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

Gamma-glutamyltransferase-associated glycoprotein patterns in human seminal plasma of normozoospermic men: a new aspect of biomarker heterogeneity

BACKGROUND

Gamma-glutamyltransferase (GGT) is a well-known laboratory biomarker. In spite of high concentration and the possible biomedical importance of estimating GGT in human seminal plasma (hSP), it has not been widely explored in reproductive physiology. This study aimed to complement existing data on its diversity, previously obtained on seminal extracellular vesicles, by analyzing matched soluble fraction of hSP. The GGT-associated patterns of selected glycoproteins were analyzed in order to establish an adjunct referent parameter for differentiation between known high molecular mass forms of GGT. Getting insight into distinct GGT-associated glycoprotein patterns should contribute to define them together as possible multimarkers.

METHODS

GGT forms in soluble, membrane-free-fraction isolated form hSP of normozoospermic men were analyzed using gel filtration and lectin blotting using WGA (wheat germ agglutinin) and Con A (concanavalin A).

RESULTS

Widely distributed GGT (with two to three partially resolved peaks), which may correspond to high molecular mass aggregates, were detected. GGT-associated patterns of selected glycoproteins (at position of big, medium, and small-GGT) all comprised high molecular mass WGA-reactive smears, but differed in the presence of Con A-reactive glycans, as well as mucin-associated antigens CA19-9 and CA125.

CONCLUSIONS

GGT contributes to several molecular patterns that differ between the soluble and extracellular vesicle fractions of hSP. Their glycobiochemical heterogeneity is due to difference in the presence of distinct sialylated and mannosylated glycans. Moreover, GGT-associated glycoprotein patterns differentiate between high molecular mass forms of GGT in the soluble fraction of hSP. They hold promise as possible targets for increasing biomarker potential of GGT.

Comparison of Enzyme Activity in Order to Describe the Metabolic Profile of Dairy Cows during Early Lactation

Enzymatic diagnostics have practical applications in diseases of the liver, heart, pancreas, muscles, blood, and neoplastic diseases. This study aimed to compare enzyme activity to describe dairy cows' metabolism during early lactation. Based on their general health symptoms, the cows were assigned to one of three groups: acidotic, healthy and ketotic. Samples of milk, blood and rumen fluid were collected at 12 ± 5 days postpartum. Ketotic cows were characterized by the highest malondialdehyde (MDA, 76.098 nM/mL), glutathione reductase (GluRed, 109.852 U/L), superoxide dismutase (SOD, 294.22 U/L) and gamma-glutamyltranspeptidase (GGTP, 71.175 U/L) activity. In comparing ketotic and acidotic cows, MDA, GluRed, SOD and GGTP activity were higher by a factor of almost: 1.85, 1.89, 0.79 and 2.50, respectively. Acidotic cows were characterized by the highest aspartate aminotransferase activity (AspAT, 125.914 U/L). In comparing acidotic and ketotic cows, AspAT activity was higher by a factor of almost 1.90. The use of enzymatic markers could limit the frequency of sampling for laboratory analyses and may result in a faster diagnosis of metabolic disorders. AspAT activity in blood serum seems to be a good indicator of acidosis; GGTP may participate in the pathogenesis of ketosis.

Assembly of tetraspanins, galectin-3, and distinct N-glycans defines the solubilization signature of seminal prostasomes from normozoospermic and oligozoospermic men

BACKGROUND

Prostasomes, extracellular vesicles (EVs) abundantly present in seminal plasma, express distinct tetraspanins (TS) and galectin-3 (gal-3), which are supposed to shape their surface by an assembly of different molecular complexes. In this study, detergent-sensitivity patterns of membrane-associated prostasomal proteins were determined aiming at the solubilization signature as an intrinsic multimolecular marker and a new parameter suitable as a reference for the comparison of EVs populations in health and disease.

METHODS

Prostasomes were disrupted by Triton X-100 and analyzed by gel filtration under conditions that maintained complete solubilization. Redistribution of TS (CD63, CD9, and CD81), gal-3, gamma-glutamyltransferase (GGT), and distinct N-glycans was monitored using solid-phase lectin-binding assays, transmission electron microscopy, electrophoresis, and lectin blot.

RESULTS

Comparative data on prostasomes under normal physiology and conditions of low sperm count revealed similarity regarding the redistribution of distinct N-glycans and GGT, all presumed to be mainly part of the vesicle coat. In contrast to this, a greater difference was found in the redistribution of integral membrane proteins, exemplified by TS and gal-3. Accordingly, they were grouped into two molecular patterns mainly consisting of overlapped CD9/gal-3/wheat germ agglutinin-reactive glycoproteins and CD63/GGT/concanavalin A-reactive glycoproteins.

CONCLUSIONS

Solubilization signature can be considered as an all-inclusive distinction factor regarding the surface properties of a particular vesicle since it reflects the status of the parent cell and the extracellular environment, both of which contribute to the composition of spatial membrane arrangements.

