Studies on matrix vesicles isolated from chick epiphyseal cartilage. Association of pyrophosphatase and ATPase activities with alkaline phosphatase

Abstract

Fractions composed primarily of cells (Fraction I), membrane fragments (Fraction II) and matrix vesicles (Fraction III) were isolated from chick epiphyseal cartilage. The characteristics of the alkaline phosphatase (EC 3.1.3.1), pyrophosphatase (EC 3.6.1.1) and ATPase (EC 3.6.1.3) activities in the matrix vesicle fraction were studied in detail. Mg-2-+ was not absolutely essential to any of the activities, but at low levels was stimulatory in all cases. Higher concentrations inhibited both pyrophosphatase and ATPase activities. Both the stimulatory and inhibitory effects were pH-dependent. Ca-2-+ stimulated all activities weakly in the absence of Mg-2-+. However, when Mg-2-+ was present, Ca-2-+ was slightly inhibitory. Thus, none of the activities appear to have a requirement for Ca-2-+, and hence would not seem to be involved with active Ca-2-+ transport in the typical manner. The distribution of alkaline phosphatase, pyrophosphatase, and Mg-2-+ ATPase activities among the various cartilage fractions was identical, and concentrated primarily in the matrix vesicles. Conversely, the highest level of (Na-+ + K-+)-ATPase activity was found in the cell fraction. All activites showed nearly identical sensitivities to levamisole (4 - 10-3 M) which caused nearly complete inhibition of alkaline phosphatase and pyrophosphatase. About 10-15% of the ATPase activity was levamisole-insensitive. The data are consistent with the concept that the Mg-2-+-ATPase and pyrophosphatase activities of matrix vesicles stem from one enzyme, namely, alkaline phosphatase.