Intramolecular crosslinking of γ-glutamyl transpeptidase

Abstract

γ-Glutamyl transpeptidase (rat kidney) is a heterodimeric glycoprotein (subunit molecular weights 52,000 and 25,000). In addition to its single-chain biosynthetic precursor ( M r 78,000), glycosylated high molecular weight forms ( M r 85,000–95,000) have been reported in various rat tissues as well as during in vitro translation of its mRNA. Studies reported here suggest that these might be attributed to the anomalous behavior of intramolecularly crosslinked species. Thus, chemical crosslinking of the purified enzyme (as well as enzyme on the renal brush border membranes) by bifunctional reagents such as dimethyl suberimidate and by an active site-directed reagent, diazotized p-aminohippurate, produces stable heterodimers which exhibit molecular weights identical to that of the native enzyme when subjected to gel filtration. However, when subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the crosslinked species exhibit apparent M r values of 85,000 to 110,000, depending upon the crosslinking agent used. Protein glycosylation alone does not account for such anomalous electrophoretic behavior; the extent and the regions of the enzyme involved in formation of crosslinks appear to exert considerable constraints upon their conformation even in denaturing media.