The transport of S-cysteine conjugates in LLC-PK1 cells and its role in toxicity.

The transport of S-cysteine conjugates was studied in the kidney cell line, LLC-PK1, using the nephrotoxin, S-(1,2-dichlorovinyl)-L-cysteine (L-DCVC), as the model compound. The saturable uptake of this conjugate did not require sodium and was selectively inhibited by the amino acid transport system L-specific substrate, 2-amino-2-norbornane carboxylic acid, as well as a variety of other S-cysteine conjugates and neutral amino acids with large, nonpolar side chains. Kinetic studies suggested the existence of both low and high affinity transport systems with Km values that differed by 25-fold. Although these uptake systems showed no discernible differences in substrate specificity, the low affinity transport was more sensitive to trans-stimulation. L-DCVC uptake in subconfluent cultures was about 3-fold that of confluent cells, suggesting either adaptive regulation to cell growth or polarization of transport to the basolateral membrane. L-DCVC toxicity in LLC-PK1 cells was inhibited in the presence of nontoxic transport substrates but was potentiated when cells were preloaded with many of the same compounds, indicating that transport may be a rate-limiting factor in L-DCVC-induced toxicity under certain circumstances. The possible role of this system L-like uptake in the transport of S-cysteine conjugates in vivo is discussed.