Inhibition of muscle actomyosin ATPase activity by mitochondrial ATPase inhibitor.

Ascites hepatoma cell line AH-130 was tested for the ability to transport various amino acids and glutathione before and after γ-glutamyl transpeptidase of the cells was affinity-labeled and inactivated by 6-diazo-5-oxo-L-norleucine, a glutamine analog. The rate of uptake of alanine, glycine, leucine and glutamine by the cells remained unchanged after γ-glutamyl transpeptidase was inactivated by this affinity label. This indicated that γ-glutamyl transpeptidase of the cell was not involved in the transport process of these amino acids tested. The uptake of glutathione was also tested before and after affinity labeling the enzyme. The total amount of the radioactivity incorporated into the cells was not significantly affected by the enzyme inactivation. However, the relative amount of incorporated intact glutathione was found to be slightly but significantly increased after membraneous γ-glutamyl transpeptidase was inactivated by the affinity label, while that of component amino acid, glycine, was found to decrease. This indicated that glutathione was taken up by the cell in its intact form as well as in degraded forms into its component amino acids, and γ-glutamyl transpeptidase in the ascites tumor cell AH-130 seemed to be involved in the metabolic process via the latter system.     

Transport of the large-neutral amino acids by the gamma-glutamyl cycle: a proposal.

The γ-glutamyl cycle has been proposed by Meister (1973) as one possible mechanism for the mediation of amino acid transport. The high energy requirement of the pathway, the very low specificity of γ-glutamyl transpeptidase and the inability to account for trans membrane stimulation of amino acid entry are but three criticisms of this hypothesis. It is proposed that the various objections can be overcome by postulating that the soluble form of γ-glutamyl transpeptidase transfers the γ-glutamyl moiety from gluthathione to glutamine (in the case of brain) and that the membrane sequestered form of this enzyme catalyzes the exchange of the γ-glutamyl group between γ-glutamyl glutamine and an entering neutral amino acid. The released glutamine leaves the cell. The γ-glutamyl amino acid then passes into the cytoplasm where it is acted upon by either γ-glutamyl cyclotransferase or the soluble γ-glutamyl transpeptidase which transfers the γ-glutamyl group to another molecule of glutamine. It is postulated that access to the membrane-bound enzyme is dependent on the relative lipophilia of the entering large-neutral amino acids. The available data support this mechanism. By regeneration of γ-glutamyl glutamine, a low expenditure of energy is required for the transport process. Specificity of transpeptidation is attained by the constraints of access to the membrane bound enzyme site.     

Modification of the acceptor binding site of gamma-glutamyl transpeptidase by the diazonium derivatives of p-aminohippurate and p-aminobenzoate

Hippurate and maleate have been shown to bind to the aminoacylglycine (acceptor) binding site of γ-glutamyl transpeptidase, thereby stimulating the hydrolysis of γ-glutamyl compounds at the expense of transpeptidation (Thompson, G. A., and Meister, A. (1979) J. Biol. Chem.254, 2956–2960; Thompson, G. A., and Meister, A. (1980) J. Biol. Chem.255, 2109–2113). It has now been found that a number of benzoate derivatives also bind and modulate rat kidney transpeptidase, as indicated by their ability to enhance the rate of inactivation of transpeptidase by the glutamine antagonist l-(αS, 5S)-α-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125). Furthermore, rapid loss of transpeptidase activity results upon preincubation of the enzyme with the diazonium derivatives of p-aminohippurate and p-aminobenzoate. The modified enzyme can still hydrolyze γ-glutamyl substrates but is no longer modulated by hippurate and maleate. Loss of transpeptidase activity was not associated with incorporation of radioactive label from diazotized [¹⁴C]p-aminohippurate. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the modified enzyme revealed a nondissociable species, M_r 68,000, shown to result from crosslinking of the two subunits of transpeptidase (M_r 46,000 and 22,000, respectively). The crosslinking of the subunits paralleled the extent of inactivation of transpeptidation activity and both crosslinking and inactivation were prevented by treatment with the diazotized derivatives in the presence of either hippurate or maleate. These and other data indicate that the diazonium derivatives of p-aminohippurate and p-aminobenzoate interact with the acceptor binding site and produce a stable bond between amino acid residues in the vicinity of this site which, thus, appears to be located in the intersubunit contact region.     

