Zinc ions were found to inhibit Ca2' uptake by rat liver mitochondria driven by succinate respiration but not that by a valinomycin-induced membrane potential. Zn'+ at 1 PM or higher concentrations induced a lowering of the membrane potential under the former but not the latter conditions. It is concluded that it is the lowered membrane potential in the presence of Zn" that reduces the rate of respiration-driven Car'. Ruthenium red was found to inhibit the uptake of Zn2+ but had no influence on its action upon the membrane potential. Zn2' did not affect the Ruthenium red-insensitive Ca'+ et&ix. Ca" stimulated the uptake of Zn 2+ . It is concluded that Zn" may be transported by the mitochondrial calcium uniporter but that it may have access to sites required for inhibition of respiration by other routes.

Polyclonal rabbit antibodies against a Ca(2+)-binding mitochondrial glycoprotein were found to inhibit the uniporter-mediated transport of Ca2+ in mitoplasts prepared from rat liver mitochondria. Spermine, a modulator of the uniporter, decreased the inhibition. This glycoprotein of M(r) 40,000, isolated from beef heart mitochondria and earlier shown to form Ca(2+)-conducting channels in black-lipid membranes, thus is a good candidate for being a component of the uniporter. Antibody-IgG was found to specifically bind to mitochondria in human fibroblasts.