Sarcoplasmic Reticulum

Abstract

Acetylphosphate is hydrolyzed by fragmented sarcoplasmic reticulum of skeletal muscle serving as energy source for the active transport of calcium. Using ³²P-acetylphosphate as substrate, the formation of a protein-bound phosphate compound was observed which is acid stable, sensitive to hydroxylamine, and accumulates to steady state concentrations of 3 to 8 moles per 10⁶ g of protein with 5 mm CaCl₂ and to 0.1 to 0.3 moles per 10⁶ g of protein with 5 mm MgCl₂ and 0.5 mm ethylene glycol bis(β-aminoethyl ether)-N,N'-tetraacetate as activators. The concentration of phosphoprotein is only slightly influenced by K⁺ and Na⁺ used alone or in combination, and steady state concentrations are reached within minutes at 25°. Using ³²P-acetylphosphate as substrate, unlabeled ATP or ADP effectively reduced the level of protein-bound radioactivity, and inhibited the hydrolysis of acetylphosphate in a manner suggesting competition for the same site. Phosphate transfer from acetyl-³²P to ADP yielding AT³²P was also observed. The hydrolysis of acetylphosphate by fragmented sarcoplasmic reticulum required Mg⁺⁺ and Ca⁺⁺, was inhibited by phospholipase C, and was partially reactivated by lysolecithin, suggesting that acetylphosphate promotes Ca⁺⁺ transport by essentially the same mechanism as ATP. Carbamylphosphate also serves as energy source for Ca⁺⁺ translocation, although the carbamylphosphatase activity, unlike the hydrolysis of ATP and acetylphosphate, is not activated by Ca⁺⁺ in the presence of Mg⁺⁺.