Gamma-glutamyl transpeptidase of rat seminal vesicles; effect of orchidectomy and hormone administration on the transpeptidase in relation to its possible role in secretory activity.

γ-Glutamyl transpeptidase was prepared from rat seminal vesicles by two methods and was found to be similar to rat kidney γ-glutamyl transpeptidase with respect to substrate specificity, stimulation of “glutaminase” activity by maleate, and apparent molecular weight. Histochemical studies demonstrated that γ-glutamyl transpeptidase is concentrated in the secretory epithelium of the seminal vesicle. Like the epithelium itself, the enzyme responds to the presence or absence of testosterone. The content and specific activities of γ-glutamyl transpeptidase and γ-glutamyl cyclotransferase in rat seminal vesicles are low in orchidectomized animals, an effect which is reversed by administration of testosterone but accentuated by estradiol administration. These enzymes may be involved in the secretory functions of the seminal vesicles.     

gamma-Glutamyl transpeptidase from WI-38 fibroblasts: purification and active site modification studies

γ-Glutamyl transpeptidase has been purified to homogeneity from WI-38 human fetal lung fibroblasts, following extraction with Triton X-100 in the absence of added proteases. The specific activity of the purified enzyme is 16 units/mg protein at the optimum of pH 8.0. Although this activity value is low, the WI-38 enzyme is very similar to previously described γ-glutamyl transpeptidases in its molecular properties. The native molecule (apparent molecular weight of 82,000) is composed of one light and one heavy subunit (apparent molecular weights of 20,000 and 62,000, respectively). Papain digestion reduces the native molecular weight to an apparent value of 73,000 by proteolysis of the heavy chain. The known active site modifying agent and glutamine analog 6-diazo-5-oxo-l-nor-leucine, completely inactivates the enzyme, coincident with its stoichiometric incorporation into the light subunit. This inactivation is accelerated by maleate and prevented by S-methylglutathione. The WI-38 γ-glutamyl transpeptidase is also inactivated by the fluorescent alkylating agent, 5-iodoacetamidofluorescein. Selective reaction of this reagent with an active site residue is suggested by prevention of the inactivation by S-methylglutathione, the stoichiometric incorporation of the fluorescein moiety, and the loss of one methionine residue per molecule of protein accompanying inactivation.     

Significance of γ-glutamyl transpeptidase and thiols in amino acid uptake by rat pancreatic islets: Studies with glutamine and leucine

The importance of γ-glutamyl transpeptidase, the key enzyme of the γ-glutamyl cycle and of thiols for the uptake of amino acids into rat pancreatic islets was investigated. Both serine–borate, an inhibitor of γ-glutamy transpeptidase, and serine which does not inhibit this enzyme, but probabaly is a competitive inhibitor of amino acid uptake, inhibited of glutamine. The inhibitory effect of serine-borate was not greater than that of serine alone. The uptake of glutamine was not affected by either GSH (reduced glutathione) or diamide (a thiol oxidant). Niether substances affected the uptake of leucine. The results indicate that the uptake of glutamine by rat pancreatic islets is not dependent on the functioning of γ-glutamyl transpeptidase and that thiols are not important for the uptake of the amino acids glutamine and leucine.     

Hydrolysis and transfer reactions catalyzed by gamma-glutamyl transpeptidase; evidence for separate substrate sites and for high affinity of L-cystine.

