The Tyrosine Hydroxylase Activity of Mammalian Tyrosinase

Abstract The tyrosine hydroxylase activity of tyrosinase from hamster melanoma has been studied by measurement of the release of tritium from tyrosine-3,5-3H. Experiments with mixtures of tyrosine-3,5-3H and tyrosine-14C show that there is little or no tritium rate effect in this reaction. Dopa is the most effective hydrogen donor and will eliminate the lag period when used in catalytic amounts. 2-Amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine eliminates the lag when it is at a concentration of 2.4 x 10-3 m and shortens the lag at lower concentrations. Substrate quantities of ascorbate, reduced tri- and diphosphopyridine nucleotide, 3,4-dihydroxyphenylethylamine, epinephrine, and norepinephrine do not abolish the lag. 3,4-Dihydroxyphenylalanine at high concentration is an inhibitor of tyrosine hydroxylation, while tyrosine acts as a substrate inhibitor at concentrations above 8 x 10-4 m. The substrate inhibition can be partially reversed by increasing the concentration of 3,4-dihydroxyphenylalanine from the catalytic level to 8 x 10-4 m. Ascorbate magnifies the inhibition by 3,4-dihydroxyphenylalanine. Inhibition by cyanide, diethyldithiocarbamate, 5-hydroxyindole, and 5-hydroxy-dl-tryptophan is also reported.