[Synthesis of pyrophosphate by rat liver cytoplasmic and mitochondrial pyrophosphatases]

The kinetics of pyrophosphate synthesis catalyzed by rat liver cytosolic and mitochondrial pyrophosphatase in the presence of Mg2+ as cofactor has been studied. According to the kinetic model, this reaction proceeds through two parallel pathways: pathway I utilizing two magnesium phosphate molecules and pathway II utilizing both magnesium phosphate and free phosphate. Pyrophosphate formation is greatly facilitated in the active sites of the enzymes ([E.PPi]/[E.2Pi] = 0.11-0.24). Both enzymes are characterized by high efficiency in pyrophosphate synthesis in solution: the catalytic constants for the cytosolic and mitochondrial pyrophosphatases are 14 and 9.3 s-1, respectively (9 and 16% relative to those for PPi hydrolysis). The results obtained demonstrate that enzyme-catalyzed synthesis of pyrophosphate can proceed at a high rate in the absence of any external energy input, such as that provided by the proton-motive force in membrane systems.