The effect of ascorbic acid on the monophenolase activity of tyrosinas
e, using tyrosine as substrate, has been studied. Over the ranges of a
scorbic acid concentration used, no direct effect on the enzyme is fou
nd. However, a shortening of the characteristic induction period of th
e hydroxylation reaction is observed. The evolution of the reaction is
dependent on the concentration of ascorbic acid. Low concentrations p
ermit the system to reach the steady state when all ascorbic acid is c
onsumed, whereas high concentrations do not. In the light of these res
ults it is proposed that the influence of ascorbic acid on the reactio
n is due to its ability to reduce the enzymically generated o-quinones
. A relationship between the ascorbic acid concentration, and the indu
ction period generated by it. with the diphenolase activity of tyrosin
ase is established, which can be used as a basis for the determination
of trace amounts of this reducing agent.