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

[摘要]

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/Mg²⁺ concentration ratio is maintained, but velocity–substrate concentration curves are sigmoid when the concentration of PP_i is increased but that of Mg²⁺ is kept constant. The enzyme is inhibited when the total PP_i concentration is greater than the total concentration of Mg²⁺. Pyrophosphatase activity is activated by Mg²⁺, but if the concentration of the metal ion is increased to a value in excess of the total PP_i concentration Mg²⁺ is then strongly inhibitory. 4. It appears that the enzyme is most active towards the complex ion MgPP_i²⁻. The enzyme probably hydrolyses PP_i⁴⁻ also, but this is a poorer substrate and its competition with MgPP_i²⁻ leads to inhibition. At high Mg²⁺ concentrations Mg₂PP_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 Mg²⁺ and PP_i⁴⁻ leading to MgPP_i²⁻ and Mg₂PP_i, and are not indicative of allosteric behaviour.