Open AccessCa 2+ Uptake, Ca 2+ ‐ATPase Activity, Phosphoprotein Formation and Phosphate Turnover in a Microsomal Fraction of Smooth Muscle
Author(s) -
RAEYMAEKERS Luc,
HASSELBACH Wilhelm
Publication year - 1981
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1981.tb05345.x
Subject(s) - phosphoprotein , calcium , endoplasmic reticulum , vesicle , atpase , calcium atpase , biochemistry , chemistry , calcium pump , microsome , phosphate , skeletal muscle , biophysics , phosphorylation , biology , enzyme , membrane , endocrinology , organic chemistry
Vesicles capable of phosphate‐stimulated calcium uptake were isolated from the microsonial 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 vesicle, 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. Tile acid‐denatured phosphoprotein is dephosphorylated by hydroxylanline, which indicates that an acylphosphate is formed. This phosphoprotein probably represents a phosphorylated transport intermediate similar to that seen with the Ca 2+ ‐ATPase of sarcoplasmic reticulum of skeletal muscle. As with the Ca 2+ ‐ATPase of sarcoplasmic reticulum vesicles, this vesicular fraction catalyses an exchange between inorganic phosphate and the γ‐phosphate of ATP (ATP‐P; exchange) which is dependent on the presence of intravesicular calcium, and an exchange of phosphate between ATP and ADP (ATP‐ADP exchange). Tile 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 the Ca 2+ ‐ATPase of sarcoplasmic reticulum of skeletal muscle.