γ-Glutamyl transpeptidase was studied with L- and D-γ-glutamyl-p-nitroanilide as γ-glutamyl donors. No autotranspeptidation occurred with the D-γ-glutamyl donor or when the L-γ-glutamyl donor was used at concentrations lower than 10 μM. The K_m values for hydrolysis were 5 and 31 μM for the L- and D-γ-glutamyl donors, respectively; the corresponding V_max values were identical. The γ-glutamyl donor site of the enzyme thus exhibits low stereospecificity (but high affinity), while the acceptor site exhibits absolute L-specificity. The K_m value for L-cystine as acceptor is very low (30 μM); the same value was obtained with L- and D-γ-glutamyl donors. The data are consistent with a ping pong mechanism and the existence of separate donor and acceptor enzyme sites. The present findings thus extend the usefulness of γ-glutamyl-p-nitro-anilide as a substrate in probing the catalytic behavior of this enzyme.     

gamma-Glutamyl transpeptidase in Hydra littoralis.

$\text{Hydra}$$\text{littoralis}$ exhibits high γ-glutamyl transpeptidase activity, i.e., about 12% of the activity (determined with glutathione) of rat kidney. Histochemical studies show that the enzyme is located mainly in the gastric and sub-hypostome regions; the enzyme is also present in the tentacles and basal disc. These results and the presence of other enzymes of the γ-glutamyl cycle suggest that the cycle plays a role in the metabolism of glutathione in hydras and that γ-glutamyl transpeptidase may function in their digestive and absorptive processes and possibly also in the behavioral response to glutathione.     

Immobilization of γ-glutamyl transpeptidase,a membrane enzyme,in gel beads via liposome entrapment

Bovine kidney γ-glutamyl transpeptidase, a membrane enzyme, was immobilized in gel beads by application of the method of Wallstén et al. (Biochim. Biophys. Acta, 982, 47–52, 1989). The gel beads were equilibrated with a dispersion of the enzyme, phospholipids, and cholate and subsequently dialyzed against a buffer for reconstitution and immobilization of enzyme-bound liposomes in the pores of the beads. From the standpoints of the immobilized contents of protein and phospholipids and of the reactivity of γ-glutamyl transpeptidase, a dialysis buffer of Tris-HCl (pH 7.5), a phospholipid concentration of 45 mg/ml in the enzyme-phospholipid-cholate dispersion, and the use of Sepharose CL-6B as the support gel were found to be most appropriate for the immobilization of γ-glutamyl transpeptidase, γ-Glutamyl transpeptidase was activated and stabilized by reconstitution in liposomes. In operation with a packed bed reactor, liposome-bound γ-glutamyl transpeptidase immobilized in Sepharose CL-6B exhibited relatively stable and constant activity for 12 h. In addition, it was found that enzyme substrates were able to pass through the pores of the gel beads to interact with the enzyme present on the outer surface of the liposome membrane in the gel beads. These results thus indicated that a novel support made up of liposomes and Sepharose CL-6B would permit efficient immobilization of lipid-requiring and/or membrane enzymes.     

gamma-Glutamyl transpeptidase of human seminal fluid.

γ-Glutamyl transpeptidase, which is present in high levels in human seminal fluid plasma, was purified about 870-fold from this source. The enzyme is present in seminal fluid plasma in particulate form. Purification by a procedure involving treatment with bromelain gave a protein (apparent molecular weight, about 70,000), which exhibited catalytic properties characteristic of γ-glutamyl transpeptidase preparations isolated from rat kidney and other mammalian tissues. The physiological significance of seminal fluid γ-glutamyl transpeptidase and its potential clinical value are considered.     

ENZYMES OF THE γ-GLUTAMYL CYCLE IN THE CHOROID PLEXUS AND BRAIN

—The presence of enzymes of the γ-glutamyl cycle in the bovine and rabbit brain and choroid plexus is described. The activities of γ-glutamyl transpeptidase, γ-glutamyl cyclotransferase and γ-glutamyl-cysteine synthetase in the choroid plexus were found to be higher than in the brain. The activity of γ-glutamyl transpeptidase in the choroid plexus was many times higher than the activity of the other enzymes. Brain and choroid plexus γ-glutamyl transpeptidase were activated by Na⁺ and K⁺. Both brain and choroid plexus showed only a very limited capacity to metabolize [¹⁴C]5-oxoproline to ¹⁴CO₂.     

gamma-Glutamyl transpeptidase in Hydra.

