Liquid chromatographic determination of acidic beta-aspartyl and gamma-glutamyl peptides in extracts of rat brain

Direct Monitoring of γ-Glutamyl Transpeptidase Activity In Vivo Using a Hyperpolarized (13) C-Labeled Molecular Probe

The γ-glutamyl transpeptidase (GGT) enzyme plays a central role in glutathione homeostasis. Direct detection of GGT activity could provide critical information for the diagnosis of several pathologies. We propose a new molecular probe, γ-Glu-[1-(13) C]Gly, for monitoring GGT activity in vivo by hyperpolarized (HP) (13) C magnetic resonance (MR). The properties of γ-Glu-[1-(13) C]Gly are suitable for in vivo HP (13) C metabolic analysis since the chemical shift between γ-Glu-[1-(13) C]Gly and its metabolic product, [1-(13) C]Gly, is large (4.3 ppm) and the T1 of both compounds is relatively long (30 s and 45 s, respectively, in H2 O at 9.4 T). We also demonstrate that γ-Glu-[1-(13) C]Gly is highly sensitive to in vivo modulation of GGT activity induced by the inhibitor acivicin.

Necessity of meningococcal gamma-glutamyl aminopeptidase for Neisseria meningitidis growth in rat cerebrospinal fluid (CSF) and CSF-like medium

The growth of a gamma-glutamyl aminopeptidase (GGT)-deficient Neisseria meningitidis strain was much slower than that of the parent strain in rat cerebrospinal fluid (CSF) and in a synthetic CSF-mimicking medium, and the growth failure was suppressed by the addition of cysteine. These results suggested that, in the environment of cysteine shortage, meningococcal GGT provided an advantage for meningococcal multiplication by supplying cysteine from environmental gamma-glutamyl-cysteinyl peptides.

Glutathione pathways in the brain

The antioxidant glutathione (GSH) is essential for the cellular detoxification of reactive oxygen species in brain cells. A compromised GSH system in the brain has been connected with the oxidative stress occuring in neurological diseases. Recent data demonstrate that besides intracellular functions GSH has also important extracellular functions in brain. In this respect astrocytes appear to play a key role in the GSH metabolism of the brain, since astroglial GSH export is essential for providing GSH precursors to neurons. Of the different brain cell types studied in vitro only astrocytes release substantial amounts of GSH. In addition, during oxidative stress astrocytes efficiently export glutathione disulfide (GSSG). The multidrug resistance protein 1 participates in both the export of GSH and GSSG from astrocytes. This review focuses on recent results on the export of GSH and GSSG from brain cells as well as on the functions of extracellular GSH in the brain. In addition, implications of disturbed GSH pathways in brain for neurodegenerative diseases will be discussed.

Assay of gamma-glutamylcysteine synthetase activity in Plasmodium berghei by liquid chromatography with electrochemical detection

This work describes a high-performance liquid chromatography (HPLC) method to determine gamma-glutamylcysteine (gamma-GC), the intermediate product of glutathione biosynthesis. Separation relies on isocratic reversed-phase chromatography using a Symmetry C18 HPLC column, particle size 5 microm, 4.6 x 250 mm i.d. The mobile phase is methanol-dibasic sodium phosphate (pH 6.6; 2.8 mM) (10:90, v/v) at the flow-rate of 0.5 ml/min and detection is operated electrochemically (+200 and +550 mV) with a pre-column derivatisation reaction using ortho-phthalaldehyde (OPA) as reagent. Under these conditions the calibration range of gamma-GC was 0.3-10 microg/ml; the limit of quantification was 0.3 microg/ml; accuracy, expressed as %Bias, was <10 and precision (%CV) was <6. The proposed HPLC assay was used to quantitate the gamma-glutamylcysteine produced by the gamma-glutamylcysteine synthetase of the rodent malaria parasite Plasmodium berghei in an in vitro enzymatic assay.

Metabolism and functions of glutathione in brain

The tripeptide glutathione is the thiol compound present in the highest concentration in cells of all organs. Glutathione has many physiological functions including its involvement in the defense against reactive oxygen species. The cells of the human brain consume about 20% of the oxygen utilized by the body but constitute only 2% of the body weight. Consequently, reactive oxygen species which are continuously generated during oxidative metabolism will be generated in high rates within the brain. Therefore, the detoxification of reactive oxygen species is an essential task within the brain and the involvement of the antioxidant glutathione in such processes is very important. The main focus of this review article will be recent results on glutathione metabolism of different brain cell types in culture. The glutathione content of brain cells depends strongly on the availability of precursors for glutathione. Different types of brain cells prefer different extracellular glutathione precursors. Glutathione is involved in the disposal of peroxides by brain cells and in the protection against reactive oxygen species. In coculture astroglial cells protect other neural cell types against the toxicity of various compounds. One mechanism for this interaction is the supply by astroglial cells of glutathione precursors to neighboring cells. Recent results confirm the prominent role of astrocytes in glutathione metabolism and the defense against reactive oxygen species in brain. These results also suggest an involvement of a compromised astroglial glutathione system in the oxidative stress reported for neurological disorders.

