Lack of correlation between glutathione turnover and amino acid absorption by the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was grown in the presence of 1.0 mM l-methionine and the half-life of degradation of glutathione determined for the strains Σ1278b (444 min) and the amino-acid-uptake deficient mutant 2512c (368 min). There is no significant difference in these values, yet the rate of uptake of l-methionine is 5–7 times lower in the mutant. In neither strain is the turnover of glutathione sufficient to account for amino acid uptake. We conclude that there is no correlation between the γ-glutamyl cycle and amino acid uptake by this east.     

Stimulation of glutathione degradation by amino acids: lack of stereospecificity

The rate of degradation of glutathione by rat kidney slices has been analysed. In the absence of exogenous amino acids a half-life of 84 min is found. In the presence of the L-isomer of three amino acids which are good substrates for gamma-glutamyl transpeptidase the rate of degradation is increased in a concentration-dependent manner. The stimulatory effect is not stereospecific, the D-isomers having a similar effect to their L-enantiomers. These findings indicate that perturbations in glutathione metabolism need not be due to the stimulation of active transport mediated by gamma-glutamyl transpeptidase.     

The amino-acid sequence of bovine glutathione peroxidase

The amino-acid sequence of the seleno-enzyme glutathione peroxidase from bovine erythrocytes was completely determined. Fragmentation of the carboxymethylated protein comprised cleavages with trypsin, with endoproteinase Lys-C, and with cyanogen bromide in 70% formic acid. The resulting peptides were separated by reversed-phase high-performance chromatography or by gel filtration. For sequence determination automated solid or liquid phase techniques of Edman degradation were used. The proper alignment of fragments was experimentally proven in all but one instance. In this case, consistent indirect evidence was provided. The monomer of glutathione peroxidase was shown to consist of 198 amino acids representing a molecular mass ob about 21 900 Da. The active site selenocysteine was localized at position 45. In addition, four cysteine residues were found at positions 74, 91, 111, and 152. The N-terminal part of the sequence obtained revealed a pronounced homology with a partial sequence of the rat liver enzyme. Moreover, tentative sequence data predicted from X-ray crystallographic analysis of bovine glutathione peroxidase were found to agree in about 80% of the residues with the sequence presented. Differences between the predicted and the experimentally determined sequence are discussed.     

Elevated glutathione production by adding precursor amino acids coupled with ATP in high cell density cultivation of Candida utilis

Aims: Three precursor amino acids and adenosine triphosphate ( ATP) are necessary for fermentative production of glutathione. In this study, our aims were to develop a strategy to enhance glutathione production by adding three precursor amino acids coupled with ATP in high cell density (HCD) cultivation of Candida utilis. Methods and Results: A high-glutathione yeast strain, C. utilis WSH 02-08, was used in this study. Whole fermentative process for glutathione production was divided into two phases of cell growth and glutathione synthesis. Cells concentration was increased by HCD cultivation. Meanwhile, intracellular glutathione content was enhanced by the addition of three precursor amino acids. Concentrations of three precursor amino acids added at stationary phase were optimized by response surface methodology. Moreover, the addition of ATP 15 h after the addition of the three amino acids can further enhance glutathione production. Based on aforementioned phenomenon, a strategy of adding three precursor amino acids coupled with ATP was developed to enhance glutathione production. Conclusion: Without the addition of three precursor amino acids and the ATP, a total glutathione of 1123 mg l⁻¹ was achieved after 60-h cultivation. In comparison, addition of three precursor amino acid counterparts resulted in a total glutathione of 1841 mg l⁻¹. Moreover, by adding amino acids combined with ATP, a total glutathione of 2043 mg l⁻¹ was achieved after 72-h cultivation, increased by 81·9% and 11%, respectively, as compared with the control and the one without ATP addition. Significance and Impact of the Study: This is the first report on investigating changes of the intracellular three precursor amino acids and ATP, and γ-glutamylcysteine synthase activity in HCD cultivation of C. utilis for glutathione production. A strategy of combining addition of three precursor amino acids with ATP was developed to enhance glutathione production in C. utilis.     

