Binding and Release of Cations by Sarcoplasmic Reticulum before and after Removal of Lipid

Abstract

The Ca2+ bound actively in the presence of ATP by sarcoplasmic reticulum isolated from rabbit skeletal muscle is released by Zn2+ (or Cd2+) as well as by phospholipases A or C and by trypsin. Zinc ions also release Mg2+ and are taken up passively by the reticulum membranes, whereas phospholipases A and C and trypsin release only the actively bound Ca2+. Phospholipase A, and to a smaller extent phospholipase C and trypsin, increase the passive cation-binding capacity of reticulum. Thus, concurrently with the loss of selectively bound Ca2+ caused by these agents, there is an increase in the passively bound Mg2+. The increase in the passive cation-binding capacity of reticulum induced by phospholipase depends on the presence of the splitting products of phospholipids, since their removal when digested reticulum is washed with a 2.5% albumin solution decreases the binding capacity to about 50% of the original value. When Ca2+, Mg2+ or Sr2+ is bound passively by reticulum, the ratio of H+ released to either of these divalent cations bound, expressed in terms of nequiv H+ released per nequiv divalent cation bound is about 1.0, but when Zn2+ is taken up, the ratio H+/Zn2+ is only about 0.5, which suggests that some of the Zn2+ is bound at sites that do not release H+ or bind Ca2+. After lipid extraction with 90% acetone, the cation-binding capacity and H+ release are decreased to about 40% of the original values. This effect is particularly evident between pH values of about 6.0 and 8.0. The apparent passive Ca2+-binding affinity is not affected by the lipid removal.