Inhibition of renal Na + ‐Pi cotransporter by mercuric chloride: Role of sulfhydryl groups

We studied the role of sulfhydryl groups in Na + ‐Pi contransport across the renal brush border membrane (BBM), using HgCl 2 , an agent which penetrates membranes freely. HgCl 2 inhibited the initial Na + ‐dependent 32 Pi transport in a dose‐dependent manner (IC 50 = 54 μM). Na + ‐independent transport was not affected. The inhibitory effect persisted under Na + equilibrium–exchange conditions. Additionally, HgCl 2 had no effect on the diffusional uptake of 22 Na up to 1 min incubation. Exposure to HgCl 2 had no effect on vesicle integrity as determined by osmotic shrinking experiments. BBM vesicle (BBMV) volume, determined by D‐glucose equilibrium uptake, was not affected at low HgCl 2 concentrations, but decreased at higher concentrations (> 100 μM). Vesicle volumes, determined by flow cytometry, were not changed after exposure to HgCl 2 . Kinetic studies showed a reduction in the apparent Vmax for Pi transport from 1.40 ± 0.13 to 0.75 ± 0.19 nmoles/mg protein/5 sec, without a significant change in the apparent Km. In protection studies, dithiothreitol (DTT) completely protected agaisnt inhibition, but Pi, phosphonoformic acid (PFA), and Na + gave no protection. The data suggest that sulfhydryl groups are essential for the function of Na + ‐Pi cotransporter of renal BBM.