Calcium incorporation in matrix vesicles isolated from chicken epiphyseal cartilage.

Matrix vesicles isolated from chicken epiphyseal cartilage displayed an uptake of Ca2+ which was linear with time and the amount of vesicle protein. The matrix vesicles stimulated the incorporation of Ca2+ even at very low Ca x P, suggesting that they could bind Ca2+ and/or increase the local Ca x P to the metastable level. This uptake was abolished by EDTA or heating, and partially inhibited by cysteine, to the same extent as the hydrolysis of ATP. There was also a certain uptake of Ca2+ without added phosphate, this being stimulated by ATP up to 3 mmol, but diminishing again with higher concentrations. The presence of ATP failed to stimulate the uptake of Ca2+ more than an equimolar amount of phosphate in the form of inorganic KH2PO4. Mg2+ activated the hydrolysis of p-nitrophenylphosphate at pH 10.5, and both Mg2+ and Ca2+ the hydrolysis of ATP from pH 7 to 9.5. Paradoxically, the omission of Mg2+ stimulated the uptake of Ca2+ several-fold.