KINETIC-PROPERTIES OF LIGHT-DEPENDENT ASCORBIC-ACID OXIDATION BY MELANIN

The kinetic properties of ascorbic acid oxidation by light-activated m elanin granules demonstrate the presence of a specific reactive site o n the melanin granule saturable by ascorbic acid. Increased light inte nsity increased the V(max) and reduced the K(m) of this reaction, indi cating increased affinity of the active site for ascorbic acid. The ki netics of this reaction are not markedly changed in a reduced-oxygen e nvironment. Ascorbic acid oxidation is competitively inhibited by isoa scorbic acid, an epimer of ascorbic acid, while other tested reducing agents are inactive. The K(i) for isoascorbic acid is 1 mM, about the same as the K(m) of ascorbic acid.