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Matrix vesicle enzymes in human osteoarthritis

The enzymatic activities and in vitro calcification properties of matrix vesicle fractions isolated from normal and osteoarthritic (OA) human articular cartilage were compared to determine the essential conditions for calcification in these tissues. Four groups of human cartilage were examined, I, normal articular cartilage from aged, nonOA joints; II, discolored or fibrillated cartilage from OA joints; III, osteophytic cartilage from OA joints; IV, loose body cartilage from OA joints. Fetal bovine growth plate cartilage was also studied. Both ATP‐ and 5′‐AMP‐dependent in vitro matrix vesicle calcification occurs in all cartilage groups examined and, for human articular cartilage, these activities increase progressively from Groups I to II to III. Calcification does not occur in the absence of either phosphate or pyrophosphate. Alkaline phosphatase, 5′‐AMPase, and ATP:pyrophosphohydrolase activities are increased in Groups III and IV cartilage compared with Group I and are detected at high levels in fetal bovine growth plate cartilage. Pyrophosphatase activity occurs in only those cartilage groups juxtaposed to areas of new bone formation (osteophytic, loose body, and bovine growth plate). These results suggest that OA, growth plate, and even normal articular cartilage all have the potential to undergo calcification as long as both phosphate and pyrophosphate ions can be generated at sufficiently high levels. However, the capacity for cartilage to deposit hydroxyapatite, as it does during bone formation, may depend on the presence of pyrophosphatase activity.