The great DOPA mystery: the source and significance of DOPA in phase I melanogenesis.

One of the important characteristics of tyrosinase is the autocatalytic nature of the oxidation of natural monohydric phenol substrates, such as tyrosine. In vitro tyrosinase exhibits a lag phase in which the maximum velocity of oxidation is attained after a period of induction. This acceleration contrasts with the kinetics of dihydric phenol oxidation which exhibit conventional Michaelis-Menten kinetics. It has been known for half a century that DOPA is a co-factor in the oxidation of tyrosine and addition of a small amount of catechol reduces the length of the lag period. The significance of DOPA is in this action, and DOPA is known to be formed in phase I melanogenesis. Until recently there has been controversy regarding the source of the DOPA in the in vitro reaction system. Most investigators have favoured a mechanism based on the generation of DOPA by a direct hydroxylation of tyrosine. However, recent evidence has suggested that DOPA is indirectly derived by reduction of dopaquinone. In this communication the evidence for the indirect mechanism derived from the use of analogue substrates is reviewed.