Ion Transport by Heart Mitochondria

The passive binding of cations to submitochondrial fragments prepared by sonic oscillation of bovine heart mitochondria has been examined. At least two different types of binding sites can be distinguished. The first site appears to be phospholipid in nature and binds the following cations in the sequence indicated: La3+ g Ca2+ g Mg2+ g K+ and Na+. Each of these cations inhibits the binding of others in the series competitively, and in each case H+ is released into the suspending medium only when large amounts of the cation react with the membrane. Cations which interact primarily with this site or type of sites saturates the membrane at about 120 neq per mg of protein. In contrast, the binding of Zn2+ and certain other cations in the absence of energy occurs preferentially at protein binding sites and saturates the lipid sites only at high concentrations of cation. The binding of cations to this site is inhibited noncompetitively by cations in the series listed above and competitively by Zn2+, Cu2+, Cd2+, Hg2+, and chlorpromazine. In a like manner this group of cations inhibits the binding of cations to the lipid sites noncompetitively. Interaction of cations such as Zn2+ with the protein binding sites results in the release of substantial amounts of H+ into the medium. Binding to the lipid sites is inhibited noncompetitively by H+, but the binding of Zn2+ to the protein site is inhibited by a mechanism which is more complicated than either simple competitive or noncompetitive binding. The Zn2+-binding capacity of the membrane is saturated at about 250 neq of Zn2+ per mg of protein at pH 7.0. A number of lines of evidence also suggest that, in addition to the rather low affinity binding which is considered in this study, small numbers of binding sites with high affinity for cations are present in submitochondrial particles.