Bovine colostral gamma-glutamyltransferase; its localization in skim milk membrane and irrelevance to secretory immunoglobulin A

Early variations of blood plasma gamma-glutamyl transferase in newborn calves--a test of colostrum intake

Fourteen newborn calves had low gamma-glutamyl transferase catalytic activity concentration in blood plasma (10 to 31 U/liter). Colostrum intake induced a dramatic increase of plasma gamma-glutamyl transferase activity, which was maximal on the 1st or 2nd day (370 to 5000 U/liter) after birth; then plasma enzyme activity slowly decreased and was stable about 100 U/liter on the 20th day. In two calves given either boiled or synthetic milk, plasma gamma-glutamyl transferase activity was not higher than 25 U/liter. Gamma-glutamyl transferase activity of blood plasma in newborn calves is an easy and inexpensive test of colostrum intake.

Purification and identification of gamma-glutamyl transpeptidase of milk membranes

Recent evidence suggests that gamma-glutamyl transpeptidase may be involved in the transport of amino acids into the lactating mammary gland. The enzyme also is secreted in milk and is associated mainly with milk membranes. The objective of this study was to purify and characterize gamma-glutamyl transpeptidase from milk membranes. The enzyme has been purified from milk membranes by solubilization with Lubrol WX; treatment with acetone, deoxylcholate, and bromelain; and chromatography on ion exchange and molecular-sieving resins. gamma-Glutamyl transpeptidase was purified over 11,000-fold from milk. Electrophoresis on sodium dodecyl sulfate polyacrylamide gels indicates that the enzyme is composed of two subunits with molecular weights of 57,000 and 25,500. Both subunits are glycoproteins and have been identified in the sodium dodecyl sulfate polyacrylamide gel electrophoresis patterns of whole milk membrane. Kinetic characteristics of the purified enzyme are similar to those determined for intact milk membranes and lactating mammary tissue indicating that the purified enzyme has not been modified functionally by the purification procedure.

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.

Identification, partial purification and biochemical characterization of gamma-glutamyltranspeptidase present as a membrane component in skimmed milk and milk fat-globule membranes, and in mammary-tumour virus from the milk of infected mice

The enzyme gamma-glutamyltranspeptidase was reproducibly found to be associated with mouse milk particles; it is present in milk fat-globule membranes and mouse mammary-tumour virus of infected Swiss mice, also in particles from the milk of uninfected mice. The enzymatic activities observed range among the highest reported for mammalian tissues. The enzyme was partially purified from mouse mammary-tumour virus, and from milk fat-globule membranes. The molecule requires the presence of detergents to remain soluble, behaves as a high molecular weight component, properties characterizing integral membrane proteins. Kinetics, and the effect of competitors as well as of specific inhibitors show this enzyme to be identical to the well-known kidney gamma-glutamyltranspeptidase ((gamma-glutamyl)-peptide:amino-acid gamma-glutamyltransferase, EC 2.3.2.2). Other oncornaviruses budding from cultured cells originally expressing the enzyme in their plasma membrane also incorporate the enzyme in their structure.

gamma-Glutamyl transpeptidase of bovine milk membranes: distribution and characterization

The distribution and characterization of gamma-glutamyl transpeptidase from bovine milk was examined. The enzyme was associated mainly with components of milk membrane. Skim milk membrane had 41% of the total activity in whole milk whereas milk fat globule membrane showed 7%. Enzyme kinetic characterization of these two membrane fractions showed they had identical amino acid specificities, pH, and sodium chloride effects, and similar apparent Michaelis constants. The main enzymatic difference between the membranes was a 2 to 3 times higher specific activity of the skim milk membranes. Skim milk membrane from milk would be a good source of this enzyme for continued studies of the function of gamma-glutamyl transpeptidase in amino acid transport during lactation.

Blood and tissue distribution of gamma glutamyl transferase in the cow

To test the hepatic specificity of gamma-glutamyl transferase in the cow, the tissue distribution of this enzyme was studied in 10 healthy adult cows. It was located mainly in kidney, pancreas, and liver. The relative hepatic specificity of this enzyme indicated that measurement of its activity in serum could be a valuable test of hepatic damage in the cow. The blood activity measured within 54 plasmas and 54 serums of apparently healthy animals was almost the same, 17.7 +/- 5.7 U/liter for plasma and 18.6 +/- 6.2 U/liter for serum.