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

Glycosylation of gamma-glutamyltransferase is modified by ethanol in H5-6 hepatoma cell line

The H5-6 cultured rat hepatoma cell line was used to investigate the post-translational maturation of gamma-glutamyltransferase (GGT) and the effects of acute ethanol administration on the expression and glycosylation of this membrane-bound glycoprotein. We found that the two subunits of H5-6 GGT with molecular masses of 55 and 33 kDa were derived from a single glycosylated precursor of 80 kDa. In addition, signals of high molecular mass (more than 90 kDa) were detected. In vitro deglycosylation experiments indicated that N-linked sugars represented about 25% of the molecular weight of the H5-6 enzyme. By use of serial lectin affinity technique, we showed that N-linked sugar chains were mainly of the biantennary complex and hybrid-type, without fucose linkage to the innermost N-acetyl-glucosamine. Ethanol treatment did not seem to affect the expression of GGT and the sialic acid content of the enzyme, but altered its oligosaccharide chain composition both quantitatively and qualitatively.

Electrophoretic behavior and partial characterization of disease-associated serum forms of gamma-glutamyltransferase

We have recently devised an improved procedure for the rapid electrophoretic separation of multiple forms of serum gamma-glutamyltransferase (GGT). This procedure is based on the separation on cellulose acetate strips, usually employed for lipoprotein electrophoresis, followed by visualization with a fluorescent reagent. The method is highly sensitive and the fractions are more clearly resolved than with other procedures. Reference intervals have been evaluated in the sera from 142 healthy subjects and the patterns (two GGT forms comigrating with alpha 1 and alpha 2-globulin) are reproducible. In 150 sera from patients with various hepatobiliary diseases (including neoplasias), acute pancreatitis and non liver-involving neoplasias, we observed some disease-specific GGT forms: an albumin comigrating enzyme (Alb-GGT) specific of liver neoplasia; a gamma-globulin comigrating GGT (gamma-GGT) and a nonmigrating isoform (dep-GGT) both specifically associated to extrahepatic jaundice. Multiple lipoprotein fraction precipitation showed that beta-, gamma- and dep-GGT are complexes between GGT and low density lipoprotein and very low density lipoproteins (LDL + VLDL), and that some of the alpha 1-GGT from cirrhotic patients is a complex between GGT and high density lipoprotein (HDL). GGT fractions from normal subjects and Alb-GGT from patients with liver neoplasia do not appear to be complexed with lipoproteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Lectin-affinity chromatography of serum gamma-glutamyltransferase in liver disease

The variation of the carbohydrate chain of gamma-glutamyltransferase was studied in 45 liver patients by means of lectin affinity chromatography. Five lectins were used: concanavalin A, Ricinus communis I and II, Maclura pomifera and Ulex europaeus agglutinin. The binding towards Con A was shown to be independent from the binding towards the other lectins. Parallel variations of binding results against the galactose- and fucose-recognizing lectins were obtained. In liver steatosis, the binding results were comparable to those obtained in normal patients. Cirrhosis and metastasis patients showed a decreased binding towards Con A, while the binding against the various galactose- and fucose-recognizing lectins was increased. After neuraminidase treatment, an increased affinity towards all lectins was observed. However, differences in RCA I and RCA II binding between patients and controls still persisted. Besides sialic acid, also galactose and fucose residues contribute to serum gamma-glutamyltransferase heterogeneity.

Partial purification of gamma-glutamyltransferase from human brain microvessels

Two forms of gamma-glutamyltransferase from human brain cortex microvessels were partially purified by gel permeation and ion-exchange and group-affinity chromatography. The specific activity of the purified preparations was 320-fold (detergent form) and 830-fold (proteolytic form) higher than that of the enzyme in the brain cortex homogenate. The relative molecular mass of the proteolytic form of the enzyme was about 90,000 as determined by gel permeation chromatography. The major part of the enzyme (about 80%) was absorbed on Con A-Sepharose 4B. The pH optima for transfer reactions with gamma-glutamyl-4-nitroanilide as donor and glycylglycine and L-cystine as acceptors were in the range of 8.2 to 9.0. The studied enzyme was inhibited by a mixture of L-serine and borate and by bromcresol green.

