How many distinct enzymes are responsible for the several cellular processes involving thiol:protein-disulphide interchange?

Mixed results with mixed disulfides

A period of research with Helmut Sies in the 1980s is recalled. Our experiments aimed at an in-depth understanding of metabolic changes due to oxidative challenges under near-physiological conditions, i.e. perfused organs. A major focus were alterations of the glutathione and the NADPH/NADP(+) system by different kinds of oxidants, in particular formation of glutathione mixed disulfides with proteins. To analyze mixed disulfides, a test was adapted which is widely used until today. The observations in perfused rat livers let us believe that glutathione-6-phosphate dehydrogenase (G6PDH), i.a. might be activated by glutathionylation. Although we did not succeed to verify this hypothesis for the special case of G6PDH, the regulation of enzyme/protein activities by glutathionylation today is an accepted posttranslational mechanism in redox biology in general. Our early experimental approaches are discussed in the context of present knowledge.

Novel thiosemicarbazide–oxadiazole hybrids as unprecedented inhibitors of yeast α-glucosidase and in silico binding analysis

Novel hybrids of thiosemicarbazide–oxadiazole as potent α-glucosidase agents.

Oxyhalogen–Sulfur Chemistry: Kinetics and Mechanism of Oxidation of N-Acetyl-ʟ-methionine by Aqueous Iodine and Acidified Iodate

The use of N-acetyl-l-methionine (NAM) as a bio-available source for methionine supplementation as well as its ability to reduce the toxicity of acetaminophen poisoning has been reported. Its interaction with the complex physiological matrix, however, has not been thoroughly investigated. This manuscript reports on the kinetics and mechanism of oxidation of NAM by acidic iodate and aqueous iodine. Oxidation of NAM proceeds by a two electron transfer process resulting in formation of a sole product: N-acetyl-l-methionine sulfoxide (NAMS=O). Data from electrospray ionization mass spectrometry confirmed the product of oxidation as NAMS=O. The stoichiometry of the reaction was deduced to be IO3– + 3NAM → I– + 3NAMS=O. In excess iodate, the stoichiometry was deduced to be 2IO3– + 5NAM + 2H+ → I2 + 5NAMS=O + H2O. The reaction between aqueous iodine and NAM gave a 1 : 1 stoichiometric ratio: NAM + I2 + H2O → NAMS=O + 2I– + H+. This reaction was relatively rapid when compared with that between NAM and iodate. It did, however, exhibit some auto-inhibitory effects through the formation of triiodide (I3–) which is a relatively inert electrophile when compared with aqueous iodine. A simple mechanism containing 11 reactions gave a reasonably good fit to the experimental data.

The chemistry and biological activities of N-acetylcysteine

BACKGROUND

N-acetylcysteine (NAC) has been in clinical practice for several decades. It has been used as a mucolytic agent and for the treatment of numerous disorders including paracetamol intoxication, doxorubicin cardiotoxicity, ischemia-reperfusion cardiac injury, acute respiratory distress syndrome, bronchitis, chemotherapy-induced toxicity, HIV/AIDS, heavy metal toxicity and psychiatric disorders.

SCOPE OF REVIEW

The mechanisms underlying the therapeutic and clinical applications of NAC are complex and still unclear. The present review is focused on the chemistry of NAC and its interactions and functions at the organ, tissue and cellular levels in an attempt to bridge the gap between its recognized biological activities and chemistry.

MAJOR CONCLUSIONS

The antioxidative activity of NAC as of other thiols can be attributed to its fast reactions with OH, NO2, CO3(-) and thiyl radicals as well as to restitution of impaired targets in vital cellular components. NAC reacts relatively slowly with superoxide, hydrogen-peroxide and peroxynitrite, which cast some doubt on the importance of these reactions under physiological conditions. The uniqueness of NAC is most probably due to efficient reduction of disulfide bonds in proteins thus altering their structures and disrupting their ligand bonding, competition with larger reducing molecules in sterically less accessible spaces, and serving as a precursor of cysteine for GSH synthesis.

GENERAL SIGNIFICANCE

The outlined reactions only partially explain the diverse biological effects of NAC, and further studies are required for determining its ability to cross the cell membrane and the blood-brain barrier as well as elucidating its reactions with components of cell signaling pathways.

