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Is ascorbic acid an antioxidant for the plasma membrane?

Departments of Medicine and Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-6303, USA

¹Correspondence: 736 Medical Research Building II, Vanderbilt University School of Medicine, 2220 Pierce Ave., Nashville, TN 37232-6303, USA. E-mail: james.may{at}mcmail.vanderbilt.edu

Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electrons to the -tocopheroxyl radical and a trans-plasma membrane oxidoreductase activity. Ascorbate-derived reducing capacity is thus transmitted both into and across the plasma membrane. Recycling of -tocopherol by ascorbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductase activity is evident in open membranes, ascorbate is the preferred electron donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic antioxidants such as -tocopherol or ubiquinol-10. Further studies are needed to elucidate the structural components, mechanism, and physiological significance of this activity. Proposed functions for the oxidoreductase include stimulation of cell growth, reduction of the ascorbate free radical outside cells, recycling of -tocopherol, reduction of lipid hydroperoxides, and reduction of ferric iron prior to iron uptake by a transferrin-independent pathway.—May, J. M. Is ascorbic acid an antioxidant for the plasma membrane?

Key Words: ascorbate free radical • dehydroascorbic acid • antioxidant • ferricyanide • oxidoreductase

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