Cysteine 474 influences the interaction of human organic cation transporter 2 (hOCT2) with TEA but not MPP

Abstract

OCT2 assists in the renal elimination of potentially toxic organic cations. Previously we showed that C474 of hOCT2 is oriented toward the aqueous milieu of the hydrophilic cleft (proposed region of substrate binding), and covalent modification of this residue with maleimide-PEO2-biotin (MB) resulted in lower TEA transport activity. Here, C474 was mutated to alanine (C474A) or phenylalanine (C474F) to determine its role in substrate binding. The kinetics of [3H]TEA and [3H]MPP uptake into CHO cells stably expressing the wild-type (WT) or mutant transport proteins was determined. C474A had a 7-fold higher affinity for TEA than WT (Kt = 8 vs 54 μM), but TEA transport by C474F was too low to determine kinetics (10% of WT). The affinity of C474A and C474F for MPP was not different from WT. Consistent with the kinetic data, covalent modification of C474 in WT hOCT2 with MB resulted in reduced TEA transport, but had no effect on MPP transport. These data are consistent with C474 being near a binding surface for TEA, and, in conjunction with previous work, suggest that hOCT2 contains distinct but overlapping sites for substrate binding. (Supported by DK58251)