Studies on the nature and regulation of the cellular thio:disulphide potential

Microsomal fractions separated from homogenates of liver, kidney and corpora lutea contain a monooxygenase (dimethylaniline monooxygenase [N-oxide forming], EC 1.14.13.8) that catalyses NADPH- and oxygen-dependent oxidation of cysteamine to cystamine. The monooxygenase purified to homogeneity from hog liver also catalyses oxygenations of diverse xenobiotics, but it does not catalyse oxidation of any other physiological sulphur- or nitrogen-containing compounds. All the available evidence indicates that cysteamine is the physiological substrate for the monooxygenase, and the oxidation of this thiol to the disulphide may be a significant source of disulphide maintaining the cellular thiol:disulphide potential. The concentration of protein-low molecular weight mixed disulphide is a function of this potential. Changes in concentration of this protein-mixed disulphide reflect changes in thiol:disulphide balance. At constant substrate concentrations the potential would depend primarily on activity of the cytosol glutathione reductase (NAD(P)H: oxidized-glutathione oxidoreductase, EC 1.6.4.2) relative to that of the membrane-bound monooxygenase. In hepatic tissue from adult mice and hamsters there is a correlation between the concentration of protein-mixed disulphide and the activity of the monooxygenase relative to the reductase. Hepatic glutathione reductase is relatively constant in mice, but the monooxygenase is much higher in the female than in the male. After gonadectomy monooxygenase activity decreases in the female and increases in the male. Activities are restored to control levels by treating males with testosterone and females with progesterone. Testosterone decreases and progesterone increases activity. These two hormones apparently regulate the level of this enzyme in hepatic tissue.

Pathways of assimilatory sulphate reduction in plants and microorganisms

Assimilatory sulphate reduction, largely restricted to plants and microorganisms where it provides reduced sulphur for the formation of amino acids and proteins, nucleic acids, and various sulphur-containing coenzymes, begins with the activation of sulphate through reaction with ATP to form adenosine 5'-phosphosulphate (APS) and adenosine 3'-phosphate 5'-phosphosulphate (PAPS). Two pathways of assimilatory sulphate reduction are known. One, found in some blue-green algae (cyanobacteria) and in all oxygen-envolving eukaryotes, begins with APS where the sulpho group is transferred via APS sulphotransferase to a thiol acceptor (glutathione (G-S-) in Chlorella) to form the organic thiosulphate (G-S-SO-3). The organic thiosulphate appears to be reduced further by an organic thiosulphate reductase employing reduced ferredoxin to form G-S-S-. The terminal sulphur is then thought to be reductively transferred to O-acetylserine via O-acetylserine sulphydrase to form cysteine. A second pathway, found in bacteria and fungi, begins with PAPS where the sulpho group is transferred via PAPS sulphotransferase to an acceptor thiol to form an organic thiosulphate. Since thioredoxin is indispensable, this molecule may be the carrier or may serve to reduce the carrier. NADPH via thioredoxin reductase or glutathione and glutathione reductase reduces thioredoxin. These reactions release sulphite which is further reduced to sulphide by sulphite reductase, employing NADPH. Sulphide is then thought to react with O-acetylserine to form cysteine via O-acetylserine sulphydrase. The cellular location and evolution of these pathways is discussed.

Sulphydryl oxidase: oxidation of sulphydryl groups and the formation of three-dimensional structure in proteins

Sulphydryl oxidase, an enzyme isolated from milk, catalyses the de novo synthesis of disulphide bonds. Thiol groups in amino acids or their derivatives, peptides, and proteins are oxidized; molecular oxygen serves as the electron acceptor and undergoes a two-electron reduction to hydrogen peroxide. Michaelis constants vary considerably amongst various substrates; glutathione is a particularly good substrate. Inhibition studies and oxidation of 1,3-diphenylisobenzofuran suggest a mechanism involving an electron transfer to singlet O2 forming an enzyme-bound hydroperoxy group. Evidence for a direct interaction of the enzyme with horseradish peroxidase was also obtained. Although protein-folding appears to be thermodynamically favoured, rates of spontaneous acquisition of functional three-dimensional structures in disulphide-containing proteins have appeared disturbingly slow. In the presence of sulphydryl oxidase, functional structure is rapidly acquired by both reductively unfolded ribonuclease A and reductively denatured immobilized chymotrypsinogen A as judged by restoration of native fluorescence characteristics and biological activity. Preliminary data suggest that unlike thiol:protein-disulphide oxidoreductase, protein-disulphide isomerase, or GSSG/GSH redox systems, sulphydryl oxidase does not permit a 'reshuffling' of disulphide bonds.

Thioltransferases

A family of small molecular weight proteins with thiol-disulfide exchange activity have been discovered, widely distributed from E. coli to mammalian systems, called thioltransferases or glutaredoxins. There are no substantiated reports of thioltransferases-glutaredoxins in plants; however, partially purified dehydroascorbate reductase from peas had thiol-disulfide exchange catalytic activity using glutathione as reductant and S-sulfocysteine as thiosulfate cosubstrate (unpublished data). Thus, this class of proteins is universally distributed. Based on mutagenesis studies, a sequence of Cys-Pro-Tyr(Phe)-Cys- followed by Arg-Lys- or Lys alone is critical for both the thiol-disulfide exchange reaction and the dehydroascorbate reductase activity. The dithiol-disulfide loop represented by this structure is unique since the cystine closer to the N-terminus has a highly acidic thiol pKa (3.8 as determined for the pig liver enzyme) that contributes to the protein's high S- nucleophilicity. Compared with the microbial enzyme, the mammalian thioltransferases (glutaredoxins) are extended at both N and C termini by 10-12 amino acid residues, including a second pair of cysteines toward the C-terminus with no known special function. Yeast thioltransferase is more like mammalian enzymes in length (106 amino acids) but more like E. coli glutaredoxin in being unblocked at the N-terminus and having only one set of cysteines; that is, at the active center. The three mammalian enzymes, for which sequences are available, are blocked at the N-terminus by an acetyl group linked to alanine with no known special function other than possibly to impart greater cellular turnover stability. A report of carbohydrate (8.6%) content in rat liver thioltransferase has not been verified by more sensitive methods of carbohydrate analysis, nor has carbohydrate been identified in samples of purified glutaredoxin from any source. Thiol transferase and glutaredoxin are two names for the same protein based on similarity of amino acid sequence, immunochemical cross-reactivity, and other enzyme properties. The inability of thioltransferase from some mammalian sources to act as an electron carrier in ribonucleotide reductase systems, whether homologous or heterologous in origin, remains to be explained in future studies.

