Multiple molecular forms of gamma-glutamyl transpeptidase during human pregnancy

Structural, functional, and clinical aspects of gamma-glutamyltransferase

GGT catalyses the transfer of gamma-glutamyl residues to amino acids or small peptides. A number of publications report the purification of GGT, the rat kidney enzyme being the best characterized. Bromelain treatment liberates an active form with a molecular weight of 68,000 separable into two nonidentical glycopeptides with molecular weights of 46,000 and 22,000; the latter contains the gamma-glutamyl binding site. GGT is intimately concerned in the synthesis and metabolism of glutathione through the gamma-glutamyl cycle. There is good evidence that this plays a role in the absorption of amino acids from the glomerular filtrate and from the intestinal lumen through a translocation mechanism. Many studies indicate that the GGT content of liver is increased by enzyme-inducing drugs and that this increase is reflected in elevated activity of the enzyme in blood serum. The serum assay has potential in monitoring drug compliance. Increased serum GGT activity encountered in chronic alcoholics seems to be partly due to microsomal enzyme induction. Utility of the assay in detecting alcoholism is controversial, but it is a useful index to compliance with therapy. Dramatic increases in activity are found in many chemically-induced animal tumors, and can be recognized in premalignant cells long before any morphological changes become evident. It has been used as a test for hepatic metastases, but its predictive value has shown a wide range in the hands of many authors. A similar controversy applies to its role in monitoring cancer therapy. Many synthetic substrates have been used to measure serum GGT activity. Currently, L-gamma-glutamyl-p-nitroanilide is the most popular. Males have higher values than females; activity is very high in the neonate and rather low in pregnancy. The most universal application of serum GGT assay is in diagnosis of liver and biliary tract disease. It is widely believed that higher values occur in biliary obstruction than in parenchymal disease. However, the percentage incidence of abnormalities and the overlap of values in individual cases in different disease categories are so great that the enzyme cannot be recommended for this purpose. Isoenzyme analyses have been performed in an attempt to improve the diagnostic specificity of the serum GGT assay. Tissue-specific patterns have not been described, and disease-specific patterns cannot be reproduced with confidence. Whereas exciting advances are being made in understanding the molecular structure, mechanism, and functions of the enzyme it has yet to find a genuinely useful diagnostic role substantiated by a convincing body of scientific data.

Electrophoretic patterns of gamma-glutamyltransferase activity eluted from liver tissue

Up to three zones of gamma-glutamyltransferase activity were present in 89 samples of human serum after agarose-gel electrophoresis at pH 8.6. Their mobilities relative to albumin were zero, 0.3--0.5, and 0.7--0.9. Incubation of human liver tissue in serum increased the activity of the zones with zero and 0.7--0.9 mobilities, and transiently, of the zone of intermediate mobility. More prolonged incubation caused the intermediate zone to decline, and produced new zones of mobility greater than that of albumin which were not seen in native sera. The mobility of partially-purified liver gamma-glutamyltransferase incubated in serum or protein-free solutions was 0.7--0.8. The intermediate zone was not produced when liver tissue was incubated in protein-free solutions, nor with the purified enzyme in serum or protein-free solutions. The possible relevance of these observations to the electrophoretic patterns of gamma-glutamyltransferase in pathological sera is discussed.

Partial purification and some properties of gamma-glutamyl transpeptidase from human bile

1. Gamma-Glutamyl transpepetidase ((5-glutamyl)-peptide: amino acid 5-glutamyltransferase, EC 2.3.2.2) from human bile has been partially purified using protamine sulphate treatment, DEAE-cellulose chromatography and Sephadex G-200 filtration. The procedure resulted in 150-fold increase in specific acitivity with a 37% yield. 2. The partially purified enzyme showed a single zone of enzyme activity by polyacrylamide gel electrophoresis and eluted in the inner volume of Sephadex G-200. 3. The enzyme had a pH optimum of 8.1 and Km of 1.52 mM using gamma-glutamyl p-nitroanilide as substrate. 4. The effects of cations and different gamma-glutamyl acceptors on the activity of the enzyme are reported. 5. As bile gamma-glutamyl transpeptidase appears to be soluble in the absence of detergents, it is suggested that bile may prove to be a useful source for further studies of the kinetic properties and physiological role of human gamma-glutamyl transpeptidase.

[Isoenzyme differentiation of gamma-glutamyltransferase by concanavalin A and Con- A-Sepharose (author's transl)]

In this investigation a new possibility of isoenzyme-differentiation of the gamma-glutamyl-transferase (GGT) (EC Nr.2.3.2.2.) was demonstrated by Concanavalin A and Con A-Sepharose. Because of the different sugar content of the glycoproteins distinction between liver- and kidney-GGT is possible. Furthermore it was possible for the first time to show a different precipitation behaviour of one glycoprotein to Concanavalin A in certain diseases. In cases of alcoholic hepatitis GGT looses its Concanavalin A-affinity because of increased neuraminic acid concentration. The possible reasons of the different behaviour to the binding affinity of Con A and Con A-Sepharose and GGT as well as additional use for enzyme-differentiation by Con A-Sepharose affinity chromatography are discussed.