The glutathione status of the rat liver

Fluorimetric determination of monobromobimane and o-phthalaldehyde adducts of gamma-glutamylcysteine and glutathione: application to assay of gamma-glutamylcysteinyl synthetase activity and glutathione concentration in liver

The reversed-phase HPLC separation of fluorescent o-phthalaldehyde (OPA) derivatives has been applied to the assay of hepatic gamma-glutamylcysteine and glutathione (GSH) levels and the enzymes producing these peptides. The method has been compared to the assay using monobromobimane (MB) as the derivatizing agent. The OPA method has the advantage of faster derivatization, the lack of need to adjust the pH, isocratic separation and selectivity for GSH and gamma-glutamylcysteine. The MB method requires pH adjustment following derivatization and gradient elution chromatography. MB is also non-selective, yielding fluorescent derivatives of all biological thiols and more interfering peaks on the chromatogram. MB-based analyses are also approximately sixty times more expensive per sample. MB yields fluorescent degradation products on exposure to light. OPA adducts are stable for up to ten days when stored at -20 degrees C. OPA detection is sensitive to 12.5 pmol in the sample, at a signal-to-noise ratio of 2.5. The two methods correlate well. Hepatic gamma-glutamylcysteine synthetase in the same liver preparation was found to be 4.85 +/- 0.47 nmol min-1 mg-1 protein by the OPA method and 4.42 +/- 0.52 nmol min-1 mg-1 protein by the MB method. GSH concentrations were found to be 90.4 +/- 6.5 nmol/mg protein for the OPA method and 92.5 +/- 3.4 for the MB method.

Glutathione measurement by high-performance liquid chromatography separation and fluorometric detection of the glutathione-orthophthalaldehyde adduct

Glutathione reacts with orthophthalaldehyde to form a stable, highly fluorescent tricyclic derivative which is easily separated and quantitated by high-performance liquid chromatography. Separation of the glutathione adduct is achieved by isocratic elution over a reverse-phase column with 7.5% methanol/92.5% 0.15 M sodium acetate, pH 7.00. The adduct is detected fluorometrically and quantitated by integration of peak area. Detection of 0.1 to 200 pmol glutathione produces a linear response and the recovery of reduced and oxidized glutathione from rat liver homogenate, bile, and plasma is quantitative. The chemical identity of the adduct was confirmed by mass spectrometry.

Biochemical parameters in various sections of bovine corpora lutea graviditatis during the course of pregnancy

The enzyme activities of glutathione peroxidase (GPO) with cumenehydroperoxide (cumene-OOH) and H2O2 as substrates, glutathione-S-transferase (GSH-S-T) with 1-chloro-2,4-dinitrobenzene (CDNB) as substrate, phosphofructokinase (PFK) and succinate dehydrogenase (SuDH) were determined for months 1 through 9 of pregnancy in the basal and peripheral sections of the corpora lutea graviditatis of Holstein-Frisean cows. The concentration of reduced glutathione (GSH) was simultaneously measured in these tissue sections. Substantial topographical differences were apparent in the enzyme activities. GPO and GSH-S-T showed activity differences during the course of pregnancy. During the 2nd month of pregnancy, minimal values for the activity of cytoplasmic GPO were observed in the basal areas. The cytoplasmic GPO in the peripheral areas displayed a contrasting dynamic with maximal values during the 6th month. GSH-S-T activities in basal and peripheral tissues appeared similar. GPO activities with H2O2 as substrate, likewise, displayed similar courses of activity in both tissue localizations. SuDH was more active in the peripheral than in the basal area. The activity of PFK displayed just the reverse course. The concentration of GSH in the peripheral area was not higher than in basal area.

Effect of isoproterenol administration of rat heart glutathione status

The intraperitoneal administration of 3, 10 and 80 mg/Kg isoproterenol produced in the cardiac muscle a dose dependent increase of GSH content and a slight elevation of GSSG content. In addition, the treatment with the catecholamine at the doses of 3 and 10 mg/Kg produced a slight decrease of the mixed glutathione disulfides level, whilst at the dose of 80 mg/Kg, this effect was more pronounced. These changes were not accompanied by modifications of the activities of the enzymes glutathione peroxidase, glutathione reductase and glutathione S-transferase.

