EFFECT OF CATECHOL-THIOL CONJUGATES ON TYROSINASE DEPENDENT TYROSINE HYDROXYLATION

Abstract

1. The tyrosinase reaction in the presence of a thiol compound was studied using mushroom tyrosinase (EC 1. 10. 3. 1) with regard to catechol-thiol conjugates.
2. Although tyrosine hydroxylation of tyrosinase was extremely decreased in the presence of a thiol compound, the inhibitory effect was removed by the addition of a pyrocatechol-cysteine conjugate, S-(2, 3-dihydroxyphenyl) cysteine, which was not oxidized by the enzyme.
3. The pyrocatechol-cysteine conjugate was also able to shorten the lag period of tyrosinase-dependent tyrosine hydroxylation.
4. The sigmoidal reaction curve of tyrosine hydroxylation observed in the presence of sulf hydryl compounds was found to be caused by the catechol-thiol conjugates, the final products of the enzyme reaction, which counteract the inhibitory effect of sulf hydryl compounds.
5. The pyrocatechol-cysteine conjugate, on the other hand, was shown to cause the decrease of the reaction rate of the enzyme during incubation.