GGT levels in type II diabetic patients with acute coronary syndrome (does diabetes have any effect on GGT levels in acute coronary syndrome?)

Elevated gamma-glutamyltransferase (GGT) level is independently correlated with conditions associated with increased atherosclerosis, such as obesity, elevated serum cholesterol, high blood pressure and myocardial infarction. It is demonstrated that serum GGT activity is an independent risk factor for myocardial infarction and cardiac death in patients with coronary artery disease. Diabetes is also a well-known cardiovascular risk factor and an equivalent of coronary artery disease. Although the relationship between GGT and coronary artery disease has been reported, there are limited data exploring the changes of GGT in acute coronary syndromes, especially in patients with diabetes. So, this study aimed to determine changes in GGT level in diabetic and non-diabetic acute coronary syndromes. This trial was carried out at Kosuyolu Cardiovascular Training and Research Hospital and Van Yuksek Ihtisas Hospital, Turkey. A total of 219 patients (177 men and 42 women) presenting with acute coronary syndrome) and 51 control subjects between September 2007 and September 2008 were included in the study. Serum γ-glutamyltransferase and serum lipoprotein levels were determined. The resuls indicated that serum GGT levels were higher in acute coronary syndrome patients compared with control. In subgroup analyses, there was no difference between diabetic and non-diabetic subgroups. There was also weak correlation between GGT and blood glucose levels. There was no correlation between GGT and serum lipoprotein levels. In conclusion, serum GGT levels were higher in acute coronary syndrome patients. In subgroup analyses, There was no difference between diabetic and non diabetic subgroup.

β-Lipoprotein- and LDL-associated serum γ-glutamyltransferase in patients with coronary atherosclerosis

Elevation of serum gamma-glutamyltransferase (GGT) activity is a risk factor for myocardial infarction and stroke. GGT activity can catalyze the oxidation of low-density lipoprotein (LDL), a process involved in the pathogenesis of atherosclerosis. Serum GGT is partially adsorbed onto circulating LDL, and catalytically active GGT has been found within atherosclerotic plaques, colocalizing with oxidized LDL and foam cells. We investigated the the nature of the LDL-associated GGT, the degree of correlation between total serum GGT levels and beta-lipoprotein (beta-LP)-associated GGT, and whether this association is altered in subjects with coronary artery disease (CAD). LDL-bound GGT showed an entire, amphiphilic heavy chain, but the association was easily lost during LDL purification by affinity chromatography. When the activity of GGT associated with polycation-precipitated beta-lipoproteins was assayed, an identical immunoreactive GGT was found in Western blot, and a statistically significant linear correlation was found between total serum GGT levels and the corresponding beta-LP-bound activities (p<0.0001) in controls and patients with CAD. Nevertheless, subjects with CAD presented a lower ratio of beta-LP-bound GGT to total serum GGT respect to controls (p<0.05) and healthy subjects with elevated serum GGT (p<0.01). In addition, a relative decrease of total serum GGT was observed in CAD subjects of older age as compared to younger ones (p<0.005).

A sandwich enzyme-linked immunosorbent assay for human seminal gamma-glutamyl transpeptidase

A sandwich enzyme-linked immunosorbent assay (ELISA) was developed for detecting human seminal gamma-glutamyl transpeptidase (gamma-GTP) using a combination of anti-seminal gamma-GTP monoclonal antibodies. These monoclonal antibodies did not react with human ovary or uterus in immunohistochemical study. Optimal assay condition, resulting in a sensitive assay with a low background, is presented. The detection limit of this assay was estimated to be 1 ng/ml of seminal gamma-GTP corresponding to about 100,000 times dilution of seminal sample. This ELISA was specific for seminal gamma-GTP, without cross-reactivity to renal or hepatic gamma-GTP, normal blood serum, non-coital vaginal fluid or saliva. The recovery of seminal gamma-GTP added to various biological fluids were also examined.

Release of membrane-bound enzymes from cells and the generation of isoforms

Enzymes bound to the surfaces of cells may be retained by a hydrophobic amino acid sequence (e.g. gamma-glutamyltransferase) or by a specific glycan phosphatidylinositol (GPI) anchor (e.g. alkaline phosphatase). In either case the attachment is by means of non-covalent hydrophobic interactions between the anchoring domain of the enzyme and lipid components of the cell membrane. Enzyme molecules released into the plasma or bile, complete with their hydrophobic domains, can undergo aggregation and complexation to give rise to high molecular weight isoforms of gamma-glutamyltransferase or alkaline phosphatase. However, the GPI domain of alkaline phosphatase can be degraded by an inositol-specific phospholipase in plasma, but not in bile, with production of the hydrophobic, non-aggregating isoform of alkaline phosphatase that predominates in plasma.

