Studies on the structure of gamma-glutamyltranspeptidase. I. Correlation between sialylation and isozymic forms

Papain solubilized gamma-glutamyltranspeptidase was purified by DEAE-cellulose column chromatography. Four fractions were obtained as single proteins, each with a single isoelectric point, and fifth and sixth fractions were obtained as mixtures of several components. The molecular seights of the subunits of each form and their amino acid compositions and carboxyl-terminals were determined and found to be identical. The isoelectric points of the four purified enzymes and the main bands of the fifth fraction were pH 8.3, 8.0, 7.5, 6.7, and 6.0, respectively. The sixth fraction gave five bands with isoelectric points of pH 5.6, 5.3, 5.1, 4.6, and 4.4. An asialo-form of the enzyme, obtained by treatment with neuraminidase, had an isoelectric point of pH 8.6. A linear relationship was found between the isoelectric points of the enzymes and their contents of sialic acid, indicating that the heterogeneity of papain-solubilized gamma-glutamyltranspeptidase is mostly due to differences in the extends of sialylation of the enzymes. The distribution of the activity in rat kidney as follows: 29% in a poorly sialylated fraction (isoelectric point (Ip) 8.3 to 6.7), 24% in a moderately sialylated fraction (Ip 6.6 to 6.0), and 45% in a highly sialylated fraction (Ip 5.6 to 4.4).