Properties of the Sarcoplasmic ATPase Reconstituted by Oleate and Lysolecithin after Lipid Depletion

Abstract

The ATPase activity of the sarcoplasmic membranes and the phosphoryl transfer reaction from ATP to the membranes depend on the composition of their lipid phase. Lipid depletion leads to a drastic (∼ 60%) reduction of phosphoprotein formation. Oleate raises and lysolecithin decreases phosphoprotein formation. In absence of calcium and/or magnesium neither the native nor the delipidated vesicular preparations are phosphorylated. The ATPase activity of lipid‐depleted vesicular preparations reactivated by oleate or lysolecithin and that of the preparation containing the lipid hydrolysis products after phospholipase A treatment reaches half‐maximal activation at concentrations of ionized calcium between 0.2 and 0.3 μM. The maximal ATP‐splitting rates, however, differ considerably. Half‐maximum activation by Mg · ATP for the oleate and lysolecithin‐reactivated ATPase is reached at 10 μM and 100 μM, respectively. The corresponding Hill coefficients are 0.5 and 0.6. For the oleate‐reactivated ATPase an activation energy of about 35 kcal/mol has been calculated which is similar to that of the extra ATPase of native vesicles. The activation energy of the lysolecithin‐reactivated ATPase is about 8 kcal lower. The ATPase activity of the reconstituted vesicular preparations shows an identical steep decline at pH values >7. An irreversible damage of the vesicular preparations at higher values could be excluded. The properties of the reconstituted sarcoplasmic ATPase are discussed in relation to the sarcoplasmic calcium transport.