Laboratory markers of ethanol intake and abuse: a critical appraisal

Laboratory markers for ethanol intake and abuse and chronic alcoholism currently in use have been critically reviewed. The merits and pitfalls of each test have been evaluated. The clinical use of the new test of carbohydrate-deficient transferrin has been particularly emphasized. Carbohydrate-deficient transferrin currently provides the highest specificity and sensitivity of all commonly used markers of alcoholism.

Purification and immunological characterization of a new form of gamma-glutamyltransferase of human semen

A new form of gamma-glutamyltransferase was purified from human seminal plasma. The purified enzyme was composed of two non-identical subunits with apparent molecular masses of 150 and 95 kDa on polyacrylamide gel electrophoresis (PAGE) in the presence of sodium dodecyl sulfate (SDS), and showed a molecular mass of 500 and 250 kDa on gel filtration in the absence and presence of 1% Triton X-100, respectively. This enzyme was different from human renal gamma-glutamyltransferase not only in apparent molecular masses, but also in amino acid compositions of both the subunits to each other. Experiments with the antisera raised against the purified enzyme revealed that the enzyme was different from the renal, hepatic and testicular enzymes in reactivity to the antibody though partially related to those enzymes. Ouchterlony double diffusion analysis indicated that both human seminal plasma and prostatic extract contained two types of gamma-glutamyltransferase, one is that we purified and the other the renal type. Hence, it is most likely that gamma-glutamyltransferase accounting for most of the enzyme activity in semen results from prostata followed by secretion to seminal plasma.

Gamma-glutamyltransferase from human hepatoma cell lines: purification and cell culture of HepG2 on microcarriers

After screening different human hepatoma cell lines, we observed that both HepG2 and PLC/PRF/5 naturally produced large amounts of gamma-glutamyltransferase. We optimized HepG2 cell culture conditions and observed that higher cell densities were obtained when cells were cultured on microcarriers, particularly when Cytodex 3 was used and that cell growth was optimal when DMEM, the basic medium, was supplemented with 5% fetal calf serum and 6 mmol/l glutamine. These culture conditions allowed us to produce the highest amounts of GGT after about 150 h of culture. The GGT obtained from HepG2 cells was partially purified and some of its physico-chemical properties characterized. Successive Con A gel chromatography separated the activity into two peaks, suggesting that GGT from HepG2 is not uniformly glycosylated. Papain-treated HepG2 GGT showed a Mr of about 120 kDa and migrated as a single-chain protein in SDS-PAGE. Immunological and kinetic properties of the GGT were similar to other human GGTs (liver, kidney and serum). It appears that HepG2 GGT could be a source for the preparation of a human enzyme reference material.

Comparative studies on the properties of purified gamma-glutamyl transferase from human reproductive system and the kidney

Gamma-glutamyl transferase (GGT) of the human seminal plasma and reproductive tissues was purified and its properties were compared to those of the enzyme from kidney. A single band of GGT was obtained by polyacrylamide gel electrophoresis. Purification was 1080-fold for seminal plasma, 206-fold for prostate, 608-fold for testis and 382-fold for kidney. Similar Km value (0.87-1.06 mM) and optimum pH (8.2-8.5) were obtained for the enzymes of the four different sources. Their thermal stabilities were identical. However, inhibitions by Zn2+ and Cu2+ were different between kidney and reproductive system GGT. Molecular mass of the native enzyme was 78 kDa for seminal plasma, prostate and testis and 79 kDa and 105 kDa for kidney. The subunit molecular masses of the enzymes from seminal plasma, prostate and kidney consisted of three proteins, suggesting the precursor form, and the heavy and light subunits of the mature form.

Diagnostic efficiency in discriminating liver malignancies from cirrhosis by serum gamma-glutamyltransferase isoforms

Total GGT and GGT complexed with low-density-lipoprotein plus very low-density lipoprotein (LDL + VLDL) have been evaluated in sera from 53 healthy subjects, 23 patients with chronic hepatitis, 87 with liver cirrhosis and 50 with liver tumors (primary and metastatic). A cut-off of 20 U/l of GGT complexed with LDL + VLDL results in a diagnostic sensitivity of 84% for liver tumor patients, and a diagnostic specificity of about 80% towards the two groups of patients affected by cirrhosis or chronic hepatitis. This test, because of its high diagnostic efficiency, is a useful addition to the battery of laboratory tests that serve to discriminate cirrhosis and chronic hepatitis from liver malignancies.

