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.

Effect of anticonvulsant drugs on urinary excretion of gamma-glutamyl transferase in women

Excretion of gamma-glutamyl transferase was found to be significantly higher in a group of epileptic patients on long-term anticonvulsant therapy than in normal subjects. The losses, which may reflect renal enzyme induction, were unrelated to drug dose or duration of therapy and were significantly lower than those found in normal healthy pregnant women at term.

The excretion of gamma-glutamyl transferase in pregnancy

A cross-sectional study of gamma-glutamyl transferase (gamma-GT) excretion in 316 healthy pregnant women showed the enzyme excretion (mU per mmol creatinine) was significantly higher than that of a non-pregnant control group. The increase was apparent before the end of the first trimester and continued therafter. These changes may be indicative of hypertrophy and/or hyperplasia of the nephron and be part of the overall renal adaptation to normal pregnancy. Even higher levels were recorded in three obstetric patients in whom renal complications were to be expected and this may indicate a specific type of renal damage.

A new method for the determination of gamma-glutamyltransferase in serum

A new, simple and sensitive method is described for assay of serum gamma-glutamyltransferase activity based on the hydrolysis of the substrate L-gamma-glutamyl-3-carboxy-4-nitranilide which offers the advantage of producing directly its own chromogen. The substrate is highly soluble and the method can therefore easily be adapted to any equipment for the automated assay of gamma-glutamyltransferase. The activities at measured optimum substrate concentration are slightly higher than with the Szasz ((1969), Clin. Chem. 15, 124-136) method in which L-glutamyl-p-nitroanilide is used as substrate.

Serum -glytamyl transpeptidase activity in liver disease

Serum gamma-glutamyl transpeptidase (GGT) activity correlates closely with the activities of alkaline phosphatase (ALP) and 5'-nucleotidase (5NT) in various forms of liver disease. Maximum elevations of all three enzyme activities are observed in diseases which particularly affect the biliary tract. Compared with the other two enzymes GGT is generally increased to a greater extent and is thus the most sensitive indicator of biliary-tract disease, while estimations of serum GGT are more reproducible than those of 5NT. However, a group of patients who had been treated with phenytoin and barbiturates were found to have elevated serum GGT activities without any other evidence of liver disease. The apparent effect of certain drugs on serum GGT activity indicates the need for caution in interpreting the results of this test.