Altered glycosylation of gamma-glutamyltranspeptidase (GGT) in seminal fluid from men with accessory gland infection

The heterogeneity of gamma-glutamyltranspeptidase (GGT) in seminal plasma has been studied using Con A-chromatography. This parameter was then related to the fructose concentration, the acid phosphatase activity, ejaculate volume, sperm density and the number of bacteria per ml. Multivariate regression analysis and stepwise elimination of the least fitting factors, revealed that Con A-binding correlated with the number of bacteria per ml of semen and the acid phosphatase activity with 49% of the variance of GGT-binding being explained by these parameters. This result suggests that glycosylation of seminal GGT is altered by accessory gland infection. Neuraminidase digestion suggests that the pattern of Con A-binding of seminal GGT depends only partly on its sialic acid content.

Variations in sialic acid content of gamma-glutamyltransferase: a consequence for immunochemical determinations?

Using specific antibodies against the human kidney enzyme, gamma-glutamyltransferase (GGT, EC 2.3.2.2) was assayed from human kidney and serum by electroimmunodiffusion. Determination of the enzyme by such a method was highly influenced by the sialic acid content of the molecule. The peaks corresponding to the sialylated GGT were higher than those corresponding to the neuraminidase-treated enzyme. In contrast, sialylation of the protein had no influence on the results observed when measuring the enzyme by radial immunodiffusion. Moreover, immunoprecipitation curves of both sialylated and neuraminidase-treated samples were identical. The varying degrees of sialylation of GGT occurring under physiological or pathological conditions are known to be partly responsible for the heterogeneity of the enzyme in organs and biological fluids. Therefore, determination of the enzyme by electroimmunodiffusion may be hazardous.

gamma-Glutamyl transpeptidase activity in liver disease: serum elevation is independent of hepatic GGTP activity

gamma-Glutamyl transpeptidase activity was measured in liver and serum from 110 patients undergoing diagnostic liver biopsy, including patients with alcoholic liver disease, fatty liver not due to alcohol, primary biliary cirrhosis, persistent hepatic disease, chronic active hepatitis and normal livers. Serum gamma-glutamyl transpeptidase was markedly elevated in patients with alcoholic liver disease and primary biliary cirrhosis while mean hepatic gamma-glutamyl transpeptidase activity was significantly increased only in the alcoholic liver disease group. There was considerable overlap of individual enzyme values among the different disease groups. There was no inhibitors or activators of liver gamma-glutamyl transpeptidase in any of these disorders. The increased liver activity was not related to the degree of hepatic fibrosis or cirrhosis. There was no correlation between hepatic and serum gamma-glutamyl transpeptidase activity. Hepatic and serum gamma activities were equally increased in individuals with alcoholic liver disease whether or not they were drinking at the time of the study. The data suggest that increased hepatic gamma-glutamyl transpeptidase activity is neither specific for alcoholic liver disease nor essential for serum GGTP to be elevated.

Variants of gamma-glutamyl transferase in human urine

When dialyzed human urine was subjected to ultracentrifugation, gamma glutamyl transferase activity amounting to one-third or less of the total activity was found associated with the microsomal fraction and the remaining in the 100 000 X g supernatant. By ion-exchange chromatography on DEAE-cellulose at pH 7.5, the urinary enzyme was resolved into two fractions. The less anionic form (eluted with 0.15 mol/l NaCl) corresponded to the 100 000 X g supernatant fraction. The more anionic form (eluted with 0.5 mol/l NaCl) corresponded to the microsomal fraction. On chromatography on concanavalin A-sepharose, the urinary transferase activity separated into the unbound fraction and the bound fraction (eluted with alpha-methyl-D-glucoside). The bound fraction was retained more tightly on DEAE-cellulose (eluted with 0.5 mol/l NaCl). The unbound fraction had weaker interaction with the ionic-exchanger (eluted with 0.15 mol/l NaCl). Each of two fractions separated by affinity chromatography on concanavalin A-sepharose, resolved into two bands on cellulose acetate electrophoresis at pH 7.t. In addition, during gel chromatography on Sephadex G-200, they separated into a high molecular weight (greater than 200 000) fraction and a low molecular weight (55 000) fraction. The band that stayed at the origin during electrophoresis was high molecular weight in nature, whereas the fraction that moved towards the anode was found to be the smaller form of the enzyme. The study of the variants of urinary gamma glutamyl transferase was not found to be of clinical significance. The nature and origin of the variant forms of the enzyme in urine are discussed.

