Stereoselectivity of human liver and intestinal cytosolic fractions as well as purified human glutathione S-transferase isoenzymes towards 2-bromoisovalerylurea enantiomers

Enantioselective determination of bromoisovalerylurea by liquid chromatography on chiral stationary phase in reversed- or normal-phase partition mode

Bromoisovalerylurea (bromvalerylurea) is a sedative-hypnotic given orally as a racemate. Enantiomers of this drug could be separated by high-performance liquid chromatography on the three chiral stationary phases (a vancomycin-bonded, beta-cyclodextrin derivative-bonded, or urea derivative-bonded phase). Biological fluids of human subjects who had ingested toxic or therapeutic doses of the racemate were chromatographed after liquid-liquid extraction. The (+)-enantiomer concentration was almost equal to the (-)-enantiomer concentration in the serum of one overdosed patient. In all the other subjects, the (+)-enantiomer was less than the (-)-enantiomer in their sera and saliva. The data suggest that the drug is absorbed non-stereoselectively from the gastrointestinal tract and eliminated from the blood stereoselectively.

Debromination of (alpha-bromoiso-valeryl)urea catalysed by rat blood

(Alpha-bromoiso-valeryl) urea, a sedative or hypnotic, is metabolized to (3-methylbutyryl)urea by reductive debromination. This study was designed to evaluate the role of blood in the debromination of (alpha-bromoiso-valeryl) urea. Rat blood containing an electron donor had significant debrominating activity toward (alpha-bromoiso-valeryl)urea. This debromination proceeded by enzymatic and non-enzymatic processes which required both NADH (or NADPH) and flavin mononucleotide (FMN), under anaerobic conditions. The debrominating activity was sensitive to inhibition by carbon monoxide, and the pH optimum was 8.5. When FMN was replaced by flavin adenine dinucleotide (FAD) or riboflavin, similar results were obtained. The optimum concentration of flavins was 10(-4) M. The reductive debromination was also mediated by rat erythrocytes, but not by plasma. When the blood or erythrocytes were boiled, the debrominating activity was not abolished, but was enhanced, suggesting that the activity arises from the haemoglobin in erythrocytes, and haemoglobin had debrominating activity when supplemented with both a reduced pyridine nucleotide and a flavin. Furthermore, haematin had significant debrominating activity in the presence of these cofactors. The activity of haematin was also observed with the photochemically reduced form of FMN. The results imply that the debromination proceeds in two steps--enzymatic or non-enzymatic reduction of a flavin such as FAD, FMN or riboflavin by NADPH or NADH, then non-enzymatic reductive debromination of (alpha-bromoiso-valeryl)urea to (3-methylbutyryl)urea catalysed by the haem group of rat haemoglobin in the presence of the reduced flavin.

Abnormal glutathione conjugation in patients with tyrosinaemia type I

Previous studies have suggested that tyrosinaemia type I may be associated with reduced glutathione availability due to conjugation of tyrosinaemia-associated reactive intermediates with glutathione. In the present study, the glutathione/ glutathione S-transferase system of two tyrosinaemia patients and three healthy controls were characterized by administering the racemic sedative drug bromisoval, a probe drug for assessing glutathione conjugation activity in vivo. Furthermore, concentrations of glutathione and glutathione S-transferase class alpha (GSTA) isoenzymes as well as the glutathione S-transferase class mu phenotype were assessed in the blood of six tyrosinaemia patients. The excretion of bromisoval mercapturates in healthy children was comparable to that observed in healthy adults. Tyrosinaemia patients were found to have a very high urinary recovery of bromisoval mercapturates (> or = 60% of the dose compared to about 30% for healthy, age-matched children and adults), which could be attributed mainly to a higher urinary excretion of the mercapturate derived from S-bromisoval. Healthy children and adults predominantly excrete the (R)-bromisoval mercapturate. The differences in amount excreted as well as in stereoselectivity of the urinary excretion of bromisoval mercapturates in tyrosinaemia patients are possibly related to an increased activity of specific glutathione S-transferase isoenzymes. Plasma glutathione and blood cell glutathione disulphide concentrations in tyrosinaemia patients were normal. Low blood cell glutathione concentrations were in general found only in two patients with a poor clinical condition. These results indicate that, in contrast to previous suggestions, reduced glutathione availability is not a generalized problem in (stabilized) tyrosinaemia patients.