Variable Ca 2+ Transport: Phosphoprotein Ratios in the Early Part of the GTP‐Driven Calcium‐Transport Reaction of the Sarcoplasmic Reticulum

Initial Ca 2+ transport and phosphoprotein formation of the sarcoplasmic reticulum‐membrane with GTP were investigated in a comparative study. While saturation of the high‐affinity sites for Ca 2+ binding and transporting as well as for GTP binding on the external surface of the membrane resulted in Ca 2+ transport and phosphoprotein formation in a molar ratio of 2, the variation of the concentrations of the two reactants yielded ratios between 1.7 and 5.7. The ratios varied with a similar dependence on the concentrations of Ca 2+ and GTP, except at 500 μM Ca 2+ , if the reaction was started ty Ca 2+ instead of GTP but the overall rates decreased. 1 mM dl ‐propranolol in the preincubation medium selectively inhibited Ca 2+ transport but had no effect on initial phosphoprotein formation. These observations indicate that: (a) phosphorylation of one enzyme molecule induces Ca 2+ transport by a variable but limited number of neighbouring molecules, (b) not all Ca 2+ bound is essential for phosphorylation but can be transported in parallel, (c) Ca 2+ bound to low‐affinity sites occupied at 500 pμM Ca 2+ in the reaction medium is also transported initially, (d) the accessibility of the high‐affinity Ca 2+ binding sites for dl ‐propranolol differs, (e) dl ‐propranolol interacts with Ca 2+ binding and transporting sites only in that conformation of the enzyme that can be phosphorylated by the nucleotide.