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There is currently no general agreement on the enzymatic basis of neuromelanin synthesis. It is generally agreed that neuromelanin synthesis must differ from melanin synthesis in skin, since "tyrosinase" (aerobic dopa oxidase) is not present in brain. Various proposals have suggested that monoamine oxidase, prostaglandin H synthetase, or peroxidase may catalyze neuromelanin formation from dopa, dopamine and norepinephrine. It has also been proposed that neuromelanin synthesis may be based on the pseudoperoxidase activity of metal ions. Our group was the first to demonstrate enzymatic peroxidase activity in brain, which was subsequently confirmed by three other laboratories. Our group was also the first to demonstrate the melanogenic activity of brain peroxidase (subsequently confirmed by another laboratory) and to propose that peroxidase is the key enzyme in neuromelanin synthesis because of this potential and because of its presence in lysosomes, the subcellular site of neuromalin.

Melanosomal "tyrosinase" (L-dopa) was isolated from trypsin digest of B-16 mouse melanoma melanosomes, using polyacrylamide gel disc electrophoresis. The enzyme was represented by a single band, having characteristics similar to the T1 dopa-positive band observed when using supernatants of crude melanoma homogenates as the source. When gels with this band were incubated in solutions containing tyrosine and dopa in varying ratios , there was no enhancement of melanin formation by tyrosine when compared with incubations in corresponding concentrations of dopa alone. These data further support previous studies in our laboratory demonstrating an inability of so-called mamalian "tyrosinase" to convert tyrosine to melanin; since this enzyme readily converts L-dopa to melanin, it seems more reasonable to term this enzyme an L-dopa oxidase.

Peroxide-dependent enzymic oxidation of tyrosine to dopachrome and melanin was demonstrated in cell-free melanoma homogenates. Histochemical methods for distinguishing peroxidase activity from aerobic dopa (3,4-dihydroxyphenylalanine) oxidase activity are not reliable with cell-free preparations. Therefore the presence of peroxidase activity in such preparations precludes assay of cresolase activity of mammalian ;tyrosinase'.