Distribution and Immunologic Cross Reactivity of a Phosphohydrolytic Activity of Calcifying Cartilage and Metaphyseal Bone

The increased phosphohydrolytic activity found in calcifying cartilage has breen implicated in the process of normal calcification. Part of this multipotential activity was found to be associated with an extracellular vesicle presumed to be the initial side of calcium salt deposition.
The phosphohydrolytic activity of water extracts from calcifying cartilage and metaphyseal bone has been resolved into three enzymatic entities by DEAE-cellulose column chromatography. The activity which was eluted first, phosphatase I (pyrophosphatase I), increases as cartilage differentiates and calcifies. This increase could serve as a marker for cartilage differentiation and/or calcification. Antibodies to this enzyme isolated from calcifying cartilage or metaphyseal bone cross-react suggesting that the enzymes might, at least in part, be similar.
Cartilage and bone also possess an inorganic pyrophosphatase, pyrophosphatase II, eluted second through the DEAE-cellulose column and another phosphatase, phosphatase II, which was eluted last. By enzymatic and immunologic criteria, it appears that bone and cartilage have the same phosphate-releasing activities indicative of tissues with common cellular origin.
The possible transformation of the differentiating chondrocyte into an osteoblast or osteocyte has been postulated as the cellular mechanism whereby calcified cartilage is replaced by bone. The similarity between the phosphatase I found in epiphyseal cartilage and metaphyseal bone suggests that such transformation is quite likely.