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Ca2+ uptake, Ca2+-ATPase activity, phosphoprotein formation and phosphate turnover in a microsomal fraction of smooth muscle

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Raeymaekers,  Luc
Max Planck Institute for Medical Research, Max Planck Society;

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Hasselbach,  Wilhelm
Emeritus Group Biophysics, Max Planck Institute for Medical Research, Max Planck Society;

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Citation

Raeymaekers, L., & Hasselbach, W. (1981). Ca2+ uptake, Ca2+-ATPase activity, phosphoprotein formation and phosphate turnover in a microsomal fraction of smooth muscle. European Journal of Biochemistry, 116(2), 373-378. doi:10.1111/j.1432-1033.1981.tb05345.x.


Cite as: https://hdl.handle.net/21.11116/0000-0004-F917-A
Abstract
Vesicles capable of phosphate-stimulated calcium uptake were isolated from the microsomal fraction of the smooth muscle of the pig stomach according to a previously described procedure which consists in increasing the density of the vesicles by loading them with calcium phosphate and isolating them by centrifugation [Raeymaekers, L., Agostini, B., and Hasselbach, W. (1981) Histochemistry, 70, 139--150]. These vesicles, which contain calcium phosphate deposits, are able to accumulate an additional amount of calcium. This calcium uptake is accompanied by calcium-stimulated ATPase activity and by the formation of an acid-stable phosphoprotein. The acid-denatured phosphoprotein is dephosphorylated by hydroxylamine, which indicates that an acylphosphate is formed. This phosphoprotein probably represents a phosphorylated transport intermediate similar to that seen with the Ca2+-ATPase of sarcoplasmic reticulum of skeletal muscle. As with the Ca2+-ATPase of sarcoplasmic reticulum vesicles, this vesicular fraction catalyses an exchange between inorganic phosphate and the gamma-phosphate of ATP (ATP-Pi exchange) which is dependent on the presence of intravesicular calcium, and an exchange of phosphate between ATP and ADP (ATP-ADP exchange). The results further indicate that the turnover rate of the calcium pump, calculated from the ratio of calcium-stimulated ATPase activity to the steady-state level of phosphoprotein, is similar to that of Ca2+-ATPase of sarcoplasmic reticulum of skeletal muscle.