Effect of glutathione depletion on sulfate activation and sulfate ester formation in rats

Characteristics of Urine Organic Acid Metabolites in Nonalcoholic Fatty Liver Disease Assessed Using Magnetic Resonance Imaging with Elastography in Korean Adults

The liver is an essential organ that manufactures energy through various metabolic pathways; thus, exploring the intermediate metabolites in nonalcoholic fatty liver disease (NAFLD) may help discover novel parameters in hepatic steatosis or fibrosis. The present study aimed to investigate the traits of urine organic acid metabolites in participants with hepatic steatosis and fibrosis in nonalcoholic Korean adults. Hepatic steatosis and fibrosis, in 68 men and 65 women, were evaluated using quantification by proton density fat fraction with magnetic resonance (MR) imaging and MR elastography, respectively. Urine metabolites were obtained using a high-performance liquid chromatography–mass spectrometry analysis. The candidate metabolites were included in the logistic regression models for hepatic steatosis and fibrosis. The association between high p-hydroxyphenyllactate levels and hepatic steatosis was not independent of body mass index and Homeostatic Model Assessment-insulin resistance. High ethylmalonate, β-hydroxybutyrate, and sulfate levels were significantly related to a low probability of hepatic fibrosis, independent of covariates. In conclusion, urine metabolites were not related to hepatic steatosis independent of obesity and insulin resistance, while several metabolites were specifically associated with hepatic fibrosis. Further study is required to verify the diagnostic value of the metabolites in a population with wide-spectrum NAFLD.

Autism spectrum disorders

Autism spectrum disorders (ASDs) are collectively the most commonly diagnosed pediatric neurodevelopmental condition. ASDs include autism, pervasive developmental disorder-not otherwise specified (PDD-NOS), Rett syndrome and Asperger disorder. ASD is characterized by impaired communication and social interaction and may involve developmental delays and seizure disorders. Recent parent-reported diagnosis of ASD in the United States put it at higher levels (1:91) than previously thought, with its diagnosis in boys occurring 4 to 5 times more frequently than in girls (1:58).¹ CDC estimates are currently 1:110;¹ up from 1:150 in 2007.² Annual medical expenditures for those affected are generally four to six times greater than for those without ASD.¹ While twin studies demonstrate that genetics play a significant role in ASD, the impact of environment should not be underestimated, given the approximate 20-fold increase in incidence over the last 20 years.³

Increase in renal glutathione in cholestatic liver disease is due to a direct effect of bile acids

Hepatic synthesis and plasma levels of glutathione are markedly decreased in chronic liver disease. Because glutathione turnover is highest in kidneys, we examined whether changes in kidney glutathione occur in chronic cholestasis and whether they are related to kidney dysfunction in liver disease. Kidney and plasma GSH and GSSG were measured 1) in bile duct-ligated (BDL) rats; 2) in healthy rats after bile acid loading to mimic cholestasis; and 3) after irreversible inhibition of glutathione synthetase with buthionine-sulfoximine (BSO), where glutathione consumption, urinary volume, and sodium excretion were also estimated. In addition, gamma-glutamylcysteine synthetase (gamma-GCS) mRNA, protein, and enzymatic specific activity were measured in kidney tissue after BDL. After BDL, kidney GSH and GSSG increased within hours by 67 and 66%, respectively. The increases were not related to plasma glutathione, which decreased below control values. Intravenous bile acid loading caused identical increases in GSH and GSSG as occurred after BDL, when glycine- or taurine-conjugated dihydroxy bile acids were administered. Glutathione consumption, as estimated after blocking of de novo synthesis with BSO, was significantly increased after BDL (127 vs. 44 nmol x g-1 x min-1). gamma-GCS mRNA and enzymatic specific activity were significantly reduced 5 days after BDL, whereas protein concentrations did not change. The urinary sodium concentration was 70% lower in BDL than in control rats. Depletion of renal glutathione normalized sodium excretion by increasing urinary sodium concentration and urinary volume. The increase in kidney glutathione after BDL seems to be mediated by an increase in plasma bile acids and is critically related to sodium retention. The increase in GSH consumption despite reduced gamma-GCS activity indicates a decreased GSH turnover tentatively due to reduced renal GSH efflux by competition with organic anions at membrane transport proteins.