γ-Glutamyl transpeptidase (EC 2.3.2.2) activity is described in the coelenterate, $\text{Hydra}$$\text{attenuata}$, using the substrate γ-glutamyl-p-nitroanilide. The properties of the γ-glutamyl donor required for binding to the transpeptidase were investigated by measuring the ability of GSH analogs to inhibit the release of p-nitroaniline. Whereas no binding was observed when the γ-glutamyl moiety was altered, analogs with substitution in the Cys residue were capable of binding to the enzyme. A specificity for the Gly residue was indicated because analogs containing Leu or Tyr in place of Gly exhibited decreased binding capacities for the hydra transpeptidase. A comparison of these data with those obtained using the same analogs in the GSH induced feeding response bioassay shows that γ-glutamyl transpeptidase activity and the GSH receptor for the hydra feeding response have different specificities.     

Identification of isopeptidase activity in the midgut of insects: Purification,properties and nutritional ecology of a Hofmannophila pseudospretella (Lepidoptera: Oecophoridae) larval enzyme

Abstract A γ-glutamyl transpeptidase (isopeptidase) has been purified 580-fold to homogeneity from the midgut of keratinophagous larvae of Hofmannophila pseudospretella. The enzyme is a single polypeptide of molecular mass 80 kDa. The enzyme was identified by its hydrolytic activity against the synthetic substrate, γ-glutamyl-AMC, its molecular mass and inhibition profile compared to other γ-glutamyl transpeptidases. The enzyme is low or absent from most other insect digestive systems apart from other keratinophagous lepidopteran larvae and predatory carabids. While isopeptide bonds are present in high levels of the proteins in the diet of keratinophages, their presence in the diet of predatory beetles has not been established.     

Isolation from bovine brain of a fraction containing capillaries and a fraction containing membrane fragments of the choroid plexus

Combined differential and density gradient centrifugation was used for the isolation of a capillary-rich fraction from the cerebral cortex and a brush border containing fraction from the bovine choroid plexus. The activities of γ-glutamyl transpeptidase and several other marker enzymes were monitored during the fractionation procedure. Electron microscopic examination showed a membrane-rich fraction in the choroid plexus high in γ-glutamyl transpeptidase and 5'-nucleotidase activities. From the brain cortex, a capillary-rich fraction was obtained which was high in γ-glutamyl transpeptidase and alkaline phosphatase activities. A histochemical examination showed γ-glutamyl transpeptidase activity localized in the capillary walls.     

Evidence for the gamma-glutamyl cycle in yeast.

Many previous studies have shown that yeast contains high concentrations of glutathione and enzymes needed for its synthesis. We report here that yeast also contains γ-glutamyl transpeptidase, γ-glutamyl cyclotransferase, dipeptidase, and 5-oxoprolinase activities, suggesting that the γ-glutamyl cycle may be operative in yeast. The presence of the cycle enzymes in yeast offers a simple free-cell system which can probably be adapted to studies on the function of this cycle.     

Stimulation of intralysosomal proteolysis by cysteinyl-glycine,a product of the action of γ-glutamyl transpeptidase on glutathione

The mechanism of the stimulatory effect of glutathione on proteolysis in mouse kidney lysosomes and a lack of an effect in lysomes from the liver was investigated. The stimulation in kidney lysosomes was inhibited by serine plus borate, a reversible inhibitor of γ-glutamyl transpeptidase. Treatment of mouse kidney lysosome suspensions with $\text{l}\text{-(αS,5S)-α-}\text{amino}\text{-3-}\text{chloro}\text{-4,5-}\text{dihydro}\text{-5-}\text{isoxazoleacetic}$ acid (acivicin), an irreversible inhibitor of the transpeptidase, also inhibited the effect of glutathione, but this inhibition was completely relieved by washing and addition of freshly prepated kidney membranes or purified γ-glutamyl transpeptidase to the incubation mixtures. Cysteinyl-glycine, a product of the action of γ-glutamyl transpeptidase, stimulated proteolysis in acivicin-inhibited kidney lysosome preparations similarly to glutathione, and cysteine had no effect at equivalent concentrations. Glutathione also stimulated proteolysis in liver lysosomes in the presence of washed kidney membranes or γ-glutamyl transpeptidase, but the effect was similar to that produced by equivalent concentrations of cysteine. These results suggest that the stimulatory effect of glutathione was mediated by the action of γ-glutamyl transpeptidase present in contaminating cell membrane fragments in the lysosome preparations, and that glutathione does not take part in intralysosomal proteolysis. However, the possibility that cysteinyl-glycine is a physiological intralysosomal disulfide reductant in kidney lysosomes has not been excluded.     

