On a possible mechanism of energy conservation in sarcoplasmic reticulum membrane.

The activation of ATP equilibrium Pi exchange, ITP equilibrium Pi exchange, and the degree of phosphorylation of the membrane of sarcoplasmic reticulum vesicles by ATP, ITP, and Pi were compared under different experimental conditions. In media containing 0.1 mM CaCl2, 6 mM Pi, and 4 mM ATP, during the period of Ca2+ accumulation the rate of ATP equilibrium Pi exchange was very low and the level of membrane phosphorylation by ATP was about 10-fold higher than the level of membrane phosphorylation by Pi. When net Ca2+ accumulation ceased and the Ca2+ concentration of the assay media had fallen to less than 5 muM, the degree of membrane phosphorylation by ATP decreased 4-fold and both the level of membrane phosphorylation by Pi and the rate of ATP equilibrium Pi exchange increase 4- to 6-fold. Contrasting with these data, when ATP was replaced by ITP, the rate of ITP equilibrium Pi exchange and the level of membrane phosphorylation by Pi were already high during the period of Ca2+ accumulation and varied slightly when the Ca2+ concentration of the incubation medium decreased to less than 5 muM. During the period of Ca2+ accumulation, the degree of membrane phosphorylation by Pi varied inversely with the NTP or NDP concentration of the medium, ATP and ADP being more effective than ITP and IDP in inhibiting the membrnae phosphorylation by Pi. Leaky vesicles incubated in media containing a high Ca2+ concentration were still able to catalyze both ATP equilibrium Pi and ITP equilibrium Pi exchange. Although the membrane of leaky vesicle was amply phosphorylated by Pi in media containing 0.1 nM CaCl2 and ITP, a significant rate of ITP equilibrium Pi exchange could only be measured in Ca2+ concentrations higher than 0.5 mM. The Ca2+ concentration required for half-maximal activation of the rate of either ITP equilibrium Pi or ATP equilibrium Pi exchange in leaky vesicles was found to be in the range of 1 to 2 mM. In leaky vesicles, the apparent Km of Pi for the ITP equilibrium Pi exchange was at least 1 order of magnitude lower than for the ATP equilibrium Pi exchange.