Electron microscopic localization of adenosine triphosphate (ATP)-hydrolyzing activity in isolated matrix vesicles and reconstituted vesicles from calf cartilage.

The presence and distribution of adenosine triphosphatase (ATPase) activity in isolated matrix vesicles and reconstituted vesicles from fetal calf epiphyseal growth plate cartilage was studied by electron microscopic cytochemical methods to determine whether phosphatase activity would be found concentrated on the inside or the outside of matrix vesicle membranes or on both sides, and whether reconstitution of vesicles from deoxycholate-solubilized substituents would lead to the reassembly of membranes with ATPase incorporated. ATPase activity was observed on both the outer and inner surfaces of the investing membranes of isolated matrix vesicles and reconstituted vesicles. A transmembrane location of ATPase could indicate phosphate transfer across the vesicle membrane. Orthophosphate released by phosphatase activity within the protected microenvironment of the matrix vesicle could combine with membrane- or lipid-bound calcium, known to be present in vesicles, to form the first hydroxyapatite mineral during calcification.