory step of the respiratory chain, findings again confirming previous data from our laboratories (59) . Considering that studies carried out predominately in animal models of insulin resistance and obesity reported increased muscle content of malonyl CoA, a strong allosteric inhibitor of CPT I (31) , a decreased level of CPT combined with an increased malonyl CoA content would create a favorable intracellular metabolic condition for a reduced or impaired capacity for FA oxidation within skeletal muscle.

Borrowing from the concept of the glycolytic to oxidative enzyme ratio, a ratio that has been used to characterize the metabolic differentiation of skeletal muscle (40) and has been shown by us to be strongly related to insulin resistance (10) , we examined the ratio between CPT activity and the content of FABPPM as a means of integrating into a single value the biochemical capacities for FA binding and entrance at the level of the plasma membrane of the muscle cell and mitochondrial oxidation. In obesity, the CPT:FABPPM ratio is reduced to one-half the value found in skeletal muscle of lean individuals. Moreover, this ratio correlated with the phenotype of insulin resistance. The inference we derive from the CPT:FABPPM ratio is that in obesity, skeletal muscle appears to be equipped to take up FA from plasma but has a disproportionately reduced capacity for their oxidation. As an increased triglyceride content within skeletal muscle occurs in obesity and is found to be a relatively strong marker of insulin resistance, our findings seem to provide a potential biochemical mechanism for an enhanced disposition for esterification of FA.

Cross-sectional studies cannot fully clarify whether a decrease in the metabolic capacities within muscle of obese subjects occurs as a consequence of obesity or, instead, might add to the risk of becoming obese. In the current study, a weight loss intervention was used to address this issue. Regrettably, we found that weight loss, at least weight loss as induced by dietary intervention, does not rectify the obesity-related metabolic derangements of skeletal muscle, suggesting that the observed perturbations most likely do not develop as a consequence of obesity. Among the eight different markers of the skeletal muscle aerobic-oxidative and FA metabolism investigated in the current study, neither CPT activity nor the activity or content of the other markers, except the reduction in COX and HADH activities in women, were modified after weight loss. There is undoubtedly a need to determine whether interindividual variations in these skeletal muscle phenotypes are under a strict control of genetic factors or whether they can be altered by strategies other than dietary restriction. At least in lean individuals, exercise training not only enhances insulin sensitivity, but also increases the capacity of skeletal muscle for FA oxidation (23 , 50) . This suggests that as exercise induces metabolic fitness in skeletal muscle, this entails enhanced capacities for both glucose and FA utilization, and that in sedentary obese individuals the `metabolic unfitness' of skeletal muscle entails a combined impaired of both glucose and FA metabolic pathways. The failure of weight loss to improve oxidative capacity of skeletal muscle, including activity of CPT, despite its positive effects on insulin sensitivity, could demarcate a biochemical risk factor for the recidivism of weight gain, which unfortunately is quite common after dietary-induced weight loss.

In summary, the metabolic capacities of skeletal muscle ascertained in obese subjects within the current study form an overall profile characterized by an imbalance between the capacity for the uptake and transport of FA within the muscle cell and a reduced capacity of their oxidation within the mitochondria. These altered patterns correlate with the phenotype of insulin resistance in weight stable obese individuals; yet unlike glucose metabolism, which improves after weight loss, the biochemical markers of FA metabolism were essentially unaffected by dietary restriction and remained different from muscle of lean individuals. This composite of muscle metabolic markers further strengthens the concept that the mitochondrial bioenergetic capacities of skeletal muscle are perturbed in human obesity and may contribute both to the expression of insulin-resistant patterns of glucose metabolism and in partitioning fat toward esterification within muscle.


   ACKNOWLEDGMENTS
 
The cooperation of our research volunteers is gratefully acknowledged. We appreciate the skill and cooperation of the nursing and nutritional staff at the General Clinical Research Center and particularly the contribution of Dr. Rena Wing at the Obesity and Nutrition Research Centers of the University of Pittsburgh. We wish to specifically acknowledge the efforts of Nancy Mazzei, Bret Goodpaster, Yves Gélinas, and Michel Lacaille of Laval University and the secretarial support of Nancy Penny. This project was supported by funding from National Institutes of Health R01 DK49200–02, 5M01RR00056 (General Clinical Research Center), and 1P30DK46204 (Obesity and Nutrition Research Center). We also acknowledge the support of Novartis Pharmaceuticals, which provided the Optifast for the weight loss intervention, and the Zumberge Research and Innovation Fund of the University of California.


