O-METHYLATION OF L-DOPA IN MELANIN METABOLISM AND THE PRESENCE OF CATECHOL-O-METHYLTRANSFERASE IN MELANOCYTES

O-Methylation of L-dopa was investigated as a possible regulatory mech anism in melanin metabolism The methylation product of L-dopa, 3-O-met hoxytyrosine was detected in extracts of cultured human melanocytes. T he enzyme catechol-O-methyltransferase is responsible for this O-methy lation and that of the dihydroxyindolic intermediates of melanogenesis . The enzyme is present in melanocytes in its soluble and membrane-bou nd isoforms. Immune-electron microscopy suggests the presence of the m embrane-bound enzyme in the endoplasmic reticulum. This localization m ay indicate a role of catechol-O-methyltransferase in protecting the m elanocyte against reactive dihydroxyphenolic intermediates of melanoge nesis leaking from the melanogenic compartments. On the other hand, th e O-methylation of L-dopa may serve as a regulatory point in melanogen esis during early stage of tyrosinase processing in the endoplasmic re ticulum.