THE EFFECT OF PH ON THE SUICIDE INACTIVATION OF FROG EPIDERMIS TYROSINASE

This paper presents a new reaction mechanism for the effect of the pH on the suicide inactivation of the diphenolase activity of tyrosinase. The applicability of the mechanism is supported by the experimental c haracterization of the kinetic behaviour of the frog epidermis enzyme acting on catechol, L-dopa and alpha-methyldopa at several pH values. Two enzyme forms 'met-' and 'oxy-' tyrosinase, but no their correspond ing enzyme-diphenol complexes, present one ionizable group with very s imilar value of K-a which has been determined. The highest values of c atalytic and inactivation efficiencies correspond to alpha-methyldopa and catechol, respectively. These kinetic studies have been carried ou t by using the transient phase approach previously developed, with neg ligible substrate consumption during the assay time. That illustrate t he usefulness of the method for multisubstrate enzyme reactions.