Net efflux of cysteine, glutathione and related metabolites from rat hippocampal slices during oxygen/glucose deprivation: dependence on gamma-glutamyl transpeptidase

Extracellular metabolism of the protective substance glutathione (gamma-glutamyl-cysteinyl-glycine) may generate cysteine, glycine, several gamma-glutamyl-containing dipeptides and possibly free glutamate, all of which could participate in neurotoxicity. In the present study, we have examined how blockage of gamma-glutamyl transpeptidase, the key enzyme in glutathione degradation, influences the extracellular concentrations of glutathione, cysteine and related metabolites during anoxia/aglycemia of rat hippocampal slices. The net efflux, i.e., the increase in extracellular concentration due to changes in release and/or uptake, of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate, gamma-glutamyl-glutamine, glutathione, gamma-glutamyl-cysteine and glutamate increased as a result of anoxia/aglycemia. These increases in net efflux of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate and gamma-glutamyl-glutamine were reduced or blocked by acivicin, an inhibitor of gamma-glutamyl transpeptidase. In contrast, acivicin caused an increase in both basal and anoxia/aglycemia-induced net efflux of glutathione whereas the basal and anoxia/aglycemia-induced efflux of glutamate was unchanged by acivicin treatment. The effect of acivicin on the efflux of gamma-glutamyl-cysteine was similar to that of glutathione although less pronounced. Addition of beta-mercaptoethanol to the incubation medium during and after 30 min of anoxia/aglycemia decreased the net efflux of cysteine sulfinate specifically, indicating that the increase in cysteine sulfinate during anoxia/aglycemia may be partly derived from the spontaneous oxidation of cysteine. The results suggest that gamma-glutamyl transpeptidase may be involved in the regulation of the extracellular concentrations of cysteine, several gamma-glutamyl-containing dipeptides and glutathione but not glutamate during ischemia.

Glycosylasparaginase-catalyzed synthesis and hydrolysis of beta-aspartyl peptides

beta-Aspartyl di- and tripeptides are common constituents of mammalian metabolism, but their formation and catabolism are not fully understood. In this study we provide evidence that glycosylasparaginase (aspartylglucosaminidase), an N-terminal nucleophile hydrolase involved in the hydrolysis of the N-glycosidic bond in glycoproteins, catalyzes the hydrolysis of beta-aspartyl peptides to form L-aspartic acid and amino acids or peptides. The enzyme also effectively catalyzes the synthesis of beta-aspartyl peptides by transferring the beta-aspartyl moiety from other beta-aspartyl peptides or beta-aspartylglycosylamine to a variety of amino acids and peptides. Furthermore, the enzyme can use L-asparagine as the beta-aspartyl donor in the formation of beta-aspartyl peptides. The data show that synthesis and degradation of beta-aspartyl peptides are new, significant functions of glycosylasparaginase and suggest that the enzyme could have an important role in the metabolism of beta-aspartyl peptides.

Comparison of anion-exchange and ion-modified reversed-phase liquid chromatography for the determination of S-sulfocysteine

A dual Hg-Au amalgam electrode is used to detect S-sulfocysteine (SSC) in this study. There exist two main components in the acetonitrile (ACN) rat brain extracts, namely, Cl- and GSSG (oxidized glutathione), that are active in our detection system (GSH is not extracted in ACN). Two strong anion-exchange columns from different companies were used to separate the samples under different conditions, but SSC and Cl- were not separated at the optimum detection pH of 5.2. The signal from Cl- was greatly decreased by lowering the potential at the downstream electrode, though it cannot be completely eliminated. While a silver cartridge removed Cl- from micromoles to several millimoles without any negative effect on the SSC signal in aqueous standards, a large negative peak which interferes with SSC detection was unfortunately introduced when a silver cartridge was applied to brain tissue samples. However, SSC and Cl- in the samples are successfully separated by ion-modified reversed-phase LC in acetate buffer at the optimum detection pH (5.2). The separation conditions are 20 mM acetic acid, 2% methanol, 0.5 mM cetyltrimethylammonium p-toluene sulfonate (CTMA) (pH 5.2). Most importantly, the sensitivity of SSC under the optimum separation conditions is not sacrificed. The detection limit is 8 nM (20 microl injected).

