Rapid equilibrium dialysis has been used to show that recombinant bovine brain inositol monophosphatase binds one equivalent of Pi per subunit of enzyme. Pi is only bound in the presence of Mg2+ ions. The dissociation constant for the equilibrium is approximately 50 μM. This value of K d is independent of the concentration of the Mg2+ ions and of the presence or absence of Li+ ions. Lithium ions which inhibit the enzyme uncompetitively are not able to support the binding of the Pi to the enzyme. The observation that Pi only binds in the presence of Mg2+ ions supports similar conclusions made in experiments which studied the protection of the enzyme from proteolytic degradation and chemical modification.
