Toxicity of the cysteine-S-conjugates and mercapturic acids of four structurally related difluoroethylenes in isolated proximal tubular cells from rat kidney. Uptake of the conjugates and activation to toxic metabolites.

Isolated proximal tubular cells from rat kidney were incubated with the cysteine-S-conjugates and corresponding mercapturates of the potent nephrotoxicants tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), 1,1-dichloro-2,2-difluoroethylene (DCDFE) and 1,1-dibromo-2,2-difluoroethylene (DBDFE). Toxicity of these S-conjugates was determined by their ability to inhibit alpha-methylglucose uptake by the cells. The cytotoxicity of the cysteine-S-conjugates and mercapturates of TFE and CTFE was similar, but the cysteine-S-conjugates of DCDFE and DBDFE were more toxic than their mercapturates. The cytotoxicity of the conjugates decreased in the following order TFE approximately CTFE greater than DCDFE greater than DBDFE, which is the same as observed in vivo. Inhibition of renal cysteine-S-conjugate beta-lyase by aminooxyacetic acid alleviated the cytotoxicity of both the cysteine-S-conjugates and the mercapturic acids of the four haloethylenes. The cytotoxicity of the mercapturates, but not of the cysteine-S-conjugates, could be reduced by probenecid, suggesting that the cysteine-S-conjugates are transported by a different carrier system than the mercapturates. The deacetylation of the mercapturates of TFE and CTFE in the cells was much higher than that of the mercapturates of DCDFE and DBDFE. The cysteine-S-conjugates of DCDFE and DBDFE were N-acetylated by the cells whereas the other cysteine-S-conjugates were not (TFE) or only marginally (CTFE) N-acetylated. The observed differences in cytotoxicity may be explained by differences in (1) the balance between acetylation/deacetylation by the cells, (2) the conversion rate of the S-conjugates to toxic metabolites by renal beta-lyase and (3) the transport into the proximal tubular cells.