Biotransformation, excretion, and nephrotoxicity of the hexachlorobutadiene metabolite (E)-N-acetyl-S-(1,2,3,4, 4-pentachlorobutadienyl)-L-cysteine sulfoxide.

Hexachlorobuta-1,3-diene (HCBD) is nephrotoxic in rodents. Its toxicity is based upon a multistep bioactivation pathway. Conjugation with glutathione by glutathione S-transferases to form (E)-S-(1,2,3,4,4-pentachlorobutadienyl)-L-glutathione (PCBG), further processing to the corresponding cysteine S-conjugate, and finally processing to a reactive thioketene are thought to be responsible for the observed nephrotoxic effects. A novel metabolite, identified as (E)-N-acetyl-S-(1,2,3,4, 4-pentachlorobutadienyl)-L-cysteine sulfoxide (N-AcPCBC-SO), was described after administration of [14C]HCBD to male Wistar rats. This metabolite is formed by sulfoxidation of N-acetyl-S-(1,2,3,4, 4-pentachlorobutadienyl)-L-cysteine (N-AcPCBC) mediated by cytochrome P450 3A and has been found to be cytotoxic to proximal tubular cells in vitro without activation by beta-lyase. In rats, given HCBD in vivo, only one diastereomer of the sulfoxide is excreted; however, in rat hepatic microsomes two diastereomers, (R)- and (S)-N-AcPCBC-SO, are formed. This study focuses on the mechanisms responsible for this discrepancy and on a possible contribution of N-AcPCBC-SO to the nephrotoxicity of HCBD in vivo. (R,S)-N-AcPCBC-SO (1:1 mixture of both diastereomers) and N-acetyl-alpha-methyl-S-(1,2,3,4,4-pentachlorobutadienyl)-d, L-cysteine sulfoxide (alpha-Me-N-AcPCBC-SO) were administered iv to male and female Wistar rats (20, 40, and 80 micromol/kg of body weight). alpha-Me-N-AcPCBC-SO cannot be cleaved by cysteine conjugate beta-lyase even if alpha-Me-N-AcPCBC-SO is deacetylated by acylases. Excretion of gamma-glutamyltranspeptidase, protein, and glucose in the urine, indicative for kidney damage, and histopathological examination of the kidneys showed marked differences in the renal damage in male and female rats after application of N-AcPCBC-SO and alpha-Me-N-AcPCBC-SO. Necroses of the kidney tubules were only found in male, but not female, rats. Major sex-specific differences were observed in the elimination of sulfoxides; the (R)-isomer was excreted in a 5-10-fold excess to the (S)-isomer after application of (R,S)-N-AcPCBC-SO. After purification, both isomers were administered to male rats resulting in the urinary excretion of (R)-N-AcPCBC-SO after giving the (R)-isomer; treatment with (S)-N-AcPCBC-SO, however, revealed the formation of (S)-N-acetyl-S-(2-glycinylcystein-S-yl-1,3,4, 4-tetrachlorobutadienyl)-L-cysteine. The results show major sex-specific differences in the nephrotoxic potency of N-AcPCBC-SO and alpha-Me-N-AcPCBC-SO. However, both N-AcPCBC-SO and alpha-Me-N-AcPCBC-SO are nephrotoxic in males, suggesting the formation of a vinyl sulfoxide as an additional, beta-lyase-independent mechanism in HCBD-caused nephrotoxicity.