title: Stoichiometries of calcium and strontium transport coupled to ATP and acetyl phosphate hydrolysis by skeletal sarcoplasmic reticulum.
abstract: The stoichiometries of Ca2+ and of Sr2+ transport by the Ca2(+)-ATPase of skeletal muscle sarcoplasmic reticulum have been previously reported to be 2 and 1, respectively, when determined by flux ratio methods (Mermier, P. and Hasselbach, W. (1976) Eur. J. Biochem. 69, 79-86; Holguin, J.A. (1986) Arch. Biochem. Biophys. 251, 9-16). We have measured transport of Ca2+ and Sr2+ by the pulsed pH-stat method, when supported by ATP or the pseudo-substrate acetyl phosphate (AcP). The stoichiometry of ATP-supported Ca2+ transport, Ca2+/ATP, was pH dependent and varied from 2.0 at pH 6.5 to 1.0 at pH 8.0. Sr2+/ATP ratios showed a similar pH dependence and were approx. 7-18% lower. Ca2+/AcP ratios showed little pH dependence and varied from 2.0 to 1.7 in the pH range 6.5 to 8.0. Sr2+/AcP ratios were 17-34% lower, with maximum differences at the pH extremes. Ruthenium red, which blocks calcium efflux from calcium release channels, increased measured stoichiometries by less than 10%. It is concluded that the transport of both Ca2+ and Sr2+, when supported by either ATP or a pseudo-substrate, have similar stoichiometrics and occurs via identical mechanisms. The relatively low Sr2+ transport ratios have been related to uncoupled reverse flux through the Ca2(+)-ATPase cation transport channel. Subintegral M2+/substrate ratios appear to be an intrinsic feature of active transport by the Ca2+ pump of skeletal muscle sarcoplasmic reticulum.
authors: Medical Research Council Biomembrane Research Unit, University of Cape Town Medical School, Republic of South Africa.
authors: M C
authors: MC
authors: Berman
authors: S B
authors: SB
authors: King
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D066298:In Vitro Techniques; D009132:Muscles / Q000378:metabolism; D010755:Organophosphates / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013324:Strontium / Q000378:metabolism*
chemical_list: D010755:Organophosphates; C011632:acetyl phosphate; D000255:Adenosine Triphosphate; D002118:Calcium; D013324:Strontium