Mechanism of transport for toxic cysteine conjugates in rat kidney cortex membrane vesicles.

ABSTRACT

Cysteine conjugate transport plays a key role in the interorgan transport of xenobitoic metabolites which are formed via the mercapturic acid pathway. In the rat, transport of cysteine conjugates could be an important factor in the selective nephrotoxicity of some toxic cysteine conjugates. However, little information is available on the molecular mechanism(s) of cysteine conjugate transport in the rat kidney. Therefore, we have investigated the polarity and the molecular driving forces for the transport of S-(1,2-dichlorovinyl)-L-cysteine (DCVC) in isolated membrane vesicles from rat kidney cortex. Our data suggest that Na+-dependent transport on the lumenal side is responsible for the uptake of cysteine conjugates across the apical membrane. No Na+-stimulated transport was found on the basolateral side and uptake of DCVC in basolateral membrane vesicles was not saturable. Na+-dependent transport in brush border membrane vesicles was inhibited by a variety of neutral amino acids and cysteine conjugates, but not by polar amino acids. Therefore, the transporter is similar to the Na+-dependent neutral amino acid transporter of rat kidney brush border membranes. The system L-specific substrate, 2-amino-2-norbornane carboxylic acid, was not inhibitory. The Km for the Na+-stimulated transport system in brush border membrane vesicles was 225 microM and the Vmax was 782 pmol/15 sec/mg of protein. We propose that the driving force for the apical transport of cysteine conjugates may be the coupling of the lumenal transport to the Na+-gradient. The data are discussed with regard to a transepithelial transport model for cysteine conjugates and the role transport plays in the molecular mechanism of cysteine conjugate toxicity.