Oxyhalogen-sulfur chemistry — Kinetics and mechanism of the oxidation of cysteamine by acidic iodate and iodine

The oxidation of cysteamine by iodate and aqueous iodine has been studied in neutral to mildly acidic conditions. The reaction is relatively slow and is heavily dependent on acid concentration. The reaction dynamics are complex and display clock behavior, transient iodine production, and even oligooscillatory production of iodine, depending upon initial conditions. The oxidation product was the cysteamine dimer (cystamine), with no further oxidation observed past this product. The stoichiometry of the reaction was deduced to be IO3–+ 6H2NCH2CH2SH → I–+ 3H2NCH2CH2S-SCH2CH2NH2+ 3H2O in excess cysteamine conditions, whereas in excess iodate the stoichiometry of the reaction is 2IO3–+ 10H2NCH2CH2SH → I2+ 5H2NCH2CH2S-SCH2CH2NH2+ 6H2O. The stoichiometry of the oxidation of cysteamine by aqueous iodine was deduced to be I2+ 2H2NCH2CH2SH → 2I–+ H2NCH2CH2S-SCH2CH2NH2+ 2H+. The bimolecular rate constant for the oxidation of cysteamine by iodine was experimentally evaluated as 2.7 (mol L–1)–1s–1. The whole reaction scheme was satisfactorily modeled by a network of 14 elementary reactions.Key words: cysteamine, cystamine, Dushman reaction, oligooscillations.

Nitric oxide exposure and sulfhydryl modulation alter L-arginine transport in cultured pulmonary artery endothelial cells

The effect of nitric oxide (NO) exposure and sulfhydryl-reactive chemicals on L-arginine transport in pulmonary artery endothelial cells was evaluated. Exposure of pulmonary artery endothelial cells to 7.5 ppm (0.4 microM) NO for 4 h resulted in a significant (p < 0.05) reduction of Na(+)-dependent but not Na(+)-independent L-arginine transport. More prolonged exposure for 12-24 h reduced both Na(+)-dependent and Na(+)-independent transport of L-arginine with maximal loss of transport after 18 h of exposure (p < 0.02 for both). Similarly, incubation of cells in the presence of 50-200 microM S-nitroso-acetyl-penicillamine (SNAP) (but not 500 microM each of nitrate or nitrite) for 2 h also reduced both the Na(+)-dependent and Na(+)-independent transport of L-arginine (p < 0.05 for all concentrations). The SNAP-induced reduction of L-arginine transport was blocked by the NO scavenger oxyhemoglobin. When cell monolayers were exposed to varying concentrations of the sulfhydryl reactive chemicals N-ethylmaleimide (NEM) and acrolein, a dose-dependent reduction of L-arginine transport by both Na(+)-dependent and Na(+)-independent processes was observed. Na(+)-dependent L-arginine transport was more susceptible to inhibition by exposure to NO and to sulfhydryl reactive chemicals. Incubation of cells with 0.5 mM of the thiol-containing agent N-acetyl-L-cysteine prior to and during NEM or acrolein exposure blocked NEM and acrolein-induced reduction of L-arginine transport by both Na(+)-dependent and Na(+)-independent processes. Similarly, NO-induced reductions of Na(+)-dependent and Na(+)-independent L-arginine transport were reversed to control levels 24 h after termination of NO exposure. Treatment with the disulfide reducing agent dithiothreitol after exposure to NO resulted in partial reversal of the decreases in L-arginine transport. These results demonstrate that exposure to exogenous NO is responsible for reversible reductions of plasma membrane-dependent L-arginine transport mediated by both the Na(+)-dependent (system Bo,+) and the Na(+)-independent (system y+) transport processes. Modulation of the sulfhydryl status of plasma membrane proteins involved in L-arginine transport, such as L-arginine transporters and/or Na+/K(+)-ATPase, may be responsible, at least in part, for reductions in overall L-arginine transport in pulmonary artery endothelial cells.

Thiol dependent oxidation of enzymes: the last chance against oxidative stress

1. A survey of known effects of oxidized thiols on enzyme activity reveals a potential concerted action on metabolic pathways determining an impairment of anabolic reduction processes and an activation of the oxidative arm of the hexose monophosphate shunt. Thus it appears that, following oxidative stress, the increase of disulphides may act in restoring a reduced state in the cell by specifically channelling the metabolic energy flux.

