Interaction of cysteine conjugates with human and rabbit organic anion transporter 1

Organic anion (OA) transport mediates accumulation of the zwitterionic nephrotoxic cysteine S-conjugates S-dichlorovinylcysteine (DCVC) and S-chlorotrifluoroethylcysteine (CTFC) in the rabbit renal proximal tubule (RPT). Although these cysteine conjugates are nephrotoxic to the human RPT, neither the role of OA transport nor the specific OA transport pathway(s) involved in cysteine conjugate accumulation are known. Since the OAT1 transporter has the characteristics of para-aminokippurate (PAH) transport that closely correlate to the native RPT, we examined the interaction of DCVC, CTFC, and the nontoxic benzothiazolylcysteine (BTC) with PAH transport mediated by human OAT1 and rabbit Oat1 expressed in Chinese hamster ovary and COS7 heterologous expression systems, respectively. Although the K_m values for PAH uptake by hOAT1 and rbOat1 (8.9 ± 3.6 and 20.7 ± 8 μM, respectively) were 5- to 10-fold less than the K_m for peritubular PAH transport into rabbit RPT, the IC₅₀ values for DCVC, CTFC, and BTC inhibition of PAH uptake mediated by either hOAT1 or rbOat1 were similar between these two transporters and to the IC₅₀ values for these conjugates measured in rabbit RPT. The IC₅₀ for inhibition of hOAT1- and rbOat1-mediated PAH uptake by the hydrophobic conjugate BTC was more than 5-fold lower than the IC₅₀ values seen with DCVC and CTFC, suggesting that hydrophobicity increases the affinity of OAT1 for cysteine conjugates. Finally, preloading cells transfected with hOAT1 with BTC significantly trans-stimulated the uptake of PAH, consistent with the conclusion that BTC and, hence, other cysteine S-conjugates are substrates for hOAT1.