培养方式对富硒产朊假丝酵母性能的影响

在摇瓶和5 L发酵罐水平上分别考察亚硒酸钠浓度及其添加方式对高性能(高有机硒含量和高谷胱甘肽含量)富硒产朊假丝酵母制备的影响.结果表明:亚硒酸钠添加质量浓度为15 mg/L时,产朊假丝酵母具有较好的富硒效果,但一次性添加对酵母细胞有较大的毒害作用.采用分批次添加亚硒酸钠的方法获得了较好的制备高性能富硒产朊假丝酵母的培养方式:发酵起始添加L-蛋氨酸10 mmol/L,并在发酵过程的12和15 h分别添加亚硒酸钠10和5 mg/L.在此培养方式下,产朊假丝酵母胞内谷胱甘肽和有机硒含量分别达到172.3 mg/L和1194 μg/g.     

Inhibition of glutathione S-transferase by bile acids.

The effects of bile acids on the detoxification of compounds by glutathione conjugation have been investigated. Bile acids were found to inhibit the total soluble-fraction glutathione S-transferase activity from rat liver, as assayed with four different acceptor substrates. Dihydroxy bile acids were more inhibitory than trihydroxy bile acids, and conjugated bile acids were generally less inhibitory than the parent bile acid. At physiological concentrations of bile acid, the glutathione S-transferase activity in the soluble fraction was inhibited by nearly 50%. This indicates that the size of the hepatic pool of bile acids can influence the ability of the liver to detoxify electrophilic compounds. The A, B and C isoenzymes of glutathione S-transferase were isolated separately. Each was found to be inhibited by bile acids. Kinetic analysis of the inhibition revealed that the bile acids were not competitive inhibitors of either glutathione or acceptor substrate binding. The microsomal glutathione S-transferase from guinea-pig liver was also shown to be inhibited by bile acids. This inhibition, however, showed characteristics of a non-specific detergent-type inhibition.     

S-腺苷甲硫氨酸和谷胱甘肽联产发酵的环境条件

在5 L发酵罐中,研究pH、搅拌转速和温度等环境条件对产朊假丝酵母CCTCC M209298联产发酵合成S-腺苷甲硫氨酸(SAM)和谷胱甘肽(GSH)的影响,发现酵母细胞生长、SAM和GSH合成各自需要最适的pH、搅拌转速和培养温度。以SAM和GSH联产量最大化为目标,获得了较为合适的联产发酵条件:pH 5.0,搅拌转速350 r/min,温度30℃。在此环境条件下,结合不低于35%的溶氧体积分数,分批培养产朊假丝酵母24 h,最终SAM和GSH联产产量可达到579.6 mg/L。     

Effect of pH on the rate of Candida utilis cell cycle initiation.

The rate of cell cycle initiation (as determined by the rate of bud emergence) in yeast Candida utilis under ammonium-limited phased cultivation was dependent on the pH at which the yeast was grown.     

Amino Acid Pools and Metabolism During the Cell Division Cycle of Arginine-Grown Candida utilis

Synchronous cultures obtained by isopycnic density gradient centrifugation are used to investigate amino acid metabolism during the cell division cycle of the food yeast Candida utilis. Isotopic labeling experiments demonstrate that the rates of uptake and catabolism of arginine, the sole source of nitrogen, double abruptly during the first half of the cycle, while the cells undergo bud expansion. This is accompanied by a doubling in rate of amino acid biosynthesis, and an accumulation of amino acids. The accumulation probably occurs within the storage pools of the vacuoles. Amino acids derived from protein degradation contribute little to this accumulation. For the remainder of the cell cycle, during cell separation and until the next bud initiation, the rates of uptake and catabolism of arginine and amino acid biosynthesis remain constant. Despite the abrupt doubling in the rate of formation of amino acid pools, their rate of utilization for macromolecular synthesis increases steadily throughout the cycle. The significance of this temporal organization of nitrogen source uptake and amino acid metabolism during the cell division cycle is discussed.     

