Tetrahydrofolic Acid Is a Potent Suicide Substrate of Mushroom Tyrosinase

The coenzyme tetrahydrofolic acid is the most rapid suicide substrate of tyrosinase that has been characterized to date. A kinetic study of the suicide inactivation process provides the kinetic constants that characterize it: lambda(max), the maximum apparent inactivation constant; r, the partition ratio or the number of turnovers made by one enzyme molecule before inactivation; and k(cat) and K-m, the catalytic and Michaelis constants, respectively. From these values, it is possible to establish the ratio lambda(max)/K-m, which represents the potency of the inactivation process. Besides acting as a suicide substrate of tyrosinase, tetrahydrofolic acid reduces o-quinOnes generated by the enzyme in its action on substrates, such as L-tyrosine and L-DOPA (o-dopaquinone), thus inhibiting enzymatic browning.