Stimulation of Mg2+-independent form of Ca2+-ATPase by a low molecular mass protein purified from goat testes cytosol

A low molecular mass protein purified from goat (Capra hircus) testes cytosol following gel filtration and anion exchange chromatographic separation stimulates Mg2+-independent Ca2+-ATPase activity without any significant effect on Mg2+-dependent Ca2+-ATPase. Stimulation of the ATPase is due to an increase in the rate of dephosphorylation of the overall reaction step of the enzyme. Binding of the stimulator increases the affinity of Ca2+-ATPase for Ca2+. An analysis of enzyme kinetics reveals a reversible type of binding of the stimulator to the ATPase and non-competitive type of stimulation with respect to the substrate. Stimulation seems due to binding of the protein at a single site following Michaelis-Menten model. The protein can also counter the effect of calcium antagonists exerted on the ATPase. The pI of the protein is 6.2 and its molecular mass has been determined to be 13, 961 by Q-TOF-MS.