The activity of pentose-shunt dehydrogenases is very low in periosteal cells of normal rat metatarsals, but increases one day post-fracture and rises linearly over the next two days. By four days post-fracture, the distribution of this activity along the bone shows two centres of high activity: the first in the region of proliferation to form callus and the second at the site where new bone is first seen, one day later. The high rate of generation of NADPH would be expected to reduce glutathione; reduced glutathione has been shown to inhibit alkaline phosphatase activity in these cells.

The quantitative changes in alkaline phosphatase activity in the periosteal cells close to the fracture in rat metatarsal bones has been measured during the first 5 days postfracture. This study has been made possible by two technological advances, firstly the development of cryostat microtomy for cutting unfixed, undemineralised bone, and secondly the use of scanning and integrating microdensitometry for quantifying the activity in each periosteal cell. The results showed a loss of alkaline phosphatase activity close to the fracture site, with activity rising to normal values 0.8-1.0 mm from the site. No alkaline phosphatase activity was found in the cells which proliferated from the periosteum. It is suggested that reduced glutathione could cause such inhibition.

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