Variable Ca2+ Transport: Phosphoprotein Ratios in the Early Part of the GTP‐Driven Calcium‐Transport Reaction of the Sarcoplasmic Reticulum

Abstract

Initial Ca2+ transport and phosphoprotein formation of the sarcoplasmic reticulum membrane with GTP were investigated in rabbit skeletal muscle in a comparative study. While saturation of the high-affinity sites for Ca2+ binding and transporting, and for GTP binding on the external surface of the membrane resulted in Ca2+ transport and phosphoprotein formation in a molar ratio of 2, the variation of the concentrations of the 2 reactants yielded ratios between 1.7 and 5.7. The ratios varied with a similar dependence on the concentrations of Ca2+ and GTP except at 500 .mu.M Ca2+, if the reaction was started by Ca2+ instead of GTP, but the overall rates decreased. One mM DL-propranolol in the preincubation medium selectively inhibited Ca2+ transport but had no effect on initial phosphoprotein formation. Phosphorylation of 1 enzyme molecule induces Ca2+ transport by a variable but limited number of neighboring molecules. Not all Ca2+ bound seems essential for phosphorylation but can be transported in parallel. Ca2+ bound to low-affinity sites occupied at 500 .mu.M Ca2+ in the reaction medium evidently is transported initially. The accessibility of the high-affinity Ca2+ binding sites for DL-propranolol apparently differs. DL-propranolol appears to interact with Ca2+ binding and transporting sites only in the conformation of the enzyme that can be phosphorylated by the nucleotide.