Lymphocyte proliferation in glutathione-depleted lymphocytes: direct relationship between glutathione availability and the proliferative response

Lymphocyte proliferation in response to mitogenic lectins is directly dependent upon glutathione (GSH) availability. Thus, proliferation can be enhanced by providing lymphocytes with excess glutathione, and strongly inhibited by limiting the quantity of intracellular GSH available during the mitogenic stimulation. Exogenous GSH, cysteine and 2-mercaptoethanol (2-ME) can all significantly enhance lymphocyte proliferation and augment intracellular GSH levels. Lymphocytes depleted of GSH by buthionine sulfoximine (BSO) fail to undergo a full blast transformation response to mitogenic lectins. In lymphocytes stimulated with mitogen in the presence of BSO, the time profile of intracellular GSH levels shows a rapid decline over the first 24-48 h and a subsequent gradual decline to levels less than 0.5 nmol/10(7) lymphocytes by 72-96 h. Exogenous GSH partially sustains intracellular GSH levels and completely restores lymphocyte proliferation even in the presence of 2000 microM BSO. Other thiols, such as cysteine and 2-ME, do not significantly alter the time profile of intracellular GSH in mitogen-stimulated lymphocytes in the presence of 2000 microM BSO, and their capacity to enhance proliferation is greatly diminished albeit not completely abolished under these conditions. Ongoing GSH synthesis is clearly essential to maintain a normal proliferative response. If intracellular GSH levels are depleted initially and lymphocytes are then stimulated with mitogen in the presence of BSO, there is a diminished capacity of cysteine and 2-ME to restore proliferation relative to exogenous GSH. There is also a diminished capacity of exogenous GSH to restore proliferation with higher concentrations of BSO. This suggests that the restoration of lymphocyte proliferation by exogenous GSH is more closely linked to effects on intracellular rather than extracellular GSH. These studies confirm the importance of intracellular GSH in lymphocyte proliferation. The essential role for intracellular GSH can be demonstrated even in the presence of other exogenous thiols, such as cysteine and 2-ME. The enhancement of lymphocyte proliferation by exogenous cysteine appears to be directly linked to effects on intracellular GSH, whereas the enhancement by 2-ME is probably more complex but clearly linked to effects on intracellular GSH.

A naturally occurring molecular form of human plasma cholinesterase is an albumin conjugate

Human plasma cholinesterase (acylcholine acylhydrolase, EC 3.1.1.8) consists of four main molecular forms designated as C1, C2, C3 and C4 according to their electrophoretic mobility on gels. The major component, C4, is the tetrameric form; C1 and C3 are the monomeric and dimeric forms, respectively. The C2 form, which has an apparent free electrophoretic mobility higher than that of the three size isomers, and, moreover, a higher isoelectric point, was found to be a covalent conjugate between the cholinesterase monomer and serum albumin. This result is supported by the following arguments: the non-catalytic subunit of C2 was found to be a carbohydrate-free protein of apparent molecular mass 65 kDa that could not be labelled by diisopropylfluorophosphonate in the labelling conditions of esterases. It possesses a high affinity for a long-chain aliphatic ligand (a substituted octadecylamine) and for Cibacron blue F3 GA, and could be adsorbed on an immunoadsorbent for albumin. The two subunits of C2 are disulfide bridge linked; the active center of the cholinesterase subunit is partly masked by the albumin molecule. The conjugation reaction very likely occurs in the hepatic cell and not in plasma.

The mechanisms of reduction of protein mixed disulfides (dethiolation) in cardiac tissue

Dethiolation of proteins (reduction of protein mixed disulfides) by NADPH-dependent and glutathione (GSH)-dependent enzymes, and by nonenzymatic reaction with GSH, was studied by electrofocusing methodology with glycogen phosphorylase b and creatine kinase as substrates. Phosphorylase b was not rapidly dethiolated by reduced glutathione alone, but a cardiac extract catalyzed rapid dethiolation by both an NADPH-dependent and a GSH-dependent process. In contrast, creatine kinase was actively dethiolated by GSH. This GSH-dependent dethiolation was not enhanced by a soluble extract of bovine heart. Creatine kinase was also not dethiolated by an NADPH-dependent process. Partial purification of the phosphorylase dethiolases showed that the NADPH-dependent dethiolase had both a high-molecular-weight and a low-molecular-weight component The properties of these components were similar to those of thioredoxin and thioredoxin reductase. These two components were sensitive to inhibition by phenylarsine oxide and inhibition was reversed by addition of a dithiol. In contrast, GSH-dependent dethiolation required a single component of low molecular weight. This process was less sensitive to phenylarsine oxide inhibition. These studies show that two cytosolic proteins, phosphorylase b and creatine kinase, were dethiolated by different mechanisms. Phosphorylase b was dethiolated by both NADPH-dependent and GSH-dependent enzymes found in a soluble extract of bovine heart. In contrast, creatine kinase was rapidly dethiolated nonenzymatically by GSH alone.

