Identification of high molecular weight antigens structurally related to gamma-glutamyl transferase in epithelial tissues

Heterologous antibodies to gamma-glutamyl transferase (γGT), an ectoenzyme associated with the apical surface of many types of epithelial cells involved in secretion and transport, have been used to identify and partially characterize the spectrum of antigens in a series of epithelial tissues that exhibit a range of enzyme activities. In addition to antigens corresponding to the subunits of the active enzyme (mol wt 55K, 30K), antigens of mol wt ∼85-≥95K have been detected using an antibody raised against the enzyme purified in nonionic detergent. The latter species are shown to share antigenic determinants with and to be structurally related to the enzyme subunits; however, they do not bind significantly to antibodies raised to protease-solubilized γGT. Further, they constitute the major antigens in tissues that exhibit relatively low levels of enzyme activity. These polypeptides are apparently larger than a recently characterized biosynthetic precursor of the γGT subunits. Although they do not have γGT activity themselves and their function is undefined, the possibility that they may represent highly glycosylated polypeptides related either to γGT precursors (that persist without processing) or to the large enzyme subunit merits consideration.