Gel-filtration of serum gamma-glutamyl-transpeptidase with HPLC in hepatobiliary diseases and its significance

Serum gamma-glutamyl-transpeptidase (gamma-GTP) from patients with various hepatobiliary diseases was fractionated by molecular size using HPLC to investigate the heterogeneity of serum gamma-GTP. Serum gamma-GTP eluted with HPLC showed essentially 3 elution peaks of different molecular size. In the sera of normal patients, there was little gamma-GTP in the high molecular fraction, and most occurred in the low molecular fraction. Intermediate molecular gamma-GTP appeared only in the sera of patients with hepatobiliary disease. In the sera of patients with extrahepatic biliary obstruction, the percentage of high molecular gamma-GTP to total serum gamma-GTP activity was higher than that of other patients. High molecular gamma-GTP increased in the sera of patients with extrahepatic biliary obstruction. Intermediate molecular gamma-GTP appeared in the relatively higher molecular fraction in the sera of patients in whom alcohol consumption might have caused serum gamma-GTP increase, and in the relatively lower molecular fraction in the sera of patients with extrahepatic biliary obstruction. It was concluded, therefore, that the position of elution of intermediate molecular gamma-GTP corresponded to the morbid state. It was shown that intermediate molecular gamma-GTP appeared in the relatively low molecular fraction, corresponding to the increase of serum total bile acids concentration.

In vitro studies upon the release of gamma-glutamyltransferase from human liver

Samples of human liver have been incubated in different fluids for up to 48 h and the released gamma-glutamyltransferase studied by gel chromatography on Sephacryl S-300 and polyacrylamide gradient gel electrophoresis. When human liver is incubated in serum, most of the released enzyme is of high Mr (greater than 1,000,000). Incubation in hepatic bile, or in a solution of glycochenodeoxycholate, results in the release of enzyme that is hydrophobic in nature and which reaggregates to a varying extent after the removal of bile salts. In contrast, incubation in saline, or in a solution of human albumin results in the release of a hydrophilic low Mr (about 120,000) form of the enzyme. These observations are discussed in relation to possible mechanisms for the release of these multiple forms.

Diuresis-dependent excretion of multiple forms of renal brush-border enzymes in urine

The urinary brush-border enzymes alanine aminopeptidase (EC 3.4.11.2), alkaline phosphatase (EC 3.1.3.1) and gamma-glutamyltransferase (EC 2.3.2.2) and their activity pattern of soluble and particulate forms separated by ultracentrifugation were investigated over 8 h after a high intake of fluid (22 ml/kg body wt.). The activity pattern of the two multiple forms was markedly influenced by the urine flow rate. The time-related excretion of soluble forms of these three enzymes increased with the extent of urine output whereas the excretion of particulate form was independent on diuresis. Diuresis-dependent excretion of total activities of brush-border enzymes can be exclusively considered to be a consequence of changed excretion of soluble forms.

Comparative structural and lectin-binding studies on gamma-glutamyltransferase from human adult liver, fetal liver and primary hepatoma

gamma-Glutamyltransferase was purified to apparent homogeneity from human adult liver, fetal liver and hepatoma by deoxycholate extraction, immunoaffinity chromatography, papain digestion, phenyl-Sepharose chromatography and preparative polyacrylamide gel electrophoresis. The purified enzyme from all three sources had an apparent Mr of 82 000 by Sephadex G-150 gel filtration and on dodecyl sulphate/polyacrylamide gel electrophoresis two nonidentical subunits of Mr 57 000 and 23 000 were obtained. The pI of all three forms was 3.85 and after neuraminidase treatment they each gave at least five bands with pI values ranging over 5.9-6.6. Sialic acid content was 188 (adult liver), 182 (fetal liver) and 188 (hepatoma) nmol/mg protein. Total neutral sugar content was 702 (adult and fetal liver) and 700 (hepatoma) nmol/mg protein. The hexosamine content of the enzyme from all the three sources was the same (354 nmol/mg protein) and galactosamine was absent. Partially purified hydrophobic and hydrophilic forms of gamma-glutamyltransferase from all the three sources were precipitated by Concanavalin A, Ricinus communis agglutinin and wheat germ agglutinin. These results show that gamma-glutamyltransferase from human adult liver, fetal liver and hepatoma are structurally similar and that the elevated levels found in fetuses and hepatoma are only a quantitative increase and are not due to a new isoenzyme.

Functional characteristics of leukotriene C4- and D4-metabolizing enzymes (gamma-glutamyl transpeptidase, dipeptidase) within human plasma

Several properties of the leukotriene C4- and leukotriene D4-metabolizing enzymes within human plasma were studied after fractionation of the plasma proteins using ammonium sulfate precipitation. Leukotriene D4-metabolizing enzymes were widely distributed among the fractions obtained. They showed different pH optima (pH 6.5, pH 7.0 and pH greater than or equal to 8.5) and revealed a different degree of thermal stability. The results indicate the presence of more than one enzyme in plasma which interacts with leukotriene D4. EDTA and L-cysteine inhibited the metabolism of leukotriene D4. Two leukotriene C4-metabolizing activities (gamma-glutamyl transpeptidases) differing in their molecular weights were detected after gel filtration. Their molecular weights were estimated to be Mr greater than or equal to 150 000 and Mr between 55 000 and 100 000.

