The pyrophosphatase activity of pig kidney alkaline phosphatase and its inhibition by magnesium ions and excess of pyrophosphate.

1. Pig kidney enzyme resembles other non-specific alkaline phosphatases in its ability to hydrolyse inorganic pyrophosphate (PP(i)). 2. Studies of enzyme velocity as a function of PP(i) concentration show that Michaelis-Menten kinetics are obeyed when a constant PP(i)/Mg(2+) concentration ratio is maintained, but velocity-substrate concentration curves are sigmoid when the concentration of PP(i) is increased but that of Mg(2+) is kept constant. The enzyme is inhibited when the total PP(i) concentration is greater than the total concentration of Mg(2+). Pyrophosphatase activity is activated by Mg(2+), but if the concentration of the metal ion is increased to a value in excess of the total PP(i) concentration Mg(2+) is then strongly inhibitory. 4. It appears that the enzyme is most active towards the complex ion MgPP(i) (2-). The enzyme probably hydrolyses PP(i) (4-) also, but this is a poorer substrate and its competition with MgPP(i) (2-) leads to inhibition. At high Mg(2+) concentrations Mg(2)PP(i) is formed. This complex appears to be a potent inhibitor. 5. Sigmoid plots of v against s and of v against i result from interactions occurring between Mg(2+) and PP(i) (4-) leading to MgPP(i) (2-) and Mg(2)PP(i), and are not indicative of allosteric behaviour.