Identification and quantitation of glutathione in hepatic protein mixed disulfides and its relationship to glutathione disulfide

The amount of glutathione present in hepatic protein mixed disulfides was determined to be 20-30 nmole/g liver. This was established using two specific enzymatic methods: (a) the coupled assay with DTNB and glutathione (GSSG) reductase and (b) a newly developed test using GSH transferase and 1-chloro-2,4-dinitrobenzene for the estimation of GSH released from proteins after borohydride treatment; further, these results were confirmed by HPLC analysis. Thus, authentic glutathione makes up only 2-6% of the value for total protein mixed disulfides. The latter were determined with the generally employed o-phthalaldehyde assay, which is not necessarily specific for GSH. The amount of glutathione mixed disulfides depends linearly on the content of glutathione disulfide in the liver cell in the range studied. By increasing the GSSG levels from 20 to about 60 nmole/g liver with paraquat, nitrofurantoin or t-butyl hydroperoxide, glutathione protein mixed disulfides are increased by a similar amount.

Evolutionary conservation and variation of protein folding pathways. Two protease inhibitor homologues from black mamba venom

The pathways of unfolding and refolding of three homologous proteins are shown to be closely related. This implies that folding pathways, as well as the final folded conformation, have been largely conserved during the presumed evolutionary divergence of these proteins from a common ancestor. The pathways of the homologous proteins I and K from black mamba venom were determined here, using the disulphide interaction between their six cysteine residues to trap and identify the intermediate states, and are compared with those determined previously in the same way for the homologous bovine pancreatic trypsin inhibitor. The major one- and two-disulphide intermediates are the same with all three proteins; their kinetic roles are similar, although there are differences in the rates at which they are interconverted and in the minor intermediates that accumulate. As a consequence, different pathways may predominate with another homologous protein, even though the various most favourable pathways are the same. The energetics of the folding transitions and the stabilities of the folded states differ substantially for the three proteins. The differences in stabilities of the fully folded states are primarily reflected kinetically in the rate-determining rearrangements of the native-like conformation; the rates and equilibria of the other steps are not affected markedly. With the less stable proteins, the direct folding pathway of sequential formation of the three correct disulphide bonds becomes significant and is the most facile when considered on a solely intramolecular basis.

Increased biliary secretion and loss of hepatic glutathione in rat liver after nifurtimox treatment

Treatment of rats with nifurtimox, a nitrofuran derivative widely used for the treatment of Chagas' disease, induced a time- and dose-dependent depletion of liver glutathione, maximal effects being obtained with 200 mg nifurtimox/kg body weight. Extra release of both oxidized (GSSG) and reduced (GSH) glutathione into bile contributed to this depletion. Glutathione excretion into bile accounted for only part of liver glutathione loss, thus indicating that, in addition to the GSH-peroxidase reaction (resulting in GSSG generation), other glutathione-related processes were involved in nifurtimox detoxification. Bile flow, bile salt excretion, liver lipid conjugated diene content, liver glutathione reductase and glutathione peroxidase activities, and serum alanine aminotransferase (ALAT) activity were not affected by the nifurtimox treatment, thus ruling out widespread damage of the liver cell by nifurtimox. Nevertheless, the extra GSH release in the nifurtimox-treated rats may indicate an alteration of the hepatocyte membrane.

Thiol-disulfide-dependent interconversion of active and latent forms of rat hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase

The activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (hydroxymethylglutaryl-CoA reductase, EC 1.1.1.34) in preparations of thiol-deficient rat liver microsomes and microsomes containing thiols have been compared. Unlike microsomes containing thiols, which possess an active hydroxymethylglutaryl-CoA reductase (Ea), thiol-deficient microsomes contain an inactive, latent enzyme (E1) which can be activated by addition of thiols. Ea can be converted to E1 by dialysis. The maximal degree of activation of E1 depends on the activating thiol with the order of effectiveness: dithioerythritol = dithiothreitol greater than glutathione (GSH) greater than cysteine. Ea is inhibited by oxidized glutathione (GSSG). The degree of the inhibition of Ea by GSSG is proportional to the ratio GSSG/thiol in the reaction. E1 was solubilized from microsomes and purified. Its molecular weight is estimated to be 104 000 by gel filtration chromatography on Sepharose 6B. The reducing agents NaBH4, dithionite and ascorbate failed to activate E1. NaBH4 did not inhibit Ea whereas only partial inhibition was caused by ascorbate and dithionite. Soluble Ea binds to both blue dextran/Sepharose 4B and agarose/hexane-3-hydroxy-3-methylglutaryl Coenzyme A affinity resins at low-salt concentrations. By contrast, soluble E1 did not bind to agarose/hexane-hydroxymethylglutaryl-CoA whereas quantitative binding of E1 to blue dextran/Sepharose 4B was still observed at low salt concentrations. These results indicate that thiols are necessary cofactors for hydroxymethylglutaryl-CoA reductase reaction. Their effect on the activation of E1 is not caused by change in the state of aggregation of the enzyme. Rather, the reversible change of the enzyme from E1 to Ea is affected by increasing the affinity of the enzyme to the substrate hydroxymethylglutaryl-CoA.

