In a previous investigation, of tyrosinase activity in the cells of the nucleus substantiae nigrae of adult cats and monkeys, diphenolase, but not monophenolase activity, was detected. This is possibly due to inhibition of monophenolase. When the copper radicle in tyrosinase is oxidized, monophenolase cannot be detected, but reducing agents activate and unmask this component of tyrosinase.

The effects of reducing agents in activating monophenolase have been investigated. Tyrosine is converted to melanin by monophenolase in adult nigra cells of the cat and monkey when the enzyme is activated by the reducing agents L-dopa, L-ascorbic acid, and L-adrenaline. Variations in physical factors influence the intensity of the activated monophenolase reaction resulting from incubation of sections in tyrosine-dopa mixtures; the reaction is inhibited by general enzyme inhibitors and specific tyrosinase inhibitors.

The significance of the results in relation to tyrosinase activity in adult nigra cells is discussed.

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