ascorbic-acid and dopachrome

The effect of ascorbic acid on the monophenolase activity of tyrosinase, using tyrosine as substrate, has been studied. Over the ranges of ascorbic acid concentration used, no direct effect on the enzyme is found. However, a shortening of the characteristic induction period of the hydroxylation reaction is observed. The evolution of the reaction is dependent on the concentration of ascorbic acid. Low concentrations permit the system to reach the steady state when all ascorbic acid is consumed, whereas high concentrations do not. In the light of these results it is proposed that the influence of ascorbic acid on the reaction is due to its ability to reduce the enzymically generated o-quinones. A relationship between the ascorbic acid concentration, and the induction period generated by it, with the diphenolase activity of tyrosinase is established, which can be used as a basis for the determination of trace amounts of this reducing agent.

Topics: Ascorbic Acid; Dihydroxyphenylalanine; Hydroxylation; Indolequinones; Indoles; Models, Chemical; Monophenol Monooxygenase; Oxygen Consumption; Polarography; Quinones; Substrate Specificity; Tyrosine

The pathway of dopachrome formation from L-dopa involves the net release of one proton for each molecule of dopachrome formed. The protons produced as a consequence of the enzymic step catalysed by tyrosinase can be measured by an electrometric device able to monitor changes in H+ concentration below 1 microM. This electrometric recording can be used as a simple, sensitive and continuous method for determining tyrosinase activity. The electrometric method can also be used in the presence of ascorbate by the spontaneous coupling of ascorbate oxidation to dopaquinone reduction, but measuring proton uptake instead of proton release.

Topics: Ascorbic Acid; Catechol Oxidase; Hydrogen-Ion Concentration; Indolequinones; Indoles; Kinetics; Levodopa; Methods; Monophenol Monooxygenase; Oxidation-Reduction; Protons; Quinones; Spectrophotometry