   FOOTNOTES
 
1 Dr. Jean-Aime Simoneau passed away on August 27, 1999, after a lengthy illness. His deep commitment, and indeed passion, for scientific inquiry will be sorely missed. In a career that was too brief, he made outstanding and original contributions to our understanding of muscle biochemistry and of its importance to obesity and diabetes.

Received for publication November 17, 1998. Revised for publication April 22, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Dagenais, G., Tancredi, R., Zierler, K. (1976) Free fatty acid oxidation by forearm muscle at rest, and evidence for an intramuscular lipid pool in the human forearm. J. Clin. Invest. 58,421-431
  2. Goodyear, L. J., Giorgino, F., Sherman, L. A., Carey, J., Smith, R. J., Dohm, G. L. (1995) Insulin receptor phosphorylation, insulin receptor substrate-1 phosphorylation, and phosphatidylinositol 3-kinase activity are decreased in intact skeletal muscle strips from obese subjects. J. Clin. Invest. 95,2195-2204
  3. Dohm, G. L., Tapscott, E. B., Pories, W. J., Dabbs, D. J., Flickinger, E. G., Meelheim, D., Fushiki, T., Atkinson, S. M., Elton, C. W., Caro, J. F. (1988) An in vitro human muscle preparation suitable for metabolic studies. J. Clin. Invest. 82,486-494
  4. Cortright, R. N., Muoio, D. M., Dohm, G. L. (1997) Skeletal muscle lipid metabolism: a frontier for new insights into fuel homeostasis. Nutr. Biochem. 8,228-245
  5. Eckel, R. H., Yost, T. J., Jensen, D. R. (1995) Sustained weight reduction in moderately obese women results in decreased activity of skeletal muscle lipoprotein lipase. Eur. J. Clin. Invest. 25,396-402[Medline]
  6. Eckel, R. H., Yost, T. J., Jensen, D. R. (1995) Alterations in lipoprotein lipase in insulin resistance. Int. J. Obes. 19,S16-S21
  7. Colberg, S. R., Simoneau, J.-A., Thaete, F. L., Kelley, D. E. (1995) Skeletal muscle utilization of free fatty acids in women with visceral obesity. J. Clin. Invest. 95,1846-1853
  8. Pan, D. A., Lillioja, S., Kriketos, A. D., Milner, M. R., Baur, L. A., Bogardus, C., Jenkins, A. B., Storlien, L. H. (1997) Skeletal muscle triglyceride levels are inversely related to insulin action. Diabetes 46,983-988[Abstract]
  9. Phillips, D. I. W., Caddy, S., Ilic, V., Fielding, B. A., Frayn, K. N., Borthwirk, A. C., Taylor, R. (1996) Intramuscular triglyceride and muscle insulin sensitivity: evidence for a relationship in nondiabetic subjects. Metabolism 45,947-950[Medline]
  10. Simoneau, J.-A., Colberg, S. R., Thaete, F. L., Kelley, D. E. (1995) Skeletal muscle glycolytic and oxidative enzyme capacities are determinants of insulin sensitivity and muscle composition in obese women. FASEB J 9,273-278[Abstract]
  11. Goodpaster, B. H., Thaete, F. L., Simoneau, J.-A., Kelley, D. E. (1997) Subcutaneous abdominal fat and thigh muscle composition predict insulin sensitivity independently of visceral fat. Diabetes 46,1579-1585[Abstract]
  12. Simoneau, J.-A., Kelley, D. E. (1999) Metabolic aspects of skeletal muscle in obesity. Ailhaud, G. Guy-Grand, B. eds. Progress in Obesity Research Vol. 8,11-414 John Libbey & Co London.
  13. Kelley, D. E., Reilly, J. P., Veneman, T., Mandarino, L. J. (1990) Effects of insulin on skeletal muscle glucose storage, oxidation, and glycolysis in humans. Am. J. Physiol. 258,E923-E929[Abstract/Free Full Text]
