Reversible inactivation of rat liver inorganic pyrophosphatase by substrate and its analogs

Dissociation of Mg2+ from one of the two metal-binding sites whose occupancy is absolutely required for catalysis by rat liver inorganic pyrophosphatase is a slow reaction (tau 1/2 = 3 h). Polycarboxylic Mg2+ complexons markedly accelerate this process due to their binding with Mg2+ on the enzyme. PPi, ATP and a number of diphosphonate analogs of PPi also bind with Mg2+ on the enzyme with concomitant decrease in enzyme activity by 75% but do not release the bound Mg2+. The resulting ternary complex rapidly (tau 1/2 of several seconds) dissociates upon dilution into substrate-free medium. PPi and imidodiphosphate, which are substrates for pyrophosphatase, decrease the rate of reactivation by at least two orders of magnitude. The results can be explained by existence of two interconvertible forms of the enzyme, of which one is inactive and is stabilized by substrate or its analogs.