[Concanavalin A- Sepharose affinity chromatography for routine microanalysis of gamma-glutamyltransferase (author's transl)]

The separation of two molecular forms of liver gamma-glutamyltransferase is achieved by Con A-Sepharose chromatography, an adult type with high affinity to Con A and a fetal type without binding capacity to this lectin. Now we present a new method using micro-columns for affinity chromatography (gel volume 2 ml). By this rapid and inexpensive procedure it is possible to study this enzyme separation for its use in the diagnosis of liver diseases.

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.

Isolation of kidney brush border gamma-glutamyl transpeptidase from urine by specific antibody gel chromatography

The IgG fraction of antiserum directed against gamma-glutamyl transpeptidase (gammaGTP, EC 2.3.2.2) isolated from human kidney brush border membranes after limited proteolysis, was covalently bound to cyanogen bromide-activated Sepharose. With this antibody-gel, gammaGTP present in the urine of patients as a result of tubular damage was immunospecifically prepared by affinity chromatography. The enzyme isolated from the urine samples gave a complete cross-reaction wiht gammaGTP artificially cleaved off from brush border fragments. Since labelled anti-gammaGTP sera gave a specific immunofluorescence only of the luminal portion of cortical tubule, the use of immunosorption chromatography appears to be an important approach for the isolation of urinary kidney tissue antigens of a defined origin.

Multiple forms of gamma-glutamyltransferase in normal human liver, bile and serum

A study of the multiple forms of gamma-glutamyltransferase ((gamma-glutamyl)-peptide:amino acid gamma-glutamyltransferase, EC 2.3.2.2) in normal human liver, bile and serum are reported. An amphiphilic form of the enzyme was demonstrated in all three samples. When solubilized with detergent, estimated values for Stoke's radius of 48 A and sedimentation coefficient of 5 S were obtained for this form. A hydrophilic form was also present in serum and bile, which showed identical properties to the enzyme form obtained after papain-treatment of the three samples. The Stoke's radius was found to be 37 A, and the sedimentation coefficient 5 S. It was concluded that the heterogeneity of enzyme activity found both on gel filtration and on electrophoresis was due to aggregates of the amphiphilic form with lipids and other proteins, and could not be ascribed to the presence of isoenzymes.

Induction and 'superinduction' of sialylation of membrane-bound gamma-glutamyltransferase during liver regeneration

The present paper shows that in the regenerating rat liver the membrane-bound-gamma-glutamyltransferase exists in two molecular forms. Depending on the state of proliferation, a sialic-acid-rich enzyme (in the fetal or regenerating liver) or a sialic-acid-poor enzyme (in the adult or quiescent liver) could be detected. In regeneration liver (24 h after 2/3 resection) only the sialic-acid-rich or fetal enzyme could be found. Since total enzyme activity (adult + fetal type) remained unchanged, it is assumed that the adult type of gamma-glutamyltransferase was modified by sialylation during the initial phase of liver regeneration. This process of sialylation was prevented by inhibitors of RNA or protein synthesis such as D-galactosamine, actinomycin D or cycloheximide, provided that the inhibitor (D-galactosamine) was given within the first 8 h after partial hepatectomy. Sialylation was not impaired by inhibitors of DNA synthesis, e.g. hydroxyurea or cytosine arabinoside. Administration of actinomycin D during a defined phase of proliferation (24 to 48 h after partial hepatectomy) stimulated the transfer of sialic acid to gamma-glutamyltransferase, a finding which describes for the first time the so-called 'superinduction' of a sialylation process.

[Biology of lectins and their application in clinical biochemistry (author's transl)]

Lectins are proteins or glycoproteins, originally isolated from plant seeds. Characteristics are their ability to bind glycoproteins or glycolipids depending on the carbohydrate residues. The present review describes the structure of the lectins, their binding specificity and their functions with respect to precipitation of glycoproteins, agglutination of cells, transformation of lymphocytes and toxic action. Recently, lectin-analogs have been described in rabbit liver, which are responsible for hepatic uptake of circulating glycoproteins. The regulation of this process is intimately linked to the terminal N-Acetylneuraminic acid (NA-NA). Moreover, its significance is shown during fetal development, oncogenic transformation, immunologic recognition as well as homostasis. Due to the different terminal carbohydrate residues, glycoproteins of adult, fetal or transformed cells can be separated using affinity chromatography. Besides the purification of glycoproteins, lectins are also used for the separation of intact cells. Therefore the use of lectins is recommended for preparative and analytical methods, for the measurements of glycoprotein-turnover and for clinical diagnostics.