Kinetic characteristics of Ca2+, Mg2+-ATPases in cells of the submandibular salivary gland of rats

Kinetic properties of Ca2+, Mg2+-ATPases membranes from acinar cells of rat submandibular salivary glands have been investigated. It was found that kinetics of ATP hydrolysis dependent on Ca2+, Mg2+-ATPases corresponds to the first-order reaction during first 2 min. It was found that the initial velocity of the reaction (V0), maximal amount of the reaction product (Pmax) and characteristic time of the reaction (T) comprised 1.8 +/- 0.4 and 1.6 +/- 0.2 mmole Pi/min per 1 mg protein, 7.5 +/- 1.3 and 1.4 +/- 0.2 mmole Pi/mg protein and 4.1 +/- 0.7 min and 1.1 +/- 0.1 for Ca2+-ATPases from plasma and endoplasmic reticulum membranes, correspondingly. High- and low-affinity sites of ATP and Ca2+-binding in Ca2+-ATPases from plasma and endoplasmic reticulum membranes were identified. Negative cooperation in ATP binding to Ca2+-ATPase from plasma membrane and a positive cooperation for Ca2+-ATPase from endoplasmic reticulum has been found. Ca2+ binding to low-affinity sites of both Ca2+-ATPases showed no cooperation, while Ca2+ binding to high-affinity sites showed the positive cooperation. Using the Hill's coordinates we have found the values of the Mg2+ Michaelis constant (K(Mg)) which yielded 3.89 x 10(-5) and 3.80 x 10(-5) mole/l for Ca2+-ATPases from plasma and endoplasmic reticulum membranes, correspondingly. It is supposed that obtained data are important for further studies of molecular and membrane mechanisms involved in the regulation of intracellular calcium signalling and secretion by salivary acinar cells.