Specific S-thiolation of a 30-kDa cytosolic protein from rat liver under oxidative stress

Thin-gel isoelectric focusing (IEF) is a simple and sensitive method of quantifying S-thiolation of individual proteins (protein mixed-disulfide formation). IEF of rat liver cytosol identified one major protein (pI 7.0) which underwent S-thiolation with glutathione disulfide to produce two acidic bands with pIs 6.4 and 6.1. The S-thiolated forms of the protein were purified by preparative isoelectric focusing. An apparent molecular mass of 30 kDa was determined by SDS/polyacrylamide gel electrophoresis. The 30-kDa protein amounted to 7 +/- 2% of the total cytosolic protein on IEF. The most abundant soluble protein of freshly isolated hepatocytes, with an identical isoelectric point to the liver 30-kDa protein, was modified in a similar manner in response to oxidative stress induced by model compounds. Addition of 50 microM tert-butyl hydroperoxide, 50 microM diamide [1,1-azobis(N,N'-dimethylformamide)] or 20 microM menadione (2-methyl-1,4-naphthoquinone) initiated the S-thiolation within less than 2 min in the hepatocytes. These compounds, at the concentrations employed, did not result in cell death. Menadione produced slowly progressive S-thiolation of the protein, while tert-butyl hydroperoxide or diamide produced rapid S-thiolation that decreased quickly after 2 min.

Inactivation of yeast hexokinase by 2-aminothiophenol. Evidence for a 'half-of-the-sites' mechanism

Yeast hexokinase is a homodimer consisting of two identical subunits. Yeast hexokinase was inactivated by 2-aminothiophenol at 25 degrees C (pH 9.1). The reaction followed pseudo-first-order kinetics until about 70% of the phosphotransferase activity was lost. About 0.65 mol of 2-aminothiophenol/mol of hexokinase was found to be bound after the 70% loss of the enzyme activity. Completely inactivated hexokinase showed a stoichiometry of about 1 mol of 2-aminothiophenol bound/mol of the enzyme. The evidence obtained from kinetic experiments, stoichiometry of the inactivation reaction and fluorescence emission measurements suggested site-site interaction (weak negative co-operativity) during the inactivation reaction. The approximate rate constants for the reversible binding of 2-aminothiophenol to the first subunit (KI) and for the rate of covalent bond formation with only one site occupied (k3) were 150 microM and 0.046 min-1 respectively. The inactivation reaction was pH-dependent. Dithiothreitol, 2-mercaptoethanol and cysteine restored the phosphotransferase activity of the hexokinase after inactivation by 2-aminothiophenol. Sugar substrates protected the enzyme from inactivation more than did the nucleotides. Thus it is concluded that the inactivation of the hexokinase by 2-aminothiophenol was a consequence of a covalent disulphide bond formation between the aminothiol and thiol function at or near the active site of the enzyme. Hexokinase that had been completely inactivated by 2-aminothiophenol reacted with o-phthalaldehyde. Fluorescence emission intensity of the incubation mixture containing 2-aminothiophenol-modified hexokinase and o-phthalaldehyde was one-half of that obtained from an incubation mixture containing hexokinase and o-phthalaldehyde under similar experimental conditions. The intensity and position of the fluorescence emission maximum of the 2-aminothiophenol-modified hexokinase were different from those of the native enzyme, indicating conformational change following modification. Whereas aliphatic aminothiols were completely ineffective, aromatic aminothiols were good inhibitors of the hexokinase. Cyclohexyl mercaptan weakly inhibited the enzyme. Inhibition of the hexokinase by heteroaromatic thiols was dependent on the nature of the heterocyclic ring and position of the thiol-thione equilibrium. The inhibitory function of a thiol is associated with the following structural characteristics: (a) the presence of an aromatic ring, (b) the presence of a free thiol function and (c) the presence of a free amino function in the close proximity of the thiol function.(ABSTRACT TRUNCATED AT 400 WORDS)

Tissue distribution and molecular heterogeneity of bovine thiol:protein-disulphide oxidoreductase (disulphide interchange enzyme)

1. The distribution of thiol:protein-disulphide oxidoreductase (disulphide interchange enzyme) in 17 bovine tissue extracts was determined by rocket immunoelectrophoresis and by measuring the reductive cleavage of insulin. 2. The relative concentration (per mg total protein) was found to be in the order: Pancreas greater than liver greater than lymph node greater than testes, fat tissue greater than parotid gland, brain, spleen, lung greater than small intestine, spinal cord, large intestine, kidney greater than paunch, aorta greater than skeletal muscle greater than heart. 3. The distribution of specific activity showed a similar pattern, irrespectively of whether glutathione or L-cysteine was used as cosubstrate. 4. The concentration varied 200-fold and the specific activity 400-fold between pancreas and heart muscle, respectively. 5. Crossed immunoelectrophoresis demonstrated that a fast-migrating form of the enzyme was the only one present in almost all tissues, but 15% of the enzyme in liver was a slow-migrating form and 50% in heart muscle a medium-migrating form. 6. The lung contains a species having partial immunological identity to the enzyme. 7. Purified enzyme from bovine liver has a somewhat lower mobility than the fast-migrating form in extract. 8. The results seem to support the general view that the enzyme is involved in synthesis of disulphide-bonded extracellular proteins, although the presence of the enzyme in tissues like fat, brain, spinal cord, skeletal muscle and heart indicates other cellular functions as well.