  14. Bogardus, C., Lillioja, S., Mott, D., Hollenbeck, C., Reaven, G. (1985) Relationship between degree of obesity and in vivo insulin action in man. Am. J. Physiol. 248,E286-E291[Abstract/Free Full Text]
  15. Felber, J. P., Ferrannini, E., Golay, A., Meyer, H., Thiebaud, D., Curchod, B., Maeder, E., Jéquier, E., DeFronzo, R. (1987) Role in lipid oxidation in the pathogenesis of insulin resistance of obesity and type II diabetes. Diabetes 36,1341-1350[Abstract]
  16. Kelley, D. E., Simoneau, J.-A. (1994) Impaired free fatty acid utilization by skeletal muscle in non-insulin-dependent diabetes mellitus. J. Clin. Invest. 94,2349-2356
  17. Mandarino, L. J., Consoli, A., Kelley, D. E. (1993) Differential regulation of intracellular glucose metabolism by glucose and insulin in human muscle. Am. J. Physiol. 265,E898-E905[Abstract/Free Full Text]
  18. Zurlo, F., Lillioja, S., A, E.-D., Nyomba, B. L., Raz, I., Saad, M. F., Swinburn, B. A., Knowler, W. C., Bogardus, C., Ravussin, E. (1990) Low ratio of fat to carbohydrate oxidation as predictor of weight gain: a study of 24-h RQ. Am. J. Physiol. 259,E650-E657[Abstract/Free Full Text]
  19. Ferraro, R., Eckel, R. H., Larson, D. E., Fontvieille, A. M., Rising, R., Jensen, D. R., Ravussin, E. (1993) Relationship between skeletal muscle lipoprotein lipase activity and 24-hour macronutrient oxidation. J. Clin. Invest. 92,441-445
  20. Zurlo, F., Nemeth, P. M., Choksi, R. M., Sesodia, S., Ravussin, E. (1994) Whole-body energy metabolism and skeletal muscle biochemical characteristics. Metabolism 43,481-486[Medline]
  21. Lean, M. E. J., James, W. P. T. (1988) Metabolic effects of isoenergetics nutrient exchange over 24 hours in relation to obesity in women. Int. J. Obes. 12,15-27[Medline]
  22. Blaak, E. E., van Baak, M. A., Kemerink, G. J., Pakbiers, M. T., Heidendal, G. A., Saris, W. H. (1994) Beta-adrenergic stimulation of skeletal muscle metabolism in relation to weight reduction in obese men. Am. J. Physiol. 267,E316-E322[Abstract/Free Full Text]
  23. Turcotte, L., Richter, E., Kiens, B. (1992) Increased plasma FFA uptake and oxidation during prolonged exercise in trained and untrained humans. Am. J. Physiol. 262,E791-E799[Abstract/Free Full Text]
  24. Turcotte, L. P. (1999) Fatty acid binding proteins and muscle lipid metabolism in skeletal muscle. Hargreaves, M. eds. Biochemistry of Exercise X ,201-215 Human Kinetics Champaign, Illinois.
  25. Kiens, B., Kristiansen, S., Jensen, P., Richter, E. A., Turcotte, L. P. (1997) Membrane-associated fatty acid binding protein (FABPPM) in human skeletal muscle is increased by endurance training,. Biochem. Biophys. Res. Commun. 231,463-465[Medline]
  26. Simoneau, J.-A. (1995) Adaptation of human skeletal muscle to exercise-training. Int. J. Obes. 19,S9-S13
  27. Hickey, M. S., Weidner, M. D., Gavigan, K. E., Zheng, D., Tyndall, G. L., Houmard, J. A. (1995) The insulin action-fiber type relationship in humans is muscle group specific. Am. J. Physiol. 269,E150-E154[Abstract/Free Full Text]
  28. Bass, A., Vondra, K., Rath, R., Vitek, V., Havaranck, T. (1975) Metabolic changes in the quadriceps femoris muscle of obese people: enzyme activity patterns of energy-supplying metabolism. Pfluegers Arch 359,325-334[Medline]
