[Membrane inorganic pyrophosphatase. "Syncatalytic" inactivation by the fluoride ion]

Two forms of inorganic pyrophosphatase isolated from the inner membrane and matrix of beef heart mitochondria as well as from the inner membrane of rat liver mitochondria lose their activity during incubation with NaF in the presence of a magnesium salt and PPi. The inactivation occurs in two steps. First, the F- ion combines with an enzyme-substrate complex in a readily reversible process with a dissociation constant of approximately 3 mM. Then, the ternary complex undergoes slow isomerization with a rate constant of 5 min-1. The half-time of the inhibitor dissociation from the complex is 8 min. The enzyme SH-group modification with maleimides or a disulfide enhances the affinity to F- at the first step. The F- ion incompetitively inhibits PPi hydrolysis. The mechanism of fluoride toxicity and the use of fluoride in the studies of PPi metabolism are discussed.