Hormones, glutathione status and protein S-thiolation

The formation of mixed disulfides between proteins and glutathione has been discussed as a potentially interesting metabolic signal. The S-thiolation of proteins with glutathione has been observed in several systems in vitro. We have correlated the increase in glutathione disulfide (GSSG) with the amount of protein mixed disulfides. The methodological aspects are briefly presented; normal values for protSSG are about 20-30 nmol per g wet weight of liver. Several processes have been related to changes in the thiol redox state. The stimulation of flux through the pentose phosphate pathway during the metabolism of t-butyl hydroperoxide is presented, and the increase in cellular activity of glucose-6-phosphate dehydrogenase is correlated with the increase in the level of protSSG. Hormonal stimulation of GSH efflux from the liver by vasopressin or by alpha-adrenergic agonists such as phenylephrine or epinephrine is presented and discussed in relation to physiological states of peripheral (non hepatic) GSH utilization. Preliminary work relates the release of GSH to the perturbations in thiol redox state in inflammation and in exercise.

tert-Butylhydroperoxide-induced toxicity in isolated hepatocytes: contribution of thiol oxidation and lipid peroxidation

Incubation of isolated rat hepatocytes with tert-butylhydroperoxide resulted in marked cytotoxicity preceded by intracellular glutathione depletion and extensive lipid peroxidation. Addition of antioxidants delayed, but did not prevent, this toxicity. A significant decrease in protein-free sulfhydryl groups also occurred in the presence of tert-butylhydroperoxide; direct oxidation of protein thiols and mixed disulfide formation with glutathione were responsible for this decrease. The involvement of protein thiol depletion in tert-butylhydroperoxide-induced cytotoxicity is suggested by our observation that administration of dithiothreitol, which caused re-reduction of the oxidized sulfhydryl groups and mixed disulfides, efficiently protected the cells from toxicity. Moreover, depletion of intracellular glutathione by pretreatment of the hepatocytes with diethyl maleate accelerated and enhanced the depletion of protein thiols induced by tert-butylhydroperoxide and potentiated cell toxicity even in the absence of lipid peroxidation.

Studies on the formation of intrachain disulphide bonds in newly biosynthesised bovine prolactin. Role of protein-disulphide isomerase

Native disulphide-bonded prolactin (band III) was distinguished from reduced prolactin (band II) and intermediate unstable disulphide-linked conformations by: (a) faster mobility of the former in sodium dodecyl sulphate/polyacrylamide gel electrophoresis, and (b) high-pressure liquid chromatography analyses of tryptic-digested peptides derived from prolactin in various conformations during its refolding pathway from reduced, unfolded to native conformation. The electrophoretic separation has been used to examine the state of disulphide bonding in newly synthesised prolactin translated from bovine pituitary mRNA in a rabbit reticulocyte translation system supplemented with nuclease-treated dog pancreatic microsomal membranes. The formation of correct disulphide pairing in prolactin (band III), synthesised in the in vitro translation system in the presence of pancreatic microsomes, required the presence of a thiol oxidant such as oxidised glutathione during the translation. The action of thiol oxidants on the in vitro biosynthesised and microsomally processed prolactin were both dose-dependent and catalytic; non-thiol oxidants such as NAD+ and NADP+ were ineffective. Examination of the time course of addition of oxidised glutathione to translating lysates showed that efficient and correct disulphide pairing in newly biosynthesised prolactin occurred when the oxidant was present co-translationally, but much lower yields of correctly disulphide-bonded prolactin were obtained when the oxidant was added after translation and processing were complete. The presence of protein-disulphide isomerase in dog pancreatic microsomes, employed in the in vitro translation system to process preprolactin, was demonstrated by (a) two-dimensional polyacrylamide gel electrophoresis of the membrane proteins, and (b) enzymic activity to accelerate reactivation of scrambled ribonuclease. Protein-disulphide isomerase activity was latent in intact microsomal vesicles, full activity being expressed upon sonication. A procedure has been devised to prepare pancreatic microsomal vesicles depleted of protein-disulphide isomerase which are active in processing and segregating in vitro biosynthesised prolactin. These membranes in the presence of low concentrations of oxidised glutathione are less active but in the presence of saturating levels of oxidised glutathione are fully competent in forming correct disulphide bridges in newly synthesised prolactin.

Comparative reduction/oxidation studies with single chain des-(B30) insulin and porcine proinsulin

The single chain des-(B30) insulin molecule (SCI) has been reduced and reoxidized together with porcine proinsulin (PPI). Yields of correctly folded and reoxidized SCI and PPI were analyzed by HPLC. The concentrations of both proteins were 10(-3) M during reduction and 10(-5) M during oxidation. The pH during reoxidation was varied from 8.6 to 9.2 and the temperature from 4 to 37 degrees. Under all conditions tested, the recovery of SCI was substantially higher than that of PPI. The recoveries peaked after 24-72 h. It is suggested that the "miniproinsulin" SCI folds correctly up more efficiently than porcine proinsulin, resulting in higher yields of reoxidized SCI.