  29. Simoneau, J.-A., Bouchard, C. (1995) Skeletal muscle metabolism and body fat content in men and women. Obes. Res. 3,23-29[Medline]
  30. Saha, A. K., Vavvas, D., Kurowski, T. G., Apazidis, A., Witters, L. A., Shafrir, E., Ruderman, N. B. (1997) Malonyl-CoA regulation in skeletal muscle: its link to cell citrate and the glucose-fatty acid cycle. Am. J. Physiol. 272,E641-E648[Abstract/Free Full Text]
  31. Ruderman, N. B., Saha, A. K., Vavvas, D., Heydrick, S. J., Kurowski, T. G. (1997) Lipid abnormalities in muscle of insulin-resistant rodents. Ann. N.Y. Acad. Sci. 827,221-230[Free Full Text]
  32. DeFronzo, R. A., Tobin, J. D., Andres, R. (1979) Glucose clamp technique: a method of quantifying insulin secretion and resistance. Am. J. Physiol. 237,E214-E223[Abstract/Free Full Text]
  33. Finegood, D. T., Bergman, R. N., Vranic, M. (1987) Estimation of endogenous glucose production during hyperinsulinemic-euglycemic glucose clamps. Comparison of unlabeled and labeled exogenous glucose infusates. Diabetes 36,914-924[Abstract]
  34. Kelley, D. E., Wing, R., Buomocore, C., Fitzsimmons, M., Sturis, J., Polonsky, K. (1993) Relative effects of calorie restriction and weight loss in noninsulin dependent diabetes mellitus. J. Clin. Endocrinol. Metab. 77,1287-1293[Abstract]
  35. Gauthier, J. M., Thériault, R., Thériault, G., Gélinas, Y., Simoneau, J.-A. (1992) Electrical stimulation-induced changes in skeletal muscle enzymes of men and women. Med. Sci. Sports Exercise 24,1252-1256[Medline]
  36. Howald, H., Pette, D., Simoneau, J.-A., Uber, A., Hoppeler, H., Cerretelli, P. (1990) Effects of chronic hypoxia on muscle enzyme activities. Int. J. Sports Med. 11,S10-S14
  37. Taskinen, M., Nikkila, E. A., Huttunen, J. K., Hilden, H. A. (1980) A micromethod for assay of lipoprotein lipase activity in needle biopsy samples of human adipose tissue and skeletal muscle. Clin. Chim. Acta 104,107-117[Medline]
  38. Belfrage, P., Vaughan, M. (1969) Simple liquid-liquid partition system for isolation of labeled oleic acid from mixtures with glycerides. J. Lipid Res. 10,341-344[Abstract]
  39. Turcotte, L. P., Srivastava, A. K., Chiasson, J.-L. (1997) Fasting increases plasma membrane fatty-acid binding protein (FABPPM) in red skeletal muscle. Mol. Cell. Biochem. 166,153-158[Medline]
  40. Pette, D., Spamer, C. (1986) Metabolic properties of muscle fibers. Federation Proc 45,2910-2914[Medline]
  41. Ferraro, R., Lillioja, S., Fontvielle, A.-M., Rising, R., Bogardus, C., Ravussin, E. (1992) Lower sedentary metabolic rate in women compared with men. J. Clin. Invest. 90,780-784
  42. Kelley, D. E., Simoneau, J.-A., Goodpaster, B., Troost, F. (1997) Defects of skeletal muscle fatty acid metabolism in obesity. Obes. Res. 5,21S
  43. Veerkamp, J. H., Maatman, G. H. J. (1995) Cytoplasmic fatty acid-binding proteins: their structure and genes. Prog. Lipid Res. 34,17-52[Medline]
  44. Calles-Escadon, J., Sweet, L., Ljungqvist, O., Hirshman, M. F. (1996) The membrane-associated 40 kD fatty acid binding protein (Berk's protein), a putative fatty acid transporter is present in human skeletal muscle. Life Sci 58,19-28[Medline]
