RATES OF ELEMENTARY STEPS CATