title: Biosynthesis and degradation of gamma-glutamyltranspeptidase of rat kidney.
abstract: gamma-Glutamyltranspeptidase (gamma GTP) of rat kidney is an intrinsic glycoprotein bound to the plasma membrane and composed of two nonidentical subunits and an amino-terminal portion of the heavy subunit anchors the enzyme to the membrane. The mechanisms of biosynthesis, post-translational processing and degradation of the enzyme were studied using mono-specific antibody raised to gamma-glutamyltranspeptidase purified from rat kidney. The following results were obtained. Double isotope labeling in vivo showed that gamma-glutamyltranspeptidase is synthesized as a precursor form with a single polypeptide chain of 78,000 daltons, and then processed post-translationally by limited proteolysis, resulting in two subunits of 50,000 and 23,000 daltons. Incorporation of [3H]leucine or [35S]methionine into the precursor form increased until 60 min after their intravenous injection, and a pulse-chase experiment showed that the half life of the precursor form was 53 min. [3H]Fucose and [3H]glucosamine could also be incorporated into the precursor form, showing that glycosylation of the enzyme occurs at the stage of the precursor form. Rat kidney labeled with [3H]fucose was subjected to subcellular fractionation. The Golgi fraction contained the glycosylated precursor form and a small amount of subunits, and the plasma membrane fraction contained mostly subunits with a significant amount of precursor, suggesting that post-translational processing of the precursor occurs on the plasma membrane. The apparent half lives of the native enzyme and the heavy and light subunits were all estimated as 4.3 +/- 0.5 days by labeling with [3H]leucine or [3H]fucose. gamma-Glutamyltranspeptidase has a different turnover rate from aminopeptidase M, which is located in the microvillus membrane close to gamma-glutamyltranspeptidase.
mesh_terms: D000217:Acyltransferases / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D006207:Half-Life; D007668:Kidney / Q000201:enzymology*; D007700:Kinetics; D046911:Macromolecular Substances; D008297:Male; D008970:Molecular Weight; D051381:Rats; D011919:Rats, Inbred Strains; D013347:Subcellular Fractions / Q000201:enzymology; D011503:Transglutaminases
chemical_list: D046911:Macromolecular Substances; D000217:Acyltransferases; D011503:Transglutaminases