In order to examine the role of the oxygen-derived free radicals in bone metabolism, the mouse calvaria organ culture system and the mouse bone marrow culture system were used. The activity of bone resorption stimulated by 1α, 25 (OH)2vitaminD3 was inhibited by the addition of catalase, an eliminating agent of H2O2. Catalase also significantly suppressed the osteoclast-like cell formation induced by 1α, 25 (OH)2vitaminD3 in a dose-dependent manner, whereas the agent had no effect on the mitogenic activity of the bone marrow-derived stromal cells. This suppression was reversed by adding H2O2 to the system. The effect of catalase was limited to the early stage precursor of the osteoclast-like cells. Moreover, when superoxide dismutase (SOD), which activates the production of H2O2, was added to the bone marrow culture, the number of osteoclastlike cells increased significantly in a dose-dependent manner. Finally, the source of the oxygenderived free radicals was examined by measuring the fluorescence of 2. 7-dichlorofluorescin (DCF), the obtained result indicated that the osteoclast-like cells could produce almost the equivalent amount of H2O2 as could the macrophages. These results strongly suggest that the oxygen-derived free radicals, particulary H2O2, may regulate bone resorption, promoting the differentiation of osteoclast precursor cells to osteoclasts.