Purification of thiol:protein-disulfide oxidoreductase from bovine liver

A rapid purification procedure of thiol:protein-disulfide oxidoreductase (EC 1.8.4.2) from bovine liver has been developed. The procedure is based on that of D. F. Carmichael, J. E. Morin, and J. E. Dixon (1977, J. Biol. Chem. 252, 7163-7167), and contains the following steps: homogenization in Triton X-100, selective heat denaturation, chromatography on CM-Sephadex C-50, and chromatography on DEAE-Sephadex A-50. The final preparation has a high specific activity and a high level of purity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Solubilization and partial characterization of the intestinal receptor for Escherichia coli heat-stable enterotoxin

Binding of Escherichia coli strain 431 heat-stable enterotoxin (STa) and activation of intestinal particulate guanylate cyclase by E. coli STa were studied with rat intestinal epithelial cells and brush border membranes (BBMs). The rates of guanylate cyclase stimulation by 431 STa in cells and BBMs were rapid, with maximal levels of cyclic GMP observed within 5 min. Specific binding of 125I-labeled STa from E. coli 431 (431 125I-STa) and activation of guanylate cyclase by unlabeled 431 STa were observed with intestinal BBMs; however, neither was detected with membranes from nonintestinal tissues. The STa receptor was solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, a nondenaturing dipolar ionic detergent, in yields of approximately 50%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the detergent-solubilized receptor-431 125I-STa complex, followed by autoradiography, showed that 431 125I-STa bound to a single BBM component with a molecular weight of about 100,000. Binding of 431 STa to its solubilized receptor was saturable, specific, and essentially irreversible. Pretreatment of the soluble receptor with trypsin and pronase but not chymotrypsin decreased binding of 431 125I-STa. The 431 STa-receptor complex was dissociated by boiling in the presence of 1% sodium dodecyl sulfate, incubation with 0.5 M acetic acid, or reduction with dithiothreitol. In contrast to the residual particulate guanylate cyclase activity of detergent-treated membranes, solubilized guanylate cyclase was not stimulated by STa. Membrane structure appears to play an important role in the coordination of STa binding and stimulation of guanylate cyclase activity.

Production and characterization of a monoclonal antibody to rat liver thiol: protein-disulfide oxidoreductase/glutathione-insulin transhydrogenase

Rat liver thiol:protein-disulfide oxidoreductase/glutathione-insulin transhydrogenase (glutathione:protein disulfide oxidoreductase, EC 1.8.4.2) was purified and found to give two bands on sodium dodecyl sulfate polyacrylamide gel electrophoresis. A monoclonal antibody was produced against this enzyme preparation and found to remove all the insulin degrading activity of purified preparations of the enzyme. This monoclonal antibody was also found to react with the two different forms of the enzyme observed on gel electrophoresis. These results suggest that glutathione-insulin transhydrogenase can exist in more than one state.

Roles of the 29-138 disulfide bond of subtype A of human alpha interferon in its antiviral activity and conformational stability

Human alpha (leukocyte) interferons contain two disulfide bonds between Cys-1 and Cys-98 and between Cys-29 and Cys-138. Reduction of interferon under native conditions leads to irreversible loss of antiviral activity; reduction in denaturant, followed by oxidation in native conditions, leads to restoration of activity. This behavior, unusual for disulfide-containing proteins, was studied by using a thiosulfonate derivative of subtype A of human alpha interferon (IFN-alpha A). The disulfide-free thiosulfonate formed at 25 degrees C has essentially no antiviral activity, while maintaining a conformation related to that of native IFN-alpha A. This species can regain activity after regeneration of its 29-138 disulfide, by thiol-disulfide interchange in native buffer. Incubation of the disulfide-free thiosulfonate under nonreducing conditions at 37 degrees C generates a monomeric species that has lost its native conformation as well as its ability to regain antiviral activity after thiol-disulfide interchange. These results explain the difficulty in obtaining, under native conditions, a reduced species that regains activity upon oxidation; complete reduction of IFN-alpha A in 100 mM 2-mercaptoethanol requires 37 degrees C, a temperature that promotes conformational decay of the disulfide-free form.

Properties of latent and thiol-activated rat hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase and regulation of enzyme activity

