Active accumulation of Sr 2+ by rat-liver mitochondria III. Stimulation of respiration by Sr 2+ and its stoichiometry

Abstract

1. 1. Addition of Sr 2+ to rat-liver mitochondria suspended in a medium of respiratory substrate, Mg 2+, and phosphate causes a large stimulation of respiration, the duration of which is proportional to the Sr 2+ added. The rate of respiration then returns to the original level. During the period of respiratory stimulations Sr 2+ is accumulated by the mitochondria; when the Sr 2+ of the medium is exhausted, respiration returns to the original low rate. Tests with β-hydroxybutyrate and succinate show that between 1.8 and 2.0 ions of Sr 2+ are accumulated as a pair of electrons traverses each of the three energy-coupling sites of the respiratory chain. 1 atom of extra oxygen was taken up during accumulation of 3.8 moles of Sr 2+ when succinate was the substrate, and 5.5 moles of Sr 2+ when β-hydroxybutyrate was the substrate. Succinate oxidation is stimulated to higher rates by addition of Sr 2+ than is β-hydroxybutyrate oxidation. Accumulation of phosphate accompanies accumulation of Sr 2+. 2. 2. When phosphate is not present in the medium, Sr 2+ also stimulates respiration, but in this case the return to the inhibited state takes place only with concentrations of Sr 2+ lower than 400–500 μM. The respiration continues indefinitely at the stimulated rate when higher concentrations of Sr 2+ are added. The accumulation of Sr 2+ in the absence of phosphate is much smaller than in its presence. Phosphate is therefore not required for stimulation of oxygen uptake but only for accumulation of large amounts of Sr 2+; when the latter is exhausted, the oxygen uptake ceases.