Alkaline and acid phosphatases in bone cells serve as phosphohydrolases at physiological pH in vivo: a histochemical implication.

Net activity of tissue-nonspecific alkaline phosphatase (TNAP) and acid phosphatase (ACP) remains to be determined since enzyme histochemistry has adopted biochemically determined optimal pH, which is not likely to represent local pH in vivo. The present study aimed to evaluate TNAP and ACP activities associated with bone cells at physiological pH. At the physiological pH of tissue fluid, intense phosphatase reactions were demonstrable in osteoblasts and osteoclasts as well as the bone matrix associated with osteoclasts. In fresh-frozen and freeze-substituted specimens, intense phosphatase reactions appeared at both alkaline and neutral pH along the entire surface of osteoblasts including the osteoidal surface, where TNAP was shown to be absent by immunohistochemistry. Combined specificity tests suggested that TNAP and ACP in bone cells can serve as phosphohydrolases at pH 7.3 and that reactions along the osteoidal surface of osteoblasts differ from that of TNAP and represent novel enzyme.