The mechanism of β-amylase inhibition by vitamin C

Summary

A critical study of the mechanism of β-amylase inhibition by vitamin C has been made. The hydrolysis of starch by the amylase is inhibited by vitamin C alone and vitamin C-Cu complex, the latter exerting greater inhibition.

The oxidised products of the vitamin prepared by Cu oxidation, also exert inhibition of the hydrolysis of starch by the amylase.

In addition to the inhibition exerted on the hydrolysis, vitamin C-Cu complex and the oxidised products of the vitamin inactivate the enzyme in the absence of substrate. Vitamin C alone brings about very feeble inactivation of the enzyme.

A variety of compounds—oxalic acid, uric acid, xanthine, guanine, theophylline, yeast nucleic acid, creatinine, cysteine, cystine, glutathione, histidine, potassium cyanide and hydrogen sulphide, which are shown to inhibit the catalytic oxidation of vitamin C by Cu⁺⁺ annul the inhibition and the inactivation caused by vitamin C, vitamin C-Cu complex and the oxidation products of the vitamin.

A suggestive correlation between the inhibition of the hydrolysis and the inactivation of the enzyme on the one hand, and the oxidation of vitamin C on the other, has been established.

The physiological significance of these findings in their relation to the influence exerted by the purine derivatives and other stabilisers of vitamin C oxidation in regulating the activity of amylases in plant life is discussed.