Characterization of gamma-glutamyltranspeptidase from human exocrine pancreatic carcinomas

Two different types of gamma-glutamyltranspeptidase (gamma-GTP) have been found in normal human pancreas following bromelain treatment. On the other hand, three human pancreatic ductal cell carcinomas have only a single type of gamma-GTP upon analysis with polyacrylamide gel electrophoresis, anion-exchange column chromatography and isoelectric focusing. Carcinoma gamma-GTPs were almost identical to one of the two types of normal pancreatic gamma-GTPs. The gamma-GTP from pancreatic carcinomas bound to anion-exchange column and was eluted at the same NaCl fractions as normal pancreatic gamma-GTP. The properties of pancreatic carcinoma-gamma-GTP, as assessed by binding to concanavalin A and lentil lectin affinity columns, were also similar to one of the two enzymes of normal pancreas. No apparent difference in isoelectric points was found between the carcinoma gamma-GTPs and one of the two normal pancreatic gamma-GTPs.

Two types of human pancreatic gamma-glutamyl transpeptidase

Two different types of gamma-glutamyl transpeptidase (gamma-GTP) were extracted from human pancreas by protease treatments such as bromelain. Furthermore, human pancreatic gamma-GTP, extracted with trypsin, was separated into two different components by additional treatment with bromelain. One component displayed fast electrophoretic mobility during polyacrylamide gel electrophoresis and an apparent affinity for an anion-exchange column, while the other showed slow electrophoretic mobility and passed through the anion-exchange column with starting buffer. In addition, the percentage affinity to concanavalin A (Con A) of the former was 52.2% and of the latter only 7.2%. On heat stability, the former was more sensitive than the latter at 56 degrees C. These results indicate the existence of two types of gamma-GTP in human pancreas.

The metabolism of leukotrienes in blood plasma studied by high-performance liquid chromatography

The metabolism of leukotrienes (B4, C4, D4, and E4) within human plasma was studied and a simple sample preparation is presented. It was demonstrated that leukotriene E4 and leukotriene B4 were stable during incubation at 37 degrees C using the in vitro system. In contrast, leukotriene C4 was metabolized by gamma-glutamyl transpeptidase activities into leukotriene D4 which was further metabolized by dipeptidase activities of plasma into leukotriene E4. The transition state inhibitor of gamma-glutamyl transpeptidase L-serine-borate decreased the metabolism of leukotriene C4 in plasma. Dilution of plasma demonstrated that the dipeptidase was more active compared to the gamma-glutamyl transpeptidase. The metabolizing activities of plasma were functionally characterized by fractionating the plasma proteins.

Differential solubilization of rabbit liver plasma membrane gamma-glutamyltransferase by proteases and detergents: effect of phenobarbital treatment

In an attempt to understand the mechanism of gamma-glutamyltransferase transfer from the liver to the plasma, potassium chloride, sodium dodecyl sulfate and proteases (papain and bromelain) were used to solubilize rabbit liver plasma membrane gamma-glutamyltransferase. Potassium chloride solutions solubilized 10-30% of membrane proteins but only 1-3% of membrane gamma-glutamyltransferase activity. However, when sodium dodecyl sulfate is used, even at low concentration (0.1-0.2%, w/v) greater than 90% of membrane gamma-glutamyltransferase activity and about 80% of membrane proteins can be solubilized. Furthermore, we showed that unlike the effect of bile salts on the membrane gamma-glutamyltransferase of phenobarbital-treated animals, the same treatment seems to have no influence on membrane gamma-glutamyltransferase solubilization by proteases. Indeed, the ratios of gamma-glutamyltransferase solubilization by papain or bromelain were the same for liver membranes obtained from control and phenobarbital-treated animals.

gamma-Glutamyltranspeptidase activity in a human pancreatic cancer cell line (HPC-Y1) in serum-free chemically defined medium

gamma-Glutamyltranspeptidase (gamma-GTP) activity was found in a human pancreatic cancer cell line, HPC-Y1, cultivated in a serum-free chemically defined medium. The gamma-GTP stained in the cytoplasms as fine granules and was produced constantly in a protein-free chemically defined medium. The detergent- or protease-solubilized gamma-GTP of HPC-Y1 cells in serum-free medium was compared with the gamma-GTPs extracted from HPC-Y1 cells in serum-containing medium, human pancreatic carcinoma and normal human pancreas. Their molecular weights, electrophoretic mobilities, affinity to Concanavalin A-Sepharose and isoelectric points were almost identical. No cancer specific properties in the gamma-GTP derived from human pancreatic carcinoma cell line were found by these analyses. However, the serum-free spent medium of HPC-Y1 cells was useful for purifying the gamma-GTP secreted from the human pancreatic carcinoma cell line, since it is not necessary to separate the contaminated serum components that are usually added for cell cultures and the extraction procedures could induce minor structural change and/or artificial modification of gamma-GTP.

