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

1α,25-Dihydroxyvitamin D3 enhances γ-glutamyl transpeptidase activity in LLC-PK1 porcine kidney epithelial cells

Mammalian γ-glutamyl transpeptidase (GGT) is expressed most highly in the kidney and serves to recover the constituent amino acids of glutathione in the proximal tubules. Serum GGT is used as a marker for obstructive jaundice and alcoholic liver disease and it has been reported that urinary GGT is a potential marker for bone resorption. The present study investigated the effect of derivatives of vitamin D3 on GGT activity in LLC-PK1 porcine renal tubular epithelial cells in order to analyze the biochemical basis of bone metabolism-associated alterations in GGT activity in the kidney. In the presence of 1α,25-dihydroxyvitamin D3 [1,25(OH)2VD3], GGT activity was observed to be significantly increased in LLC-PK1 cells, with an increase in GGT activity also found in the cell medium. While the stimulatory effect of 1-OH-VD3 was similar to that of 1,25(OH)2VD3, vitamin D3 and 25-OH-VD3 had no effect on GGT activity. The increased GGT activity caused by 1,25(OH)2VD3 in LLC-PK1 cells was the result of long-term stimulation of the cells, in contrast to the GGT-induced increase in alkaline phosphatase, which is more transient. Polymerase chain reaction analysis revealed that the 1,25(OH)2VD3‑induced increase in GGT activity was due to prolonged GGT turnover, rather than increased GGT expression, as no increase in GGT mRNA expression was observed. Thus, it is likely that the expression of GGT is not entirely constitutive in the kidney, but is altered under certain conditions, including under hormonal regulation.

Immunological and ultrastructural analysis of loss of tubular membrane-bound enzymes in patients with renal damage

The pathophysiological background of shedding of membrane-bound enzymes from the proximal tubule was assessed in urine specimens of patients with renal damage applying immunospecific affinity-chromatography, immunotitration, ultracentrifugation, electroimmunodiffusion, immunohistology, as well as negative staining technique. Compared to healthy controls, patients with kidney injury, e.g. after administration of potentially nephrotoxic drugs (cytostatics, contrast media) revealed an increased excretion rate of vacuolar membrane fragments (50-500 nm) into urine. The brush border (BB) of the proximal tubule was identified as a main source of urinary blebs as concluded from immunoelectrophoretic and immunohistochemical analysis. In addition, the marker enzyme profile of urinary vacuolar blebs was similar to that of the BB membrane from human kidney. The results further evidenced that, during the initial phase of tubular injury, 5-10 nm surface glycoproteins of the BB, among them Ala-Leu-aminopeptidase and portions of gamma-glu-transpeptidase, are released into urine; this might be followed by increased blebbing of macromolecular BB fragments, indicating more severe membrane disruption.

Studies on urinary gamma-glutamyl transpeptidase in nephrotic syndrome patients

(1) Variations in the levels of GGT were measured in urine specimens taken in the early morning in control and in 20 consecutive adult patients with uncomplicated nephrotic syndrome. (2) The urinary GGT activity was increased in all cases of nephrotic syndrome patients investigated with different etiology. (3) A significant correlation was found between urinary GGT activity and serum albumin (r = 0.727) but not with serum cholesterol (r = 0.129). (4) These findings suggest that enhanced excretion of urinary GGT may be stimulated by decreased albumin concentration or oncotic pressure but does not appear to be due to leakage from plasma. (5) A systematic study on urinary GGT showed that GGT activity was decreased to the upper limit of normal control values in nephrotic syndrome patients after remission.

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.

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.

Purification of detergent-solubilized form and membrane-binding domain of rat gamma-glutamyltransferase by immuno-affinity and hydrophobic chromatography

A new method to purify papain- or detergent-solubilized form (papain or detergent form) of gamma-glutamyltransferase from rat hepatomas as well as from rat kidney by immuno-affinity column chromatography is presented. The antibody-column was prepared by coupling the anti-kidney papain form antibody, which had been purified by using a kidney papain form-Sepharose column, to CNBr-activated Sepharose 4B. The enzyme bound to the antibody-column was eluted with 0.04 M NH4OH. By this method, detergent forms were purified 300 and 1600-fold in approx. 50% yields from rat kidney and rat ascites hepatoma AH 13, respectively, and the papain form was also purified 16 000-fold in a similar yield from primary hepatoma which has a very low activity of this enzyme. Preparations thus obtained apparently did not contain any peptide other than heavy and light subunit peptides of this enzyme on SDS-polyacrylamide gel electrophoresis. The detergent form of kidney enzyme was preferentially absorbed to a hydrophobic column of aminooctyl-Sepharose, while the papain form was not, suggesting that the detergent form might be adsorbed to the column through hydrophobic interaction of the membrane-binding domain. The domain peptide was also purified by the hydrophobic column after release from the detergent form by papain treatment. The molecular weight of the peptide was estimated to be about 16 000 on SDS-polyacrylamide gel electrophoresis. On double immunodiffusion, the domain peptide reacted with anti-detergent form antibody but not with anti-papain form antibody. The domain-specific antibody was also purified from the anti-detergent form antibody.

Development of a radioimmunoassay for a high molecular mass tubular antigen in urine--its application for early detection of tubular damage

In order to isolate urinary kidney antigens, the gammaglobulins fraction of an antiserum against human kidney cortex plasma membranes was coupled to Sepharose 4B. By immunospecific affinity chromatography an antigen fraction was obtained from the urine of a patient suffering from severe kidney disease. After gel filtration, the main fraction, eluted with the exclusion volume of a Sephadex G-200 column and enriched 16 000-fold, was labelled with 131I and used in a radioimmunoassay system. Soluble kidney antigens, presumably of proximal tubular origin, could be detected and quantified by the assay system in urine samples of patients with various diseases. The samples did not need to be treated, either concentrated or dialyzed, before application. The results of our experiments show a correlation between antigen excretion and kidney damage. Rejection episodes in patients with kidney transplants could be recognized early by enhanced antigen excretion. Potentially nephrotoxic drugs caused antigen excretion as well. In normal, healthy subjects output of the antigen was very low. The assay system might be of value for monitoring renal 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.