  45. Bonen, A., Luiken, J. J., Liu, S., Dyck, D. J., Kiens, B., Kristiansen, S., Turcotte, L. P., van der Vusse, G. J., Glatz, J. F. (1998) Palmitate transport and fatty acid transporters in red and white muscles. Am. J. Physiol 275,E471-E478[Abstract/Free Full Text]
  46. Turcotte, L. P., Swenberger, J. R., Tucker, M. Z., Bonen, A. (1998) Palmitate transport and binding in rat muscle are saturable and inhibited by antibodies to FABPPM. FASEB J. 12,A231(abstr.)
  47. Zechner, R. (1997) The tissue-specific expression of lipoprotein lipase: implications for energy and lipoprotein metabolism. Curr. Opin. Lipidol. 8,77-88[Medline]
  48. Eckel, R. H. (1989) Lipoprotein lipase. A multifunctional enzyme relevant to common metabolic diseases. N. Engl. J. Med. 320,1060-1068[Abstract]
  49. Fielding, B. A., Frayn, K. N. (1998) Lipoprotein lipase and the disposition of dietary fatty acids. Br. J. Nutr. 80,495-502[Medline]
  50. Seip, R. L., Angelopoulos, T. J., Semenkovich, C. F. (1995) Exercise induces human lipoprotein lipase gene expression in skeletal muscle but not adipose tissue. Am. J. Physiol. 268,E229-E236[Abstract/Free Full Text]
  51. Oscai, L. B., Essig, D. A., Palmer, W. K. (1990) Lipase regulation of muscle triglyceride hydrolysis. J. Appl. Physiol. 69,1571-1577[Abstract/Free Full Text]
  52. Evans, D. J., Murray, M., Kissebah, A. H. (1984) Relationship between skeletal muscle insulin resistance, insulin-mediated glucose disposal, and insulin binding: effects of obesity and body fat topography. J. Clin. Invest. 74,1515-1525
  53. Pollare, T., Vessby, B., Lithell, H. (1991) Lipoprotein lipase activity in skeletal muscle is related to insulin sensitivity. Arteriosclerosis Thromb 11,1192-1203[Abstract/Free Full Text]
  54. Van Nieuwenhoven, F. A., Kleine, A. H., Wodzig, W. H., Hermens, W. T., Kragten, H. A., Maessen, J. G., Punt, C. D., Van Dieijen, M. P., van der Vusse, G. J., Glatz, J. F. C. (1995) Discrimination between myocardial and skeletal muscle injury by assessment of the plasma ratio of myoglobin over fatty acid-binding protein. Circulation 92,2848-2854[Abstract/Free Full Text]
  55. Vork, M. M., Glatz, J. F. C., Surtel, D. A. M., Knubben, H. J. M., van der Vusse, G. J. (1991) A sandwich enzyme linked immuno-sorbent assay for the determination of rat heart fatty acid-binding protein using the streptativin-biotin system. Application to tissue and effluent samples from normoxic rat heart perfusion. Biochim. Biophys. Acta 1075,199-205[Medline]
  56. Veerkamp, J. H., van Moerkerk, H. T. B., Vandenborn, J. (1996) No correlation between changes in fatty acid-binding protein content and fatty acid oxidation capacity of rat tissues in experimental diabetes. Int. J. Biochem. Cell Biol. 28,473-478[Medline]
  57. McGarry, J. D., Brown, N. F. (1997) The mitochondrial carnitine palmitoyltransferase system: from concept to molecular analysis. Eur. J. Biochem. 244,1-14[Medline]
  58. Brady, P. S., Ramsay, R. R., Brady, L. J. (1993) Regulation of the long-chain carnitine acyltransferases. FASEB J 7,1039-1044[Abstract]
  59. Simoneau, J.-A., Kelley, D. E. (1997) Skeletal muscle and obesity. Bray, G. A. Bouchard, C. James, W. P. T. eds. Handbook of Obesity ,539-553 Marcel Dekker, Inc. New York.



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