Enzymatic synthesis of glutathione using yeast cells in two-stage reaction

In the present study, permeated yeast cells were used as the catalyst to synthesize glutathione. When waste cells of brewer’s yeast were incubated with the three precursor amino acids and glucose for 36 h, 899 mg/L of glutathione were produced. To release the feedback inhibition of γ-glutamylcysteine synthetase caused by glutathione, two-stage reaction was adopted. In the first stage, glycine was omitted from the reaction mixture and only γ-glutamylcysteine was formed. Glycine was then added in the second stage, and 1,569 mg/L of glutathione were produced. The conditions of the two-stage reaction were optimized using Plackett–Burman design and response surface methodology. Under the optimized condition, commercially available baker’s yeast produced 3,440 mg/L of glutathione in 30 h, and most of the produced glutathione was in the medium. The two-stage reaction could effectively reduce the feedback inhibition caused by glutathione, but degradation of glutathione was significant.     

Thiram and dimethyldithiocarbamic acid interconversion in Saccharomyces cerevisiae: a possible metabolic pathway under the control of the glutathione redox cycle.

A rapid decrease of intracellular glutathione (GSH) was observed when exponentially growing cells of Saccharomyces cerevisiae were treated with sublethal concentrations of either dimethyldithiocarbamic acid or thiram [bis(dimethylthiocarbamoyl) disulfide]. The underlying mechanism of this effect possibly involves the intracellular oxidation of dimethyldithiocarbamate anions to thiram, which in turn oxidizes GSH. Overall, a linear relationship was found between thiram concentrations up to 21 microM and production of oxidized GSH (GSSG). Cytochrome c can serve as the final electron acceptor for dimethyldithiocarbamate reoxidation, and it was demonstrated in vitro that NADPH handles the final electron transfer from GSSG to the fungicide by glutathione reductase. These cycling reactions induce transient alterations in the intracellular redox state of several electron carriers and interfere with the respiration of the yeast. Thiram and dimethyldithiocarbamic acid also inactivate yeast glutathione reductase when the fungicide is present within the cells as the disulfide. Hence, whenever the GSH regeneration rate falls below its oxidation rate, the GSH:GSSG molar ratio drops from 45 to 1. Inhibition of glutathione reductase may be responsible for the saturation kinetics observed in rates of thiram elimination and uptake by the yeast. The data suggest also a leading role for the GSH redox cycle in the control of thiram and dimethyldithiocarbamic acid fungitoxicity. Possible pathways for the handling of thiram and dimethyldithiocarbamic acid by yeast are considered with respect to the physiological status, the GSH content, and the activity of glutathione reductase of the cells.     

Glutathione utilization by Candida albicans requires a functional glutathione degradation (DUG) pathway and OPT7, an unusual member of the oligopeptide transporter family

Candida albicans lacks the ability to survive within its mammalian host in the absence of endogenous glutathione biosynthesis. To examine the ability of this yeast to utilize exogenous glutathione, we exploited the organic sulfur auxotrophy of C. albicans met15Δ strains. We observed that glutathione is utilized efficiently by the alternative pathway of glutathione degradation (DUG pathway). The major oligopeptide transporters OPT1-OPT5 of C. albicans that were most similar to the known yeast glutathione transporters were not found to contribute to glutathione transport to any significant extent. A genomic library approach to identify the glutathione transporter of C. albicans yielded OPT7 as the primary glutathione transporter. Biochemical studies on OPT7 using radiolabeled GSH uptake revealed a K(m) of 205 μm, indicating that it was a high affinity glutathione transporter. OPT7 is unusual in several aspects. It is the most remote member to known yeast glutathione transporters, lacks the two highly conserved cysteines in the family that are known to be crucial in trafficking, and also has the ability to take up tripeptides. The transporter was regulated by sulfur sources in the medium. OPT7 orthologues were prevalent among many pathogenic yeasts and fungi and formed a distinct cluster quite remote from the Saccharomyces cerevisiae HGT1 glutathione transporter cluster. In vivo experiments using a systemic model of candidiasis failed to detect expression of OPT7 in vivo, and strains disrupted either in the degradation (dug3Δ) or transport (opt7Δ) of glutathione failed to show a defect in virulence.     

