Biomedical subjects

W Dekant

Publications and source records attributed to W Dekant.

132 records · Page 8Linked to original sources

PubMedDate not supplied

Metabolism of hexafluoropropene. Evidence for bioactivation by glutathione conjugate formation in the kidney.

We investigated the metabolism of hexafluoropropene, a nephrotoxic fluoroalkene, in rat liver and kidney subcellular fractions and in rats in vivo. Incubation of hexafluoropropene (1 mM) with microsomes and cytosol in the presence of glutathione (GSH) yielded S-(1,2,3,3,3-pentafluoropropenyl)glutathione (PPFG) and S-(1,1,2,3,3,3-hexafluoropropyl)glutathione (HFPG) as identified by thermospray mass spectrometry and 1H-NMR. In liver microsomes, PFPG formation was predominant (240 nmol/min/mg protein) over HFPG (36 nmol/min/mg), whereas in cytosol, HFPG was the only hexafluoropropene metabolite (136 nmol/min/mg) detectable. In kidney microsomes, GSH-conjugate formation could not be detected; in kidney cytosol, HFPG was exclusively formed (46 nmol/min/mg). Hexafluoropropene inhalation (800 ppm for 1 hr) in rats fitted with a biliary cannula resulted in the biliary elimination of PFPG without detectable formation of HFPG; the exclusively formed urinary metabolite, identified by GC/MS, was N-acetyl-S-(1,1,2,3,3,3-hexafluoropropy)-L-cysteine. The results show that hexafluoropropene is metabolized to two different GSH-conjugates in rat liver and kidney. The data also suggest that hexafluoropropene metabolites formed in the liver and eliminated with bile are not translocated to the kidney and that intrarenal bioactivation by GSH-conjugation may be responsible for hexafluoropropene-induced nephrotoxicity.

Animals↗

PubMedDate not supplied

Bioactivation of tetrachloroethylene. Role of glutathione S-transferase-catalyzed conjugation versus cytochrome P-450-dependent phospholipid alkylation.

The metabolism of [14C]tetrachloroethylene (Tetra) and its metabolite S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) was investigated with in vitro systems to substantiate metabolic pathways of Tetra deduced from in vivo experiments. In the presence of NADPH, rat hepatic microsomal fractions metabolized Tetra to soluble metabolites, which were identified as trichloroacetic acid and oxalic acid by gas chromatography/mass spectroscopy and a metabolite largely bound to microsomal macromolecules. The majority of the alkylated macromolecules were identified as N-trichloroacetylated phospholipids by high performance liquid chromatography and GC/MS. When Tetra was incubated with hepatic microsomes and cytosol in the presence of 10 mM glutathione, but in the absence of NADPH, the formation of a polar metabolite other than trichloroacetic acid and oxalic acid was observed. This metabolite was identified, after hydrolysis to the corresponding cysteine conjugate, as S-(1,2,2-trichlorovinyl)-glutathione (TCVG). Microsomal GSH S-transferases catalyzed TCVG formation more efficiently than cytosolic GSH S-transferases; the competitive substrate 1-chloro-2,4-dinitrobenzene inhibited TCVG formation. In the presence of both NADPH and GSH, TCVG formation in microsomes was decreased, indicating that oxidative metabolism and GSH conjugation of Tetra are competitive reactions. The Tetra metabolite TCVC was cleaved by bacterial cysteine conjugate b-lyase to dichloroacetic acid and pyruvate. The obtained results substantiate the postulated pathways of Tetra biotransformation and demonstrate that both oxidative and conjugative reactions occur in hepatic Tetra metabolism. Phospholipid alkylation, which occurs during oxidative metabolism, may be a deactivation reaction, whereas TCVG formation, renal metabolism to TCVC, and cleavage of TCVC by b-lyase under formation of mutagenic intermediates may contribute to the nephrocarcinogenic effect of Tetra.

Alkylation↗

PubMedDate not supplied

Enzymatic conjugation of hexachloro-1,3-butadiene with glutathione. Formation of 1-(glutathion-S-yl)-1,2,3,4,4-pentachlorobuta-1,3-diene and 1,4-bis(glutathion-S-yl)-1,2,3,4-tetrachlorobuta-1,3-diene.

The glutathione-dependent metabolism of the nephrotoxin and nephrocarcinogen hexachloro-1,3-butadiene (HCBD) was investigated in subcellular fractions from rat liver and kidney. HCBD was metabolized by hepatic glutathione S-transferases to (E)- and (Z)-1-(glutathion-S-yl)-pentachlorobuta-1,3-diene (GPCB) in a ratio of 20:1, which were identified by secondary ion MS and by GC-MS after acid hydrolysis. The formation of GPCB was dependent on time and on protein and glutathione concentrations. Microsomal glutathione S-transferases from rat liver catalyzed GPCB formation more efficiently than did cytosolic glutathione S-transferases; very low rates of GPCB formation were observed in kidney subcellular fractions. GPCB is also a substrate for glutathione S-transferases and is metabolized to a diglutathione conjugate, which was identified by secondary ion MS and 13C NMR spectrometry as 1,4-bis(glutathion-S-yl)-1,2,3,4-tetrachlorobuta-1,3-diene (BTCB). BTCB formation from GPCB was dependent on time and on protein, glutathione, and GPCB concentrations. Hepatic cytosol catalyzed BTCB formation more efficiently than did hepatic microsomes; significant amounts of BTCB were also formed in kidney cytosol. Hepatic formation of glutathione S-conjugates, translocation of the S-conjugates to the kidney, and renal processing to form reactive intermediates may be the cause of HCBD-induced nephrotoxicity and, perhaps, nephrocarcinogenicity.

