Multiple molecular forms of rat mammary glucose-6-phosphate dehydrogenase: proposed role in turnover of the enzyme

Abstract

Polyacrylamide gel electrophoresis shows that more than 80% of the total glucose-6-phosphate dehydrogenase activity of rat mammary homogenates exists as three dimeric forms. During lactation all three forms are present in equivalent amounts, but 24 h after the removal of the pups to stop lactation, there is a shift to the fastest migrating form (band I). During this period the ratio of enzyme activity to tissue glutathione concentration decreased. Using partially purified preparations of the enzyme it was shown that dithiothreitol could convert all the activity to the slowest migrating form (band III) and that diamide could reverse this change giving rise to band I only. Reduced glutathione could partially convert the enzyme to the band III form. Experiments using iodoacetic acid, iodoacetamide, p-chloromercuribenzoate, and mercuric chloride are also described. Based on these observations it is proposed that band I and band III represent fully oxidized and fully reduced forms of the enzyme, respectively, and band II a partially oxidized form. The oxidized and reduced forms are catalytically equivalent. However, the oxidized form is the most rapidly inactivated by chymotrypsin or mammary microsomes. Mammary microsomes can also catalyze the oxidation of the enzyme. The oxidation precedes the inactivation by microsomes and also occurs at lower microsome concentrations. It is proposed that the microsomal oxidation of the enzyme is the initial step in the turnover of the enzyme.