HRPE cells expressing cysteinesubstituted mutants were preincubated for 10 min with sodium (Na) or choline buffer (Ch) with or without 10 mm succinate (S) and with or without MTSET. The preincubation solutions were washed away, and the remaining uptake activity of [3H]succinate was measured. Uptake activities of cells pretreated with MTSET are shown as a percentage of the uptakes in control cells pretreated with the same buffers without MTSET. MTSET concentrations were 10 μM (T484C, L485C, I489C, and M493C) or 1 mm (C476S, T483C, L487C, and Y503C), depending on the sensitivity of each mutant. Bars represent means ± S.E. (n = 2–6 separate experiments). *, significant difference compared with sodium group (p < 0.05).