ALLOSTERIC MODULATION BY ATP, CALCIUM AND MAGNESIUM-IONS OF RAT OSSEOUS PLATE ALKALINE-PHOSPHATASE

Alkaline phosphatase from rat osseous plate is allosterically modulate d by ATP, calcium and magnesium at pH 7.5. At pH 9.4, the hydrolysis o f ATP and PNPP follows Michaelis-Menten kinetics with K0.5 values of 1 54 muM and 42 muM, respectively. However, at pH 7.5 both substrates ex hibit more complex saturation curves, while only ATP exhibited site-si te interactions. Ca2+-ATP and Mg2+-ATP were effective substrates for t he enzyme, while the specific activity of the enzyme for the hydrolysi s of ATP at pH 7.5 was 800-900 U/mg and was independent of the ion spe cies. ATP, but not PNPP, was hydrolyzed slowly in the absence of metal ions with a specific activity of 140 U/mg. These data demonstrate tha t in vitro and at pH 7.5 rat osseous plate alkaline phosphatase is an active calcium or magnesium-activated ATPase.