Subunit structure and isozymic forms of gamma-glutamyl transpeptidase.

Abstract

.gamma.-Glutamyl transpeptidase is associated with the membranes of a number of epithelial and lymphoid cells. When the enzyme is isolated from rat kidney by a method involving detergent extraction and affinity chromatography, an aggregate of MW > 200,000 (heavy form) is obtained. Treatment of the heavy form with bromelain yields a light form of the enzyme (MW .apprx. 68,000) which is separable by isoelectric focusing into 12 enzymatically active isozymes which are very similar with respect to catalytic behavior, content of amino acids, hexoses and aminohexoses, but which differ significantly in sialic acid content. Treatment with neuraminidase converts the acidic isozymes to more basic forms. Each isozyme dissociates in sodium dodecyl sulfate into 2 nonidentical glycopeptides (MW of 46,000 and 22,000) which can be cross-linked with dimethylsuberimidate to yield a species with an apparent MW of 70,000, which indicates that the isozymes are dimers. Physical and immunological studies indicate that the heavy form of the enzyme contains the dimeric light form as well as other membrane proteins.