The glutathione dependence of inorganic sulfate formation from L- or D-cysteine in isolated rat hepatocytes

The GSH dependence of the metabolic pathways involved in the conversion of cysteine to sulfate in intact cells has been investigated. It was found that hepatocyte-catalysed sulfate formation from added L-cysteine did not occur if hepatocyte GSH was depleted beforehand, but was restored when GSH levels recovered. Furthermore, sulfate formation did not recover in GSH-depleted hepatocytes if GSH synthesis was prevented with buthionine sulfoximine. Thiosulfate formation was, however, markedly enhanced in GSH-depleted hepatocytes. These results suggest that thiosulfate is an intermediate in the formation of inorganic sulfate from L-cysteine and that GSH was required for the conversion of thiosulfate to inorganic sulfate. Much less sulfate was formed if the cysteine was replaced with cysteinesulfinate. Furthermore, sulfate formation from L-cysteine was markedly inhibited by the addition of the transaminase inhibitor DL-cycloserine or the gamma-cystathionase inhibitor DL-propargylglycine. The major routes of sulfate formation from L-cysteine therefore seems to involve pathways that do not involve L-cysteinesulfinate. Similar amounts of sulfate were formed from D-cysteine as L-cysteine. Thiosulfate instead of sulfate was also formed in GSH-depleted hepatocytes. However, sulfate formation from D-cysteine differed from L-cysteine in that it was inhibited by the D-aminoacid oxidase inhibitor sodium benzoate and was not affected by transaminase or gamma-cystathionase inhibitors. These results suggest that thiosulfate is an intermediate in sulfate formation from D-cysteine and involves the oxidation of D-cysteine by D-amino acid oxidase to form beta-mercaptopyruvate.

Regulation of glucuronidation by glutathione redox state through the alteration of UDP-glucose supply originating from glycogen metabolism

The effect of altered redox state of glutathione was investigated on p-nitrophenol glucuronidation in isolated mouse hepatocytes. Decrease of GSH/GSSG ratio provoked by various agents caused increased glucuronidation which was accompanied by stimulated glycogenolysis and elevated UDP-glucose content. The stimulation of glycogenolysis and glucuronidation by glutathione consumption could be prevented by the reduction of oxidized glutathione with dithiothreitol and by the glycogenolysis inhibitor fructose. In permeabilized hepatocytes glycogen metabolism, bypassed by the addition of UDP-glucose, stimulated glucuronidation which was insensitive to glutathione depletion. In liver microsomes either UDP-glucuronosyltransferase activity or UDP-glucuronic acid transport was not influenced by GSH/GSSG ratio. These results suggest that alteration of the GSH/GSSG ratio regulates glucuronidation by affecting enzymes of the glycogen metabolism via the modification of UDP-glucuronate supply.

Nutritionally and chemically induced impairment of sulfate activation and sulfation of xenobiotics in vivo

Sulfation requires 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as the sulfate donor. In the search for methods to inhibit sulfation reactions via impairment of PAPS synthesis, two experimental conditions have been tested in rats. A low-sulfur diet, which does not deplete hepatic glutathione, reduced inorganic sulfate but not PAPS levels in the liver and moderately decreased sulfation of acetaminophen. Administration of molybdate, which is an alternative substrate for intestinal and renal sulfate transport as well as for ATP-sulfurylase, depleted both sulfate and PAPS in liver and markedly inhibited sulfation of acetaminophen. Therefore, administration of molybdate may be used as an experimental tool to study the role of sulfation in the fate and effect of xenobiotics.

Synthesis rates of glutathione and activated sulphate (PAPS) and response to cysteine and acetaminophen administration in glutathione-depleted rat hepatocytes

