Metabolism of cysteine and cysteinesulfinate in rat kidney tubules

In studies with rat hepatocytes, hypotaurine plus taurine production accounted for less than 5% of the total amount of cysteine (CYS) catabolized, whereas more than 90% of the metabolized cysteinesulfinate (CSA) was converted to taurine plus hypotaurine. Similar studies have been carried out with kidney tubules isolated from fed rats and incubated with 2 mM [1-14C]CYS or 25 mM [1-14C]CSA at 370C for up to 40 min. The production of 14CO2 from CSA (3.1 +/- 1.3 nmol/sup ./ min-1/sup ./ mg dry wt-1) was equivalent to the accumulation of N in NH4+ plus glutamate. Substantial oxidation of CYS was observed (16 +/- 11 nmol CO2 x min-1 x mg dry wt-1), but only 12% of the expected amount of N was recovered as NH4+ plus glutamate. Accumulation of hypotaurine plus taurine was equivalent to 20% of the observed rate of 14CO2 production from CSA but accounted for only 2% of the observed rate of 14CO2 production from CYS. Addition of unlabeled CSA to incubations with varying levels of CYS had no effect on production of 14CO2. Addition of 2 mM α-ketoglutarate to the incubation mixtures resulted in an increased in 14CO2 production from CSA to 290% of the control level but had no effect on CYS oxidation. In agreement with the authors findings for rat hepatocytes, these data suggest that most metabolism of CYS by the rat kidney tubule occurs by a CSA-independent pathway. However, in contrast to the metabolism of CSA almost entirely to taurine in the hepatocyte, kidney tubules appeared to metabolize CSA primarily by the transamination pathway