Optimal fermentation conditions for enhanced glutathione production by Saccharomyces cerevisiae FF-8

The influence of feedstock amino acids, salt, carbon and nitrogen sources on glutathione production by Saccharomyces cerevisiae FF-8 was investigated. Glucose, yeast extract, KH2PO4, and L-cysteine were found to be suitable feedstock. Highest glutathione production was obtained after cultivation with shaking for 72 h in a medium containing glucose 3.0% (w/v), yeast extract 3.0%, KH2PO4 0.06% and L-cysteine 0.06%. The glutathione concentration achieved using this medium increased 2.27-fold to 204 mg/l compared to YM basal medium.     

Pathways of glutathione degradation in the yeast Saccharomyces cerevisiae

The degradation of glutathione (GSH) in the yeast Saccharomyces cerevisiae appears to be mediated only by γ-glutamyltranspeptidase and cysteinylglycine dipeptidase. Other enzymes of the γ-glutamyl cycle, γ-glutamyl cyclotransferase and 5-oxo-l-prolinase, are not present in the yeast. In vivo transpeptidation was shown in the presence of a high intracellular level of γ-glutamyltranspeptidase, but only when the de-repressing nitrogen source was a suitable acceptor of the transferase reaction. In contrast, when the de-repressing source was not an acceptor of the transferase reaction (e.g. urea), only glutamate was detected. Intracellular GSH is virtually inert when the level of γ-glutamyltranspeptidase is low. Possible roles for in vivo transpeptidation are discussed.     

The fate of extracellular glutathione in the rat.

When intravenously administered to rats, [U-14C]glycine-labelled GSSG, GSH and its analogue ophthalmic acid were rapidly removed from the blood. In perfusion studies with isolated liver, however, the compounds did not enter the liver tissue. Thus, uptake by this tissue is obviously not responsible for the removal of gamma-glutamyl tripeptides from the blood. Instead, rapid hydrolysis of the tripeptides was observed. The undegraded tripeptides were only detected in the blood immediately after administration. Within tissue the degradation product glycine accounted for all the radioactivity. After intravenous injection of the labelled tripeptides the radioactivity accumulated first in the kidney, as shown by autoradiographic studies and chemical analysis of different tissues. The hydrolysis of the gamma-glutamyl tripeptides decreased markedly after the renal arteries were clamped. These observations strongly suggest that renal tissue is the principal site of the degradation of the tripeptides. Inhibition studies and experiments with isolated renal tubules revealed that gamma-glutamyl transpeptidase catalyses the fast hydrolysis of the extracellular peptides. The results indicate that, when entering the extracellular space, glutathione and its analogues are completely hydrolysed and must be resynthesized after reuptake of the constituent amino acids. It is concluded that the degradation occurs mainly on the luminal surface of the renal brush-border membrane and that gamma-glutamyl transpeptidase is a glutathionase acting on extracellular glutathione.     

Extracellular glutathione fermentation using engineered Saccharomyces cerevisiae expressing a novel glutathione exporter

A novel extracellular glutathione fermentation method using engineered Saccharomyces cerevisiae was developed by following three steps. First, a platform host strain lacking the glutathione degradation protein and glutathione uptake protein was constructed. This strain improved the extracellular glutathione productivity by up to 3.2-fold compared to the parental strain. Second, the ATP-dependent permease Adp1 was identified as a novel glutathione export ABC protein (Gxa1) in S. cerevisiae based on the homology of the protein sequence with that of the known human glutathione export ABC protein (ABCG2). Overexpression of this GXA1 gene improved the extracellular glutathione production by up to 2.3-fold compared to the platform host strain. Finally, combinatorial overexpression of the GXA1 gene and the genes involved in glutathione synthesis in the platform host strain increased the extracellular glutathione production by up to 17.1-fold compared to the parental strain. Overall, the metabolic engineering of the glutathione synthesis, degradation, and transport increased the total (extracellular + intracellular) glutathione production. The extracellular glutathione fermentation method developed in this study has the potential to overcome the limitations of the present intracellular glutathione fermentation process in yeast.     

