gamma-Glutamyltranspeptidase is a glycoprotein composed of heavy and light subunits and associated with the brush border membrane of the kidney and small intestine. gamma GTP solubilized with papain is a hydrophilic enzyme which has lost the membrane binding segments but its catalytic activity is not altered, whereas gamma GTP solubilized with Triton X-100 is a hydrophobic enzyme which contains hydrophobic domain binding to the membrane. Amino acid compositions of these two forms were compared and Triton solubilized enzyme was found to contain 52 amino acid residues more than the papain solubilized form. This difference is due to the heavy subunit not light subunit. Then, end group analysis was carried out and the carboxyl-termini of their light subunits were found to be phenylalanine and those of their heavy subunits were tyrosine, respectively. Although light subunits of two forms contain a common sequence, Thr-Ala (X)-Leu as an amino-terminal portion, that of heavy subunit of Triton X-100 solubilized form contains the sequence Met-Lys-Asn-Arg-Phe-Leu-Val-Leu-Gly-Leu-Val-Ala-Val-Val-Leu-Val-Phe-Val- Ile-Ile-Gly-Leu and the papain-solubilized form contains completely different amino-terminal sequence Gly-Pro-Pro-Leu. It is concluded that an amino-terminal portion of the heavy subunit is a hydrophobic domain consisting of about 20 hydrophobic amino acids and contributes to anchor the enzyme to the membrane. gamma GTP has been known to show great heterogeneity in charge and multiple forms with different isoelectric points are found to be mainly due to differences of their sugar chains. Then the structures of the oligosaccharides attached to gamma GTP were determined. They were found to be all asparagine linked and consisted of neutral and acidic oligosaccharides with remarkable heterogeneity. A correlation between the contents of the acidic oligosaccharides and the isoelectric points of multiple forms of gamma GTP was observed. In addition, multiple forms of gamma GTP were immunologically identical and their protein structures were identical. Next, the mechanisms of biosynthesis of gamma GTP were examined and it was found that two subunits of gamma GTP are synthesized as a precursor protein with a single polypeptide chain of 78,000 daltons. Then processing by limited proteolysis occurs post-translationally, and it is a rather slow process. Since the precursor form is already core glycosylated and fucosylated, proteolytic processing could be carried out after completion of terminal glycosylation at the Golgi membrane or the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)
"GGT activity is also present in epithelial cell secretions . A hydrophilic form resulting from proteolytic removal of the transmembrane domain in hepatocytes accounts for the GGT normally found in the blood , while the amphipathic form found in bile duct is due to its solubilization by the Fig. 2. Lung T2 cell GGT expression. Epithelial T2 and T1 cells and the alveolar macrophage (MO) are denoted and the results of Northern blot for GGT mRNA expression along with RT-PCR with P3 specific primers shown at left (Lv = liver, K = kidney, T 2 = lung T2 cell, Lg = lung, H = heart). "
[Show abstract][Hide abstract] ABSTRACT: This article also appeared in Thiol Metabolism and Redox Regulation of Cellular Functions, A. Pompella, G. Bánhegyi and M. Wellman-Rousseau, eds, IOS Press, Amsterdam.
[Show abstract][Hide abstract] ABSTRACT: The subcellular distributions of the precursor form and mature form of gamma-glutamyltranspeptidase of rat kidney were studied by labeling the enzyme with [3H]fucose in vivo. In trans Golgi elements and basolateral membranes, gamma-glutamyltranspeptidase was present as a precursor form with a single polypeptide chain. However, the brush border membranes contained the heavy and light subunits as well as precursor. These results suggest that the precursor is converted to the mature form after its transport to the brush border membranes.
Biochemical and Biophysical Research Communications 08/1983; 114(2):889-95. DOI:10.1016/0006-291X(83)90864-1 · 2.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Heterologous antibodies to gamma-glutamyl transferase (gamma GT), 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 approximately 85-greater than or equal to 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 blind significantly to antibodies raised to protease-solubilized gamma GT. 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 gamma GT subunits. Although they do not have gamma GT activity themselves and their function is undefined, the possibility that they may represent highly glycosylated polypeptides related either to gamma GT precursors (that persist without processing) or to the large enzyme subunit merits consideration.
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