Physical properties of gamma-glutamyltransferase in human serum

Using gel chromatography on Sephacryl S300 we have separated serum gamma-glutamyltransferase into three fractions with estimated relative molecular masses of (a) greater than 1000 000, (b) 250 000-500 000 and (c) about 120 000. Similarly, serum leucine aminopeptidase has been separated into three fractions. We have studied, particularly, the gamma-glutamyltransferase fraction of intermediate relative molecular mass (250 000-500 000) in serum from patients with a number of liver diseases. We have shown, both by polyanion and immuno-precipitation, that it consists significantly of a complex between gamma-glutamyltransferase and high density lipoprotein. The physical properties of this fraction, namely its mass and charge, can be altered by incubating serum with either bile or bile salts.

Gamma-glutamyl transferase activity in fetal serum, maternal serum, and amniotic fluid during gestation

Gamma-glutamyl transferase activity was measured in fetal serum, maternal serum, and amniotic fluid in 173 pregnancies from 15 to 40 weeks' gestation. Fetal serum was obtained in the second trimester by fetoscopy and in the third trimester by umbilical cord puncture at caesarian section or vaginal delivery. Enzyme activities in maternal blood (10 IU/1, SD 2) and fetal blood (88 IU/1, SD 20) remained relatively constant throughout gestation, whereas in the amniotic fluid there was a significant decrease at term from the value in the second trimester (p less than 0.001). Electrophoretic separation of the enzyme showed one isoenzyme in the fetal blood and at least two in the amniotic fluid. The fetal isoenzyme had the same mobility as the major isoenzyme in the amniotic fluid.

Separation of hydrophobic and hydrophilic forms of gamma-glutamyltransferase from human serum by hydrophobic chromatography on phenyl-Sepharose CL-4B: studies on normal sera and sera of patients with liver disease

A new method is described for the separation of hydrophobic and hydrophilic forms of gamma-glutamyltransferase (GGT) present in human serum using hydrophobic chromatography on phenyl-Sepharose CL-4B columns. The analysis of sera of 30 normal healthy people showed that 71.49 +/- 8.73% (mean +/- SD) of the GGT was in the hydrophilic form. In sera from 52 patients with various liver diseases the major part of the total enzyme was hydrophobic GGT, contributing 67.38 +/- 13.8% of the total GGT activity. Elevated total serum GGT and a hydrophobic to hydrophilic ratio of more than one helped in diagnosing hepatobiliary disease in 51 of 52 patients. Papain digestion of serum converted hydrophobic to hydrophilic GGT, which was similar to soluble form of liver GGT. The results are discussed in relation to the proposed mechanisms of release of GGT from the liver to the circulation.

Variant gamma-glutamyltransferases in colorectal carcinomas

Two variants of gamma-glutamyltransferase were demonstrated in colorectal carcinomas. When compared to the enzyme in normal large bowel mucosa, one of the variants showed reduced affinity to concanavalin A and considerable charge heterogeneity largely due to variable sialic acid content. The other variant appeared to be an asialo form with no affinity to concanavalin A. The two novel forms were of identical size and antigenicity compared to the normal enzyme. They might therefore reflect different post-translational modifications of the carbohydrate moiety of the enzyme.

The nature of gamma-glutamyltransferase and other hepatocyte plasma membrane enzymes in human bile

High molecular mass gamma-glutamyltransferase in human bile is converted by physiological concentrations of the bile salts glycocholate and glycochenodeoxycholate to a form with a molecular mass estimated by gel chromatography to be 165,000. A molecule of intermediate molecular mass estimated to be 330,000 is obtained following treatment with the non-ionic detergent Triton X-100. When human bile is centrifuged at 150,000 X g, between 25-73% of gamma-glutamyltransferase activity is recovered in the supernatant. This proportion is increased following prior addition of bile salts to the bile. Analogous results are obtained for the high molecular mass enzymes alkaline phosphatase and leucine aminopeptidase, present in human bile, and also considered to originate from the hepatocyte plasma membrane. From these results it is suggested that these high molecular mass enzymes found in bile may, in part at least, represent artefacts following aggregation of the enzymes in aqueous media containing no bile salts.

Multiple forms of serum gamma-glutamyltransferase. Association of the enzyme with lipoproteins

The association of gamma-glutamyltransferase activity with lipoproteins was investigated in serum from hepatobiliary diseased patients. From 60 to 80% of the total activity in such sera was bound in lipoprotein complexes. These complexes or multiple forms of gamma-glutamyltransferase could be separated into two main fractions by gel filtration or agarose gel electrophoresis. One fraction was characterized by gamma-glutamyltransferase of high molecular mass (Mr greater than 600,000) and beta-mobility and was detected in increased amounts in serum from patients with cholestasis. The main part of this activity was associated with lipoprotein-X and this complex could amount to 50% in some sera. The other fraction contained gamma-glutamyltransferase eluting in the molecular mass range of 250,000 to 450,000 and migrating with alpha 1 alpha 2-mobility. In this fraction complexes of gamma-glutamyltransferase and high-density lipoprotein were detected which could amount to 70% in non-icteric sera. However, heterogeneity in size, charge and density could be demonstrated in both fractions.

