Possible functional states of the enzyme of the sarcoplasmic calcium pump

The calcium accumulation performed by the sarcoplasmic vesicles isolated from the skeletal muscle is a unique example for an active ion transport process by biomembranes. The high transport activity on the one hand and the nearly complete absence of other enzymatic activities on the other hand, made it possible to perform detailed kinetic studies of calcium translocation and of the coupled ATP (NTP) hydrolysis. In this respect the sarcoplasmic membrane proved to be superior to most other membranes with transport activity. On the basis of such studies, a number of reaction mechanisms of the sarcoplasmic calcium pump were proposed by several authors with more or less experimental support [1 -8 ] . In the last years, it was demonstrated that the calcium accumulation by the sarcoplasmic vesicles is a reversible process [9 -11 ]. The translocation of calcium in both directions across the membrane is provided by the formation of an activated in t e rmed ia te a phosphoprotein [ 12,13]. Recently, it has been shown that, even in the absence of energy-rich phosphate donators, the vesicle protein can be phosphorylated with orthophosphate after formation of a steep concentration gradient of calcium ions across the membrane [ 12,13]. This phosphoprotein has the same chemical properties as those of the phosphoproteins obtained in the presence of energy rich phosphate donators and was assumed to be an acyl phosphate. These observations indicate directly that the sarcoplasmic membrane acts as an energy converter which transduces osmotic energy into chemical I06