Increase in hepatic mixed disulphide and glutathione disulphide levels elicited by paraquat

Paraquat (1 mM), when added to isolated haemoglobin-free perfused rat liver, leads to an increase of intracellular mixed disulphides from 1.3 mumole GSH equivalents per g wet weight in the controls to 2.5 mumole/g. This raises the proportion of mixed disulphides to total glutathione equivalents from about 0.2 at the beginning of the perfusion to about 0.4. The mixed disulphides are predominantly protein-bound, with low molecular weight compounds being quantitatively negligible. The content of intracellular glutathione disulphide (GSSG) is increased from 17 nmole/g in the controls to 38 nmole/g in the presence of paraquat. In addition, there is an increased rate of release of GSSG into the extracellular (biliary) space, reported previously. It is suggested that, in a reaction catalysed by thioltransferase(s), the rise in GSSG is correlated with the rise in mixed disulphides (reaction 1). Occupancy of potential cellular mixed disulphide sites is about 1/2 in the controls, and rises to about 2/3 in the presence of paraquat. THe ratio of cellular contents, NADPH/NADP+, is decreased from 5.1 in the controls to 2.3 in the presence of paraquat, while the sum of NADPH plus NADP+ remains unaltered. The perturbation in the glutathione status may be related to metabolic effects such as the stimulation of the pentose-phosphate pathway activity, and possibly also to the expression of toxic effects.

The stimulus-secretion coupling of glucose-induced insulin release. Thiol: disulfide balance in pancreatic islets

An increase in the production rate of reduced pyridine nucleotides is currently considered as a coupling factor between metabolic and distal events in the process of glucose-stimulated insulin release. The possible participation in such a coupling of thiol: disulfide interchanges was investigated in rat pancreatic islets. NADPH-dependent glutathione reductase and glutathione-cystine transhydrogenase activities were present in islet homogenates, whereas no glutathione peroxidase activity could be detected. In intact islets, glucose (16.7 mM) augmented both the GSH/GSSG ratio (from a basal value of 6.7 +/- 0.6 to 8.4 +/- 0.4) and the tissue content of sulphydryl groups (from a basal value of 119 +/- 7 to 170 +/- 9 pmol/microgram protein). The latter effect was mimicked by D-glyceraldehyde, 2-ketoisocaproate, anoxia and KCN; it failed to be reproduced by L-glucose or D-fructose, was unaffected by theophylline, and was inhibited by D-mannoheptulose, iodoacetate, menadione, cytochalasin B and the absence of extracellular Ca2+. These findings support the view that a glucose-induced reduction of disulphide bridges to sulphydryl groups participates in the stimulus-secretion coupling of nutrient-induced insulin release.

Biotransformation of nitrosobenzene, phenylhydroxylamine, and aniline in the isolated perfused rat liver

1. Haemoglobin-free single-pass perfusion of isolated rat liver with [14C]aniline, [14C]phenylhydroxylamine, and [14C]nitrosobenzene was carried out. 2. Perfusion with aniline revealed apparent enzyme kinetics for 4-aminophenol formation with Km = 144 microM, Vmax = 51 nmol/min per g liver wet; for 2-aminophenol Km = 144 microM, Vmax = 16 nmol/min per g; for acetanilide Km = 33 microM, Vmax = 25 nmol/min per g. Formation of phenylhydroxylamine and nitrosobenzene was observed at a rate of 1.5 nmol/min per g provided that these metabolites had been trapped within red cells. 3. Perfusion with phenylhydroxylamine displayed a metabolic pattern similar to aniline with apparent phenylhydroxylamine reduction kinetics of Km = 260 microM and Vmax = 600 nmol/min per g. In addition an acid-labile phenylhydroxylamine glucuronide was formed. 4. Perfusion with nitrosobenzene showed very rapid reduction to phenylhydroxylamine and to the metabolites observed with phenylhydroxylamine. In postmicrosomal supernatant, enzymic reduction of nitrobenzene by NADH and NADPH showed Km = 12 microM nitrosobenzene and Vmax = 5000 nmol/min per g. 5. Three per cent of nitrosobenzene was irreversibly bound to liver proteins. After 20 min perfusion with nitrosobenzene, 0.95 mumol of liver glutathione was lost per 10 mumol nitrosobenzene infused; 0.16 mumol of glutathione was released with effusate and bile, 0.46 mumol of glutathionesulphinanilide was produced, the rest, 0.33 mumol, may have formed mixed disulphides.