gamma-Glutamyl transpeptidase: relation to immunoglobulin A and free secretory component.

The experiments reported show that bovine γ-glutamyl transpeptidase can be separated from free secretory component. An ion-exchange Chromatographic procedure was developed to analyze the incubation mixtures of the enzyme with glutathione or S-(2-acetamido)-glutathione and glycylglycine. Using this system or the γ-glutamyl p-nitroanilide assay, no significant transpeptidase activity could be detected in the free secretory component-containing fractions of DEAE-cellulose chromatography. Gel filtration on Biogel A-5M showed that the bovine whey transpeptidase chromatographed in the void volume suggesting an aggregate of a minimum molecular weight of about 5 × 10⁶. The transpeptidase could be separated from all immunoglobulins in bovine whey and human colostrum by a combination of agarose gel filtration and immunoadsorption. Concentrated samples of human and sheep saliva showed normal amounts of secretory component, but no detectable γ-glutamyl transpeptidase activity. These experiments show that (1) the transpeptidase and secretory component are two different proteins, and (2) the transpeptidase is present in bovine and human milk as a high molecular weight aggregate which does not include any of the immunoglobulins.     

Localization of gamma-glutamyl transpeptidase in the retinal pigment epithelium and visual receptor cells.

The activities of γ-glutamyl transpeptidase and other enzymes of the γ-glutamyl cycle, a series of reactions that catalyzes the synthesis and utilization of glutathione, were studied in the rabbit retina. Histochemical studies demonstrated that γ-glutamyl transpeptidase is localized in the visual receptor cells and the retinal pigment epithelium. Rat and mouse retinas revealed similar localizations of transpeptidase. These findings are in accord with the view that γ-glutamyl transpeptidase is involved in the transport of amino acids between the retinal pigment epithelium and the avascular visual receptor cells.     

gamma-glutamyl transpeptidase and related enzyme activities inthe reproductive system of the male rat.

The γ-glutamyl transpeptidase activity of the epididymis is much higher than that of the several other organs of the reproductive system of the male rat. The epididymal caput has much more activity than the epididymal cauda. Relatively low activity was found in spermatozoa. The enzyme is present in the epididymal fluid in a particulate form suggesting that it originates from membranes of epididymal epithelial cells. The epididymal caput exhibits high γ-glutamylcysteine synthetase activity indicating an active γ-glutamyl ycle in this tissue, which plays an important role in transport phenomena.     

Isolation of anthglutin, an inhibitor of gamma-glutamyl transpeptidase from Penicillum oxalicum.

Anthglutin, a new inhibitor of γ-glutamyl transpeptidase, has been isolated from the cultured medium of Penicillium oxalicum and its structure established as l-γ-l-glutamyl-2-(2-carboxyphenyl)hydrazine. The isolation of anthglutin was achieved by ion-exchange chromatography. Anthglutin inhibited γ-glutamyl transpeptidase specifically and the kinetic analysis of the inhibition showed that anthglutin inhibited the enzyme competitively with regard to the glutamyl donor, γ-glutamyl-p-nitroanilide, and noncompetitively with regard to the glutamyl acceptor, glycylglycine. K₁ values were 5.7 μm for the hog kidney enzyme, 18.3 μm for the human kidney enzyme, 13.6 μm for the human liver soluble enzyme, and 10.2 μm for the bound enzyme. After oral administration of [¹⁴C]methionine and anthglutin to rats, no effect of anthglutin was observed on the absorption of methionine in the intestine.