Evidence for the Degradation of Nicotinamide Adenine Dinucleotide Phosphate-Dependent Glutamate Dehydrogenase of Candida utilis During Rapid Enzyme Inactivation

The nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase (NADP-GDH) from the food yeast Candida utilis was found to be rapidly inactivated when cultures were starved of a carbon source. The addition of glutamate or alanine to the starvation medium stimulated the rate of inactivation. Loss of enzyme activity was irreversible since the reappearance of enzyme activity, following the addition of glucose to carbon-starved cultures, was blocked by cycloheximide. A specific rabbit antibody was prepared against the NADP-GDH from C. utilis and used to quantitate the enzyme during inactivation promoted by carbon starvation. The amount of precipitable antigenic material paralleled the rapid decrease of enzyme activity observed after transition of cells from NH(4) (+)-glucose to glutamate medium. No additional small-molecular-weight protein was precipitated by the antibody as a result of the inactivation, suggesting that the enzyme is considerably altered during the primary steps of the inactivation process. Analysis by immunoprecipitation of the reappearance of enzyme activity after enzyme inactivation showed that increase of NADP-GDH activity was almost totally due to de novo synthesis, ruling out the possibility that enzyme activity modulation is achieved by reversible covalent modification. Enzyme degradation was also measured during steady-state growth and other changes in nitrogen and carbon status of the culture media. In all instances so far estimated, the enzyme was found to be very stable and not normally subject to high rates of degradation. Therefore, the possibility that inactivation was caused by a change in the ratio of synthesis to degradation can be excluded.     

Role of the N-terminus of glutathione in the action of yeast glyoxalase I.

A number of S-substituted glutathiones and the corresponding N-substituted S-substituted analogues have been found to be linear competitive inhibitors of yeast glyoxalase I at 26 degrees C over the pH range 4.6-8.5. N-Acetylation of S-(p-bromobenzyl)glutathione weakens binding by 13.7-fold. N-benzoylation by 25.6-fold, N-trimethylacetylation by 53.3-fold and N-carbobenzoxylation by 7.8-fold, indicating a minor steric component in the binding at the N-site. The Ki-weakening effect of N-substitution of glutathione depends on the chemical nature of the S-substituent, indicating flexibility in the glutathione and/or glyoxalase I contributions to the binding site for glutathione derivatives. The effect of N-acylation on Ki is in accord with a charge interaction of the free enzyme with S-blocked glutathione in a region of reasonably high dielectric constant. There is a slight pH effect on Ki for S-(m-trifluoromethylbenzyl)glutathione but not for S-(p-bromobenzyl)glutathione.     

The putative electrogenic nitrate-proton symport of the yeast Candida utilis. Comparison with the systems absorbing glucose or lactate.

Strain N.C.Y.C. 193 of Candida utilis was grown aerobically at 30 degrees C with nitrate as limiting nutrient in a chemostat. The washed yeast cells depleted of ATP absorbed up to 5 nmol of nitrate/mg dry wt. of yeast. At pH 4-6, extra protons and nitrate entered the yeast cells together, in a ratio of about 2:1. Charge balance was maintained by an outflow of about 1 equiv. of K+. Nitrate stimulated the uptake of about 1 proton equivalent during glycolysis or aerobic energy metabolism. Studies with 3,3'-dipropylthiadicarbocyanine indicated that the proton-linked absorption of nitrate, amino acids or glucose depolarized the yeast cells. Proton uptake along with lactate led neither to net expulsion of K+ nor to membrane depolarization.     

Utilization of glutathione as an exogenous sulfur source is independent of gamma-glutamyl transpeptidase in the yeast Saccharomyces cerevisiae: evidence for an alternative gluathione degradation pathway

gamma-Glutamyl transpeptidase (gamma-GT) is the only enzyme known to be responsible for glutathione degradation in living cells. In the present study we provide evidence that the utilization of glutathione can occur in the absence of gamma-GT. When disruptions in the CIS2 gene encoding gamma-GT were created in met15Delta strains, which require organic sulfur sources for growth, the cells were able to grow well with glutathione as the sole sulfur source suggesting that a gamma-GT-independent pathway for glutathione degradation exists in yeast cells. The CIS2 gene was strongly repressed by ammonium and derepressed in glutamate medium, and was found to be regulated by the nitrogen regulatory circuit. The utilization of glutathione as a sulfur source was, however, independent of the nitrogen source in the medium, further underlining that the two degradatory pathways were distinct.