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Constitutive and UV-induced metabolism of melatonin in keratinocytes and cell-free systems

Tobias W. Fischer^*,, Trevor W. Sweatman, Igor Semak^*,||, Robert M. Sayre, Jacobo Wortsman^# and Andrzej Slominski^*,1

^* Department of Pathology and Laboratory Medicine,

Department of Pharmacology, and

Division of Dermatology, University of Tennessee Health Science Center, Memphis, Tennessee USA;

Department of Dermatology, University Hospital Schleswig-Holstein, University of Lübeck, Lübeck, Germany;

^|| Department of Biochemistry, Belarus State University, Minsk, Belarus; and

^# Department of Internal Medicine, Southern Illinois University, Springfield, Illinois, USA

¹Correspondence: Department of Patholology and Laboratory Medicine, University of Tennessee Health Science Center, 930 Madison Ave., Memphis, TN, 38163. E-mail: aslominski{at}utmem.edu

ABSTRACT

Melatonin, which can be produced in the skin, exerts a protective effect against damage induced by UV radiation (UVR). We have investigated the effect of UVB, the most damaging component of UVR, on melatonin metabolism in HaCaT keratinocytes and in a cell-free system. Four metabolites were identified by HPLC and LC-MS: 6-hydroxymelatonin, N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK), 2-hydroxymelatonin (the main intermediate between melatonin and AFMK), and 4-hydroxymelatonin. Concentrations of these photoproducts were directly proportional to UVR-dose and to melatonin substrate content, and their accumulation was time-dependent. The UVR-dependent increase of AFMK and 2-hydroxymelatonin was also detected in keratinocytes, where it was accompanied by simultaneous consumption of intracellular melatonin. Of note, melatonin and its two major metabolites, 2-hydroxymelatonin and AFMK, were also detected in untreated keratinocytes, neither irradiated nor preincubated with melatonin. Thus, intracellular melatonin metabolism is enhanced under exposure to UVR. The additional biological activity of these individual melatonin metabolites increases the spectrum of potential actions of the recently identified cutaneous melatoninergic system.—Fischer, T. W., Sweatman, T. W., Semak, L., Sayre, R. M., Wortsman, J., Slominski, A. Constitutive and UV-induced metabolism of melatonin in keratinocytes and cell-free systems.

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