ABSTRACT
The various theories put forward to explain the characteristic lag kinetics of oxidation of L-tyrosine by tyrosinase a rate regulatory step in the biosynthesis of melanin are reviewed Examination of the evidence in the literature and from experiments in the author's laboratory indicate that one of the hypotheses, that is, competition of tyrosine and dopa for met-tyrosinase and the formation of a dead-end complex of met-enzyme with tyrosine as explanation for lag kinetics is not consistent with available information. The alternative hypothesis that tyrosinase is an allosteric enzyme with tyrosine having negative effector site on the enzyme and dopa competing for it as an explanation for lag kinetics of tyrosinase is not yet disproved. Irrespective of the actual explanation for the lag kinetics of tyrosinase, it is suggested that the highly conserved lag kinetics may serve a physiological function. It is suggested that this function is to keep the enzyme essentially inactive during its transport to the specific organelle, namely the melanosome, in which an acidic environment exists. Only at acidic pH is the enzyme able to catalyze the biosynthesis of melanin.