The gamma-glutamyl transpeptidase inhibitor acivicin preserves glutathione released by astroglial cells in culture

The release of glutathione from astroglial cells was investigated using astroglia-rich primary cultures prepared from the brains of newborn rats. These cells release glutathione after onset of an incubation in a glucose-containing minimal medium. The amount of extracellular glutathione increased with the time of incubation, although the accumulation slowed down gradually. An elevated rate of increase of the glutathione concentration in the incubation medium was found if the astroglial ectoenzyme gamma-glutamyl transpeptidase was inhibited by acivicin. The activity of gamma-glutamyl transpeptidase in astroglia-rich primary cultures, which was found to be 1.9 +/- 0.3 nmol/(min x mg protein), was markedly reduced if the cells had been incubated in the presence of acivicin. After 2 h of incubation with acivicin half-maximal and maximal inhibition of gamma-glutamyl transpeptidase activity was found at concentrations of about 5 microM and 50 microM, respectively. In the presence of acivicin at a concentration above 10 microM the glutathione content found released from astroglial cells apparently increased almost proportional to time for up to 10 h. Under these conditions the average rate of release was 2.1 +/- 0.3 nmol/(h x mg protein) yielding after a 10 h incubation an extracellular glutathione content three times that of the medium of cells incubated without inhibitor. Half-maximal and maximal effects on the level of extracellular glutathione were found at 4 microM and 50 microM acivicin, respectively. After a 10 h incubation with acivicin the intracellular content of glutathione was reduced to 75% of the level of untreated astroglial cultures. These results suggest that glutathione released from astroglial cells can serve as substrate for the ectoenzyme gamma-glutamyl transpeptidase of these cells.

Studies on the identity of the rat optic nerve transmitter

The possible role of glutamate, aspartate, sulfur-containing excitatory amino acids and gamma-glutamyl peptides as major transmitters in the rat optic nerve was evaluated. Four days following optic nerve lesion the K(+)-evoked Ca(2+)-dependent glutamate release was reduced to 31 +/- 16% (+/- S.D., n = 9) comparing release from slices of the denervated (contralateral to the lesion) and non-denervated (ipsilateral) superior colliculus, indicative of a major transmitter function for glutamate. However, significant decreases in glutamate release could not be detected seven days following the lesion (n = 5). Other studies have shown that optic nerve denervation induce formation of synapses of non-retinal origin and cause other cellular changes which may reduce the effect of deafferentation on glutamate release after 7 days. No significant change was observed in aspartate release following the lesion. The concentrations of cysteine sulfinate, cysteate, homocysteine sulfinate, homocysteate and O-sulfo-serine in the optic layers of the superior colliculus were below 1 nmol/g tissue (n = 6). Theoretical considerations indicate that this level is too low for a function of any of these as a major optic nerve transmitter. All postsynaptic components in the rat superior colliculus response, evoked by electrical optic nerve stimulation, were reduced by kynurenate (1-10 mM), a broad spectrum glutamate-receptor antagonist. The study gives further support for the view that glutamate is a major transmitter in the rat optic nerve.

The neurotransmitter candidature of sulphur-containing excitatory amino acids in the mammalian central nervous system

While L-glutamate (L-Glu) is considered to be the predominant excitatory amino acid transmitter in the mammalian CNS, other amino acids have come under scrutiny as possible rivals for such a role. These include four sulphur-containing analogues of L-Glu and L-aspartate known as the SAAs. The L-Glu analogues are L-homocysteic acid and L-homocysteine sulphinic acid, while the L-aspartate analogues are L-cysteic acid and L-cysteine sulphinic acid. They are mixed agonists of excitatory amino acid receptors on a variety of neurones and are reported to be present in and released from mammalian CNS tissue. This review serves to summarize the current state of research into the possibility that one or more of these compounds is indeed a transmitter within the mammalian CNS.

Site-specific N-terminal auto-degradation of human serum albumin

Human serum albumin prepared by blood fractionation for clinical purposes was found to degrade when stored at or above 30 degree C. Mass spectrometry and N-terminal sequencing of the protein identified degradation corresponding to the loss of the first two residues, aspartic acid and alanine. The reaction was shown to be dependent upon temperature and the N-terminal alpha-amino group. In addition, comparison with serum albumins derived from other species showed that the instability of the N-terminus was specific to the human albumin sequence. An intact aspartyl-alanyl dipeptide, purified from degraded albumin solutions, differed substantially from a synthetic dipeptide on amino acid analysis, N-terminal sequencing and NMR. It is suggested that the released dipeptide may be cyclic, implying a novel cleavage mechanism.