The effects of cysteine and acetaminophen (AA) on the synthesis rates of glutathione (GSH), adenosine 3'-phosphate 5'-phosphosulphate (PAPS, activated sulphate) and the AA metabolites, AA-GSH and AA-sulphate were studied in rat hepatocytes depleted of GSH by diethyl maleate (DEM). The synthesis rates were determined simultaneously by a previously described radioactive tracer method. Preincubation of the hepatocytes with 0.7 mM DEM for 30 min depleted GSH by 59% (P < 0.05) and PAPS by 28% (P < 0.05). Incubation with a toxic AA concentration resulted in GSH synthesis at a rate of 95 nmol/(10(6) cells.min) which increased to 281 nmol/(10(6) cells.min) (P = 0.05) after addition of cysteine. However, increased GSH synthesis was not followed by increased AA-GSH synthesis [4.7 vs 4.8 nmol/(10(6) cells.hr)]. Also, PAPS synthesis increased after cysteine administration [10.2 to 19.1 nmol/(10(6) cells.min)] (P < 0.05) without any change in AA-sulphate synthesis 18.5 vs 18.3 nmol/(10(6) cells.hr)]. Thus, in contrast to hepatocytes with normal GSH concentration, cysteine stimulated both GSH and PAPS synthesis rates in GSH-depleted rat hepatocytes incubated with a toxic AA concentration without stimulation of AA-GSH or AA-sulphate synthesis rates, indicating that the hepatoprotective effect of cysteine on AA toxicity is primarily due to stimulation of a GSH-mediated reduction of the reactive AA metabolite N-acetyl-p-benzoquinoneimine back to AA.

Simultaneous measurements of glutathione and activated sulphate (PAPS) synthesis rates and the effects of selective inhibition of glutathione conjugation or sulphation of acetaminophen

The aim of the present study was to examine the effects of the hepatotoxic drug acetaminophen (AA) on the synthesis rates of glutathione (GSH), activated sulphate (PAPS; adenosine 3'-phosphate 5'-phosphosulphate) and the AA metabolites AA-GSH and AA-sulphate after selective inhibition of GSH biosynthesis or sulphation in isolated rat hepatocytes. Selective inhibition of the two interdependent metabolic pathways was accomplished by buthionine sulphoximine (BSO) and 2,6-dichloro-4-nitrophenol (DCNP). The synthesis rates of GSH and PAPS were determined simultaneously by a previously described method based on trapping of radioactivity (35S) in the pre-labelled GSH and PAPS pools. Pre-incubation with 10 mM BSO for 30 min depleted GSH by 38% (P < 0.05) and PAPS by 27% (P < 0.05). The depletion resulted in increased PAPS synthesis at low, non-toxic [5-19 nmol/(10(6) cells.min)] (P < 0.05) and at high, toxic [7-30 nmol/10(6) cells.min)] (P < 0.05) AA concentrations. In both cases sulphur is diverted from GSH biosynthesis to sulphoxidation and PAPS synthesis, thereby maintaining the PAPS pool and preserving the sulphation capacity. This corresponds to the finding that AA sulphation was unaffected by BSO irrespective of AA concentration [6 vs 5 and 20 vs 17 nmol/(10(6) cells.hr), respectively]. Even though the GSH synthesis was halved after BSO pre-incubation, the GSH conjugating capacity of AA was well preserved. Incubation with 200 microM DCNP and 5 mM AA diminished PAPS synthesis from 24 to 10 nmol/(10(6) cells.min) (P < 0.02) and reduced AA-sulphate synthesis by 67% compared to experiments without DCNP incubation [4.8 vs 14.7 nmol/(10(6) cells.hr)] (P < 0.05). GSH and AA-GSH synthesis rates did not change compared to control experiments in which sulphation was not inhibited [1165 vs 1487 nmol/(10(6) cells.min), respectively] and [1.7 vs 1.7 nmol/(10(6) cells.hr), respectively]. This indicates that increased sulphur availability due to decreased PAPS synthesis is unable to raise the cysteine pool and stimulate the gamma-glutamyl cycle and GSH synthesis.

Effect of various non-hepatotoxic compounds on urinary and liver taurine levels in rats

Administration of compounds which alter protein synthesis or sulphur amino acid metabolism in rats results in changes in the excretion of urinary taurine. Treatment with diethylmaleate (DEM) or phorone, which will deplete glutathione (GSH), reduces taurine excretion, whereas treatment with buthionine sulphoximine (BSO), which will inhibit glutathione synthesis, increases taurine excretion. Treatment with cycloheximide, an inhibitor of protein synthesis, increases taurine excretion, whereas pretreatment with phenobarbital, which will increase protein synthesis, decreases taurine excretion. Administration of agents which damage organs other than the liver such as the kidney, heart and testes, does not increase urinary taurine.