Hydrophilic form of gamma-glutamyltransferase: proteolytic formation in liver homogenates and its estimation in serum

By incubating liver homogenates at pH 7.0 a proteolytic modification of gamma-glutamyltransferase was demonstrated. The amphiphilic, membrane-bound enzyme was converted to a hydrophilic form whose size and electrophoretic mobility were identical to those of the hydrophilic form present in human serum. The relative amounts of the hydrophilic and the amphiphilic gamma-glutamyltransferase in various pathological sera were analyzed. In the majority of these sera the hydrophilic form constituted less than 15-20% of the total activity. High amounts (30-55%) were demonstrated in serum samples from two patients with necrosis of the liver. However, the determination of this form of gamma-glutamyltransferase appears to have little clinical significance.

Comparative ontogenesis of gamma-glutamyl transpeptidase in rat tissues

In rats, placental gamma glutamyl transpeptidase (GGTP) specific activity declined linearly during the last third of gestation. In contrast, the kidney enzyme activity progressively increased 15-fold. In the gut, peak activity was reached at mid-lactation (age 12 days). Hepatic GGTP levels were maximal during the foetal stage. Brain GGTP was negligible at birth and attained its highest specific activity at weaning.

Biochemistry of alkaline phosphatase isoenzymes

The review discusses the similarities and differences between the common isoenzymatic forms of ALP. Methods for separating, measuring, and purifying the isoenzymes on the basis of these differing properties are described. The evidence is reviewed for the existence of different genes coding for different isoenzymes, and the current state of knowledge is surveyed concerning the location, development, function, and regulation of the isoenzymes. Finally, some unusual forms of ALP which may appear in the circulation are described.

Gamma-glutamyltransferases in bile and sera from patients with extrahepatic biliary obstruction

Gamma-glutamyltransferase has been studied in bile and sera from patients with extrahepatic biliary obstruction. In both fluids gamma-glutamyltransferase activity is found largely in the high molecular mass fraction present in the void volume following G200 gel chromatography and which remains at the origin after 7% polyacrylamide gel electrophoresis. When bile, sera and liver microsomes are treated with deoxycholate, a different form of gamma-glutamyltransferase, with an approximate molecular mass of 157 000 estimated by gel chromatography and of 125 000 by gradient gel electrophoresis, is obtained. After treatment with papain, all three types of specimen gave rise to a form of gamma-glutamyltransferase with a molecular mass of 115 000 estimated by gel chromatography and of 98 000 by gradient gel electrophoresis. The possible relationship between these various forms of gamma-glutamyltransferase, and their relevance to the increases in activity in serum seen in extrahepatic biliary obstruction, are discussed.

Separation and characterization of human gamma-glutamyltransferases

gamma-Glutamyltransferase (EC 2.3.2.2) from human liver, kidney and pancreas could be separated by differences in their carbohydrate moieties. Differences in electrophoretic mobility, presumably due to different sialic acid content, and in affinity to several lectins, were revealed. Apparent identity reactions could be demonstrated by double immunodiffusion for the enzymes from these organs. Of the molecular properties studied, no differences were demonstrated between the serum and liver enzymes, or between the urine and kidney enzymes.

The molecular forms of gamma-glutamyl transferase in bile and serum of icteric rats

Alterations in the molecular form of gamma-glutamyl transferase (5-glutamyl)-peptide:amino-acid 5-glutamyltransferase, EC 2.3.2.2.) were studied in the bile and serum of rats under surgical ligation of the bile duct. Polyacrylamide gel electrophoresis of the bile, followed by the enzyme stain, revealed a major, slowly migrating broad band and a minor, faster migrating band. The former was converted to the latter upon limited proteolysis of the bile with a very small amount of papain. This conversion was accompanied by a decrease in molecular size of the enzyme. Both enzyme forms were specifically adsorbed to a concanavalin A-Sepharose column. Most of the papain-treated enzyme preparation could be eluted from the column by alpha-methyl-D-glucoside, a haptenic sugar of this lectin. On the other hand, the predominant form of the enzyme in the untreated bile was eluted only in the presence both of the sugar and Triton X-100. Based on the chromatographic behavior of the two enzyme forms (detergent-solubilized and protease-solubilized form) purified from rat renal brush border membrane on concanavalin A-Sepharose column, it was concluded that the predominant form of the enzyme in the bile was the detergent-solubilized form and that the minor component represents the protease-solubilized enzyme. The serum from icteric rats was also found to contain both types of the enzyme. However, the relative amount of the protease-solubilized form to the detergent-solubilized form in the serum was much greater than that in the bile. These findings suggested that gamma-glutamyl transferase in the hepatobiliary membrane systems was solubilized into the bile mainly as the detergent-solubilized form, and that, during the process of translocation into the blood circulation, the enzyme was partly converted to the protease-solubilized form by some protease-like action.