The effect of the thiols glutathione (GSH), dithiothreitol (DTT), and dithioerythritol (DTE) on the conversion of an inactive, latent form (El) of rat liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.34) to a catalyticaly active form (Ea) is examined. Latent hepatic microsomal HMG-CoA reductase is activated to a similar degree of activation by DTT and DTE and to a lower extent by GSH. All three thiols affect both Km and Vmax values of the enzyme toward HMG-CoA and NADPH. Studies of the effect of DTT on the affinity binding of HMG-CoA reductase to agarose-hexane-HMG-CoA (AG-HMG-CoA) resin shows that thiols are necessary for the binding of the enzyme to the resin. Removal of DTT from AG-HMG-CoA-bound soluble Ea (active enzyme) does not cause dissociation of the enzyme from the resin at low salt concentrations. Substitution of DTT by NADPH does not promote binding of soluble El (latent enzyme) to AG-HMG-CoA. The enzymatic activity of Ea in the presence of DTT and GSH indicates that these thiols compete for the same binding site on the enzyme. Diethylene glycol disulfide (ESSE) and glutathione disulfide (GSSG) inhibit the activity of Ea. ESSE is more effective for the inhibition of Ea than GSSG, causing a higher degree of maximal inhibition and affecting the enzymatic activity at lower concentrations. A method is described for the rapid conversion of soluble purified Ea to El using gel-filtration chromatography on Bio-Gel P-4 columns. These combined results point to the importance of the thiol/disulfide ratio for the modulation of hepatic HMG-CoA reductase activity.

Immunological identity between bovine preparations of thiol:protein-disulphide oxidoreductase, glutathione-insulin transhydrogenase and protein-disulphide isomerase

Five preparations of bovine thiol:protein-disulphide oxidoreductase/glutathione-insulin transhydrogenase (EC 1.8.4.2) and one preparation of bovine liver protein-disulphide isomerase (EC 5.3.4.1) from four different laboratories showed immunological identity in double immunodiffusion and rocket-line immunoelectrophoresis. Consequently, thiol:protein-disulphide oxidoreductase/glutathione-insulin transhydrogenase and protein-disulphide isomerase, formerly classified as two separate enzymes, should be considered as alternative activities of the same enzyme.

Structural properties of homogeneous protein disulphide-isomerase from bovine liver purified by a rapid high-yielding procedure

Protein disulphide-isomerase from bovine liver was purified to homogeneity as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, two-dimensional electrophoresis and N-terminal amino acid analysis. The preparative procedure, a modification of that of Carmichael, Morin & Dixon [(1977) J. Biol. Chem. 252, 7163-7167], is much faster and higher-yielding than previous procedures, and the final purified material is of higher specific activity. The enzyme has Mr 57 000 as determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, both in the presence and in the absence of thiol compounds. Gel-filtration studies on Sephadex G-200 indicate an Mr of 107 000, suggesting that the native enzyme is a homodimer with no interchain disulphide bonds. Ultracentrifugation studies give a sedimentation coefficient of 3.5S, implying that the enzyme sediments as the monomer. The isoelectric point, in the presence of 8 M-urea, is 4.2, and some microheterogeneity is detectable. The amino acid composition is comparable with previous analyses of this enzyme from bovine liver and of other preparations of thiol:protein disulphide oxidoreductases whose relation to protein disulphide-isomerase has been controversial. The enzyme contains a very high proportion of Glx + Asx residues (27%). The N-terminal residue is His. The pure enzyme has a very small carbohydrate content, determined as 0.5-1.0% by the phenol/H2SO4 assay. Unless specific steps are taken to remove it, the purified enzyme contains a small amount (5 mol/mol of enzyme) of Triton X-100 carried through the purification.

Kinetics and specificity of homogeneous protein disulphide-isomerase in protein disulphide isomerization and in thiol-protein-disulphide oxidoreduction

The protein disulphide-bond isomerization activity of highly active homogeneous protein disulphide-isomerase (measured by re-activation of 'scrambled' ribonuclease) is enhanced by EDTA and by phosphate buffers. As shown for previous less-active preparations, the enzyme has a narrow pH optimum around pH 7.8 and requires the presence of either a dithiol or a thiol. The dithiol dithiothreitol is effective at concentrations 100-fold lower than the monothiols reduced glutathione and cysteamine. The enzyme follows Michaelis-Menten kinetics with respect to these substrates; Km values are 4,620 and 380 microM respectively. The enzyme shows apparent inhibition by high concentrations of thiol or dithiol compounds (greater than 10 X Km), but the effect is mainly on the extent of reaction, not the initial rate. This is interpreted as indicating the formation of significant amounts of reduced ribonuclease in these more reducing conditions. The purified enzyme will also catalyse net reduction of insulin disulphide bonds by reduced glutathione (i.e. it has thiol:protein-disulphide oxidoreductase or glutathione:insulin transhydrogenase activity), but this requires considerably higher concentrations of enzyme and reduced glutathione than does the disulphide-isomerization activity. The Km for reduced glutathione in this reaction is an order of magnitude greater than that for the disulphide-isomerization activity, and the turnover number is considerably lower than that of other enzymes that can catalyse thiol-disulphide oxidoreduction. Conventional two-substrate steady-state analysis of the thiol:protein-disulphide oxidoreductase activity indicates that it follows a ternary-complex mechanism. The protein disulphide-isomerase and thiol:protein-disulphide oxidoreductase activities co-purify quantitatively through the final stages of purification, implying that a single protein species is responsible for both activities. It is concluded that previous preparations, from various sources, that have been referred to as protein disulphide-isomerase, disulphide-interchange enzyme, thiol:protein-disulphide oxidoreductase or glutathione:insulin transhydrogenase are identical or homologous proteins. The assay, nomenclature and physiological role of this enzyme are discussed.