Protection against alpha-naphthylisothiocyanate-induced liver injury by decreased hepatic non-protein sulfhydryl content

alpha-Naphthylisothiocyanate (ANIT) injures bile duct epithelium and hepatic parenchymal cells in rats. It is commonly believed that ANIT must undergo bioactivation by hepatic, cytochrome P450-dependent mixed-function oxidases (MFO), since agents which are inducers or inhibitors of hepatic MFO activity enhance or attenuate, respectively, the liver injury associated with ANIT. Several of these agents also affect hepatic glutathione (GSH) content and/or GSH S-transferase activity in a manner to suggest a causal role for GSH in ANIT-induced hepatotoxicity. To determine whether GSH might be involved in the mechanism of injury, buthionine sulfoximine (BSO), diethyl maleate (DEM), or phorone was used to reduce hepatic non-protein sulfhydryl (NPSH) content, an indicator of GSH content. Twenty-four hours after ANIT treatment, rats exhibited cholestasis and elevations in serum of total bilirubin concentration, total bile acid concentration, aspartate aminotransferase (AST) activity, and gamma-glutamyltransferase activity. Cotreatment of rats with BSO decreased NPSH content by 70% at 24 hr and prevented the cholestasis and elevations in serum markers of liver injury caused by ANIT. Likewise, cotreatment of rats with DEM afforded protection against markers of liver injury. Phorone treatment attenuated ANIT-induced elevations in serum total bilirubin concentration and AST activity. Although BSO treatment afforded protection against ANIT-induced liver injury at 24 hr, the injury was evident at 48 hr, and it appeared to coincide with a return of hepatic NPSH content. These results suggest that GSH plays a causal or permissive role in the liver injury caused by ANIT.

Measurement of reduced and oxidized glutathione in cultures of adult rat hepatocytes

A reversed-phase ion-exchange high-performance liquid chromatographic technique, suitable for the separate measurement of reduced (GSH) and oxidized (GSSG) glutathione in cultures of adult rat hepatocytes, is described. A commercially available Nucleosil 120-7NH2 column was used. A complete run took ca. 22 min. The retention times for GSH and GSSG were 10.6 and 12.7 min, respectively, providing a resolution coefficient of 1.4. The coefficients of variation for GSH and GSSG were ca. 5 and 25%, respectively, for freshly isolated hepatocytes, and 16 and 15%, respectively, for 24-h cultured hepatocytes. The detector response was linear as a function of GSH and GSSG concentration and the hepatocytes concentration studied. Addition of up to 1.5 mg/ml bovine serum albumin to the culture medium had no effect on the linearity. The recovery for standards, ranging from 0 to 150 nmol of GSH or GSSG per millilitre in the presence of hepatocytes, was 98% for GSH and 80% for GSSG. The detection limit of the method was between 0.5 and 1.0 nmol of GSH and GSSG per millilitre. In cultured rat hepatocytes, the GSH content increased during the first 24 h of culture, followed by a slow decrease. After six days of culture, the GSH content was less than 50% of the value found for freshly isolated hepatocytes. GSSG was present in cultured rat hepatocytes in only small amounts and becomes unmeasurable after four days of culture.

Effect of sulfhydryl-deficient diets on hepatic metallothionein, glutathione, and adenosine 3'-phosphate 5'-phosphosulfate (PAPS) levels in rats

Low dietary concentrations of methionine and cysteine are known to decrease hepatic glutathione content. However, it is not known if restricting the dietary content of these sulfur containing amino acids also affects hepatic levels of adenosine 3'-phosphate 5'-phosphosulfate (PAPS), the cofactor for sulfation, or metallothionein, a protein rich in sulfhydryl groups. Rats were fed diets lacking cysteine and containing various concentrations of methionine (0.15, 0.3, or 0.6%) for 8 days. Control diet contained 0.3% each of methionine and cysteine. Hepatic glutathione levels were decreased approximately 75% in rats fed diets containing 0.15 or 0.3% methionine. In contrast, PAPS and hepatic metallothionein concentrations were not decreased by the low sulfhydryl diets. Additionally, rats on the various diets were challenged by the administration of ZnCl2 (3 mmol/kg. sc). In both control rats and rats maintained on sulfhydryl-deficient diets, ZnCl2 increased hepatic metallothionein to the same level. However, significantly lower levels of PAPS were observed after ZnCl2 in rats receiving sulfhydryl-deficient diets than in controls. In summary, restriction of dietary sulfhydryl markedly decreases the hepatic content of glutathione and has a minor effect on PAPS concentration, but does not decrease the basal hepatic concentration of metallothionein or its induction by ZnCl2.