Characterization of human serum gamma-glutamyltranspeptidase

The contribution of proteolysis to leakage of gamma-glutamyltranspeptidase from the liver into the serum was examined by comparing the properties of human serum gamma-glutamyltranspeptidase with those of partially purified enzyme preparations solubilized from human liver with Triton X-100 and with papain. The Triton-solubilized enzyme was hydrophobic, whereas the papain-solubilized enzyme was hydrophilic. Serum gamma-glutamyltranspeptidase was found to be hydrophilic, since it could be extracted without addition of detergent after acetone treatment to remove bound cholate. The affinity of serum-gamma-glutamyltranspeptidase to concanavalin A-Sepharose was compared with those of the Triton-solubilized and papain-solubilized enzymes. All three enzymes were adsorbed to concanavalin A-Sepharose in 50 mmol/l imidazole buffer, pH 7.2. The serum enzyme and papain-solubilized enzyme were eluted with 100 mmol/l alpha-methylmannoside in 50 mmol/l imidazole buffer containing 1% Triton X-100. The molecular weights of the Triton-solubilized, papain-solubilized and serum-gamma-glutamyltranspeptidases were estimated as 98 000 +/- 2000, 90 000 +/- 2000 and 90 000 +/- 2000, respectively, by electrophoresis. These results show chemical similarities between the serum-gamma-glutamyltranspeptidase and papain-solubilized gamma-glutamyltranspeptidase.

Isolation and purification of multiple forms of gamma-glutamyl transpeptidase from rat brain

Four different forms of the enzyme gamma-glutamyl transpeptidase were isolated from rat brain by chromatography on concanavalin A. An approximate 1500-fold purification was achieved. The four forms were characterized with respect to molecular weight, Km for gamma-glutamyl-p-nitroanilide, mobility on polyacrylamide gels, and inhibitory effects of borate-serine. The multiple forms of the enzyme were found to have molecular weights ranging from 74,000 to 234,000 and Kms of 0.07 to 8.6 mM. It was determined that in brain, the major portion of the enzyme activity is associated with plasma membrane fragments and endoplasmic reticulum.

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.

Serum gamma-glutamyltransferase isoenzymes in extrahepatic biliary obstruction

The gamma-glutamyltransferase isoenzymes in the sera of patients with extrahepatic biliary obstruction have been studied, using electrophoretic, gel filtration, and ultracentrifugation techniques, and compared with those present in normal sera. Five isoenzymes were shown to exist in patients' sera, three of which were not demonstrated in normal sera. The observations are discussed in relation to the influence of biliary regurgitation and the possible solubilisation of membrane-bound enzymes. The results are compared with those of previous studies on alkaline phosphatase.

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.

Subcellular localization of gamma-glutamyltransferase activity in guinea pig liver. Effect of phenobarbital on the enzyme activity levels

The localization of gamma-glutamyltransferase activity in guinea pig liver was studied after subcellular fractionation. The enzyme activity was essentially connected with plasma membranes whereas only low activity was found in the endoplasmic reticulum. A similar activity distribution was demonstrated for 5'-nucleotidase. Highest specific activity of gamma-glutamyltransferase was found in plasma membranes enriched in bile canaliculi. In this fraction the specific activity was 35 times greater than the specific activity of the total homogenate, a value similar to the relative specific activity of (Na+,K+)-ATPase. More than 90% of the total gamma-glutamyltransferase activity in guinea pig liver was connected with parenchymal cells and the enzyme seemed to have an outside orientation. Animals treated with phenobarbital showed moderate increased in gamma-glutamyltransferase activity in serum and liver, whereas high activities were found in most bile samples. No particular liver subfraction showed substantial accumulation of gamma-glutamyltransferase activity. The present findings do not support the suggested use of serum gamma-glutamyltransferase measurements as a direct index of "microsomal enzyme induction".

The activity of gamma-glutamyltransferase after bile duct ligation in guinea pig

The activity of gamma-glutamyltransferase was studied in guinea pig after bile duct ligation. In serum, an abrupt increase in activity up to 10--20 times the normal value was found 3 h after obstruction and the mean activity over the first 3 days following the operation was some 8 times the normal value. In liver, however, a small decline in activity could be demonstrated. The administration of cycloheximide did not influence the acute increase in serum activity. Bile duct ligation caused marked increases in serum bile acid levels which initially paralleled the serum gamma-glutamyltransferase activity. It is suggested that the increased serum activity may arise from the solubilization by bile acids of liver membrane-bound enzyme.