RESUMEN
The indoleamine melatonin is ubiquitously distributed, and because of its small size and amphiphilic nature, it is able to reach easily all cellular compartments. The highest intracellular melatonin concentrations are found in the mitochondria, suggestive of local metabolism and/or direct participation in organelle function. In mitochondria cytochrome c (cyt c) could represent a melatonin target since it has the capability to oxidize organic molecules in the presence of H2O2, and mitochondria are the main site of H2O2 production in nonphagocytic cells. Therefore, we investigated oxidation of melatonin by cyt c/H2O2 couple as a potential pathway for its metabolism in the mitochondria. We found melatonin conversion into N(1)-acetyl-N(2)-formyl-5-methoxykynuramine via sequential steps that generate the intermediates 2-hydroxymelatonin and 2,3-dihydroxymelatonin. We experimentally excluded mediation by a Fenton/Haber-Weiss-type reaction and documented the dependence on oxoferryl heme for melatonin oxidation. Given the high mitochondrial concentrations of both melatonin and cyt c as well as the continuous generation of H2O2 during respiration, it is entirely possible that mitochondrial cyt c-mediated oxidation of melatonin may be a plausible pathway of its biotransformation in vivo.
Asunto(s)
Citocromos c/metabolismo , Melatonina/metabolismo , Animales , Peróxido de Hidrógeno/metabolismo , Masculino , Oxidación-Reducción , Ratas , Ratas Wistar , Espectrofotometría UltravioletaRESUMEN
We have recently uncovered the full expression of novel cutaneous serotoninergic and melatoninergic systems in the human and hamster skin. In this work, we have characterized serotonin metabolism in the rat skin using liquid chromatography-mass spectrometry and found that serotonin undergoes acetylation in the presence of acetyl coenzyme A. Inhibition of serotonin acetylation with Cole bisubstrate inhibitor shows that rat skin expresses both arylalkylamine and arylamine N-acetyltransferase activities. The serotonin degradation product-5-hydroxyindole acetic acid is also detected and pargyline (monoaminooxidase inhibitor) suppresses almost completely 5-hydroxyindole acetic acid accumulation. Together with previous data, the present study clearly demonstrates that biotransformation of serotonin in mammalian skin follows two alternate pathways. In the first pathway, serotonin is acetylated by arylalkylamine and arylamine N-acetyltransferases to generate the precursor of melatonin. Alternately, serotonin may undergo oxidative deamination by monoaminooxidase followed by enzymatic degradation by aldehyde dehydrogenase into 5-hydroxyindole acetic acid, which is presumably devoid of biological activity. Thus, the current methodological development of a liquid chromatography-mass spectrometry-based assay allows rapid resolution of the cutaneous metabolism of serotonin.