Variable Ca2+ transport:phosphoprotein ratios in the early part of the GTP-driven calcium-transport reaction of the sarcoplasmic reticulum.

Initial Ca2+ 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 Ca2+ binding and transporting as well as 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 two 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 microM Ca2+, if the reaction was started by Ca2+ instead of GTP but the overall rates decreased. 1 mM DL-propranolol in the preincubation medium selectively inhibited Ca2+ transport but had no effect on initial phosphoprotein formation. These observations indicate that:L (a) phosphorylation of one enzyme molecule induces Ca2+ transport by a variable but limited number of neighbouring molecules, (b) not all Ca2+ bound is essential for phosphorylation but can be transported in parallel, (c) Ca2+ bound to low-affinity sites occupied at 500 microM Ca2+ in the reaction medium is also transported initially, (d) the accessibility of the high-affinity Ca2+ binding sites for DL-propranolol differs, (e) DL-propranolol interacts with Ca2+ binding and transporting sites only in that conformation of the enzyme that can be phosphorylated by the nucleotide.