Acetylphosphate-Induced Ca2+-Ca2+ Exchange that Is Mediated by (Ca2+, Mg2+)-ATPase in Sarcoplasmic Reticulum Vesicles1

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Abstract
Sarcoplasmic reticulum vesicles were preloaded with either 45 Ca 2+ or unlabeled Ca 2+ . 45 Ca 2+ efflux and influx were determined in the presence and absence of acetyl phosphate. Phosphorylation of the membrane-bound (Ca 2+ , Mg 2+ )-ATPase by [ 32 P]acetylphosphate was also determined. The rate of efflux with acetylphos-phate was considerably higher than that without acetyl phosphate. When the acetyl-phosphate concentration was greatly reduced by diluting the reaction mixture after the start of the reaction, the rate of the efflux decreased markedly. These results demonstrate the acceleration of 45 Ca 2+ efflux by acetylphosphate. This acetyl-phosphate-induced efflux required external Ca 2+ . The external Ca 2+ concentration giving half-maximum activation of efflux was 3.8 μM. The Ca 2+ concentration dependence of the efflux coincided with that of phosphorylation. When the acetylphosphate concentration was varied, the rate of acetylphosphate-induced efflux changed approximately in proportion to the phosphoenzyme concentration. These and other findings show that acetylphosphate-induced 45 Ca 2+ efflux represents Ca 2+ -Ca 2+ exchange (between the external medium and the internal medium) mediated by the phosphoenzyme and further demonstrate the direct dissociation of Ca 2+ from the Ca 2+ -bound phosphoenzyme to the external medium in Ca 2+ -Ca 2+ exchange.