Animals↗

PubMedDate not supplied

Role of cytochrome P450 2E1 in the metabolism of 1,1,2,3,3,3-hexafluoropropyl methyl ether.

1,1,2,3,3,3-Hexafluoropropyl methyl ether is developed as an alternative to replace ozone-depleting chlorofluorocarbons. The metabolism of 1,1,2,3,3,3-hexafluoropropyl methyl ether was studied in rat and human liver microsomes and in rats in vivo. Rat and human liver microsomes metabolized 1,1,2,3,3,3-hexafluoropropyl methyl ether to inorganic fluoride and formaldehyde in a ratio of 2:1. 2,3,3,3-Tetrafluoropropionic acid was also identified as a metabolite by GC/MS and 19F-NMR. In rat liver microsomes, formation of inorganic fluoride and formaldehyde from 1,1,2,3,3,3-hexafluoropropyl methyl ether was dependent on the presence of NADPH, time, substrate concentration and protein concentration, and was linear for up to 35 min. Microsomes from unpretreated rats oxidized 1,1,2,3,3,3-hexafluoropropyl methyl ether at low rates (3.56 nmol fluoride.20 min-1.mg-1). Pretreatment of rats with pyridine and ethanol, inducers of cytochrome P450 2E1, increased the rate of fluoride formation. The rates of fluoride formation from 1,1,2,3,3,3-hexafluoropropyl methyl ether correlated well with the relative amount of cytochrome P450 2E1 in the microsomes as quantified by immunoblotting. Coincubation of 1,1,2,3,3,3-hexafluoropropyl methyl ether with microsomes and diethyldithiocarbamate (100 microM), an inhibitor of cytochrome P450 2E1, reduced fluoride production by > 80%. In different samples of human liver microsomes, rates of fluoride formation were higher than observed in liver microsomes from unpretreated rats and correlated well to the content of cytochrome P450 2E1 protein as determined by immunoblotting and chlorzoxazone 6-hydroxylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMedDate not supplied

Gas-uptake pharmacokinetics of 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123).

The in vivo metabolic rate constants for the metabolism of the chlorofluorocarbon replacement 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123) were determined for both male and female rats with a physiologically based pharmacokinetic model. Uptake studies with 500-5,000 ppm HCFC-123 indicated that a single saturable component was involved in both sexes, and no significant differences were observed in in vivo metabolic rate constants between male and female rats. The in vivo metabolic rate constants obtained from computer simulation studies were: for male rats--KM = 1.2 mg liter-1 (7.85 mumol liter-1) and Vmaxc = 7.20 +/- 0.28 mg kg-1 hr-1 (47.1 +/- 1.83 mumol kg-1 hr-1); for female rats--KM = 1.2 mg liter-1 (7.85 mumol liter-1) and Vmaxc = 7.97 +/- 0.30 mg kg-1 hr-1 (52.1 +/- 1.96 mumol kg-1 hr-1). The physiologically based pharmacokinetic model failed to simulate the reduction in HCFC-123 uptake in female rats at 2,000-5,000 ppm. The production and excretion of trifluoroacetic acid, the major urinary metabolite of HCFC-123, was also predicted by the physiologically based pharmacokinetic model with in vivo metabolic rate constants obtained in the gas-uptake simulation studies. Diallyl sulfide, a selective, mechanism-based inhibitor of cytochrome P450 2E1, inhibited the metabolism of HCFC-123, as indicated by a decreased uptake of HCFC-123 and by a lowered urinary excretion of trifluoroacetic acid in diallyl sulfide-treated rats.

Administration, Oral↗

PubMedDate not supplied

Glutathione-dependent biosynthesis and bioactivation of S-(1,2-dichlorovinyl)glutathione and S-(1,2-dichlorovinyl)-L-cysteine, the glutathione and cysteine S-conjugates of dichloroacetylene, in rat tissues and subcellular fractions.

Dichloroacetylene is neurotoxic, nephrotoxic, and nephrocarcinogenic. The present experiments were designed to test further the hypothesis that the glutathione conjugate of dichloroacetylene, S-(1,2-dichlorovinyl)glutathione (DCVG), may be involved in dichloroacetylene-induced neurotoxicity. Hence, the biosynthesis of DCVG from dichloroacetylene and glutathione was studied in cytosolic, microsomal, and mitochondrial fractions of liver, lung, brain, and kidney. Enzymatic formation of DCVG was observed in liver microsomes and mitochondria, but only nonenzymatic formation of DCVG was seen in lung, brain, and kidney. The KM and Vmax for DCVG biosynthesis with glutathione as the variable substrate were 0.40 +/- 0.28 mM and 487 +/- 90.3 nmol/mg protein/min, respectively. No region-specific differences in the rates of DCVG biosynthesis in the brain were observed. In whole brain homogenates, DCVG was hydrolyzed to DCVC, which was biotransformed to pyruvate and S-(1,2-dichlorovinyl)-3- mercaptopyruvate, indicating cysteine conjugate beta-lyase-catalyzed beta-elimination and transamination reactions in cerebellar tissues. These findings indicate that the glutathione-dependent bioactivation of dichloroacetylene may be involved in the dichloroacetylene-induced neurotoxicity.

Acetylene↗