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

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Abstract
We studied the role of sulfhydryl groups in Na+-Pi contransport across the renal brush border membrane (BBM), using HgCl2, an agent which penetrates membranes freely. HgCl2 inhibited the initial Na+-dependent 32Pi transport in a dose-dependent manner (IC50 = 54 μM). Na+-independent transport was not affected. The inhibitory effect persisted under Na+ equilibrium–exchange conditions. Additionally, HgCl2 had no effect on the diffusional uptake of 22Na up to 1 min incubation. Exposure to HgCl2 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 HgCl2 concentrations, but decreased at higher concentrations (> 100 μM). Vesicle volumes, determined by flow cytometry, were not changed after exposure to HgCl2. 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.

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