Studies on the Myrosinase in Mustard Seed

Abstract

In order to clarify whether the myrosinase containes only thioglucosidase, or thioglucosidase and sulfatase, chromatographic behaviors of the myrosinase on cellulose ion exchanger were studied. Myrosinase showed two peaks by the chromatography on TEAE-cellulose at pH 8.5, but the fractionation of the two enzymes was not accomplished. It was concluded that the two peaks was not ascribable to an artifact but the two closely resembled species of myrosinase existing in the sample solution. Chromatographic separation of the two enzymes on DEAE-cellulose resulted in failure. These results confirmed that the myrosinase was a single β-thioglucosidase originally. The mechanism of activation of plant myrosinase by ascorbic acid was studied. The oxidation-reduction reaction of ascorbic acid had nothing to do with the reaction by the activated myrosinase. Presence of firmely bound ascorbic acid in the enzyme protein was doubtful from the result of incubalion with ascorbate oxidase. Presence of the effector site for ascorbic acid was presumed by the kinetic measurement and the instabilization by ascorbic acid in heating. But the effector site and the substrate site could not be distinguishable experimentally. The affinity of ascorbic acid to the myrosinase was not influenced by the substrate. The differences of the activation mechanism of the myrosinase by ascorbic acid and the allosteric enzyme were discussed.