Study Of Tyrosine And Dopa Enantiomers As Tyrosinase Substrates Initiatingl- Andd-Melanogenesis Pathways

Tyrosinase starts melanogenesis and determines its course, catalyzing the oxidation by molecular oxygen of tyrosine to dopa, and that of dopa to dopaquinone. Then, nonenzymatic coupling reactions lead to dopachrome, which evolves toward melanin. Recently, it has been reported thatd-tyrosine acts as tyrosinase inhibitor and depigmenting agent. The action of tyrosinase on the enantiomers of tyrosine (l-tyrosine andd-tyrosine) and dopa (l-dopa andd-dopa) was studied for the first time focusing on quantitative transient phase kinetics. Post-steady-state transient phase studies revealed thatl-dopachrome is formed more rapidly thand-dopachrome. This is due to the lower values of Michaelis constants forl-enantiomers than ford-enantiomers, although the maximum rates are equal for both enantiomers. A deeper analysis of the inter-steady-state transient phase of monophenols demonstrated that the enantiomerd-tyrosine causes a longer lag period and a lower steady-state rate, thanl-tyrosine at the same concentration. Therefore,d-melanogenesis fromd-tyrosine occurs more slowly than doesl-melanogenesis froml-tyrosine, which suggests the apparent inhibition of melanin biosynthesis byd-tyrosine. As conclusion,d-tyrosine acts as a real substrate of tyrosinase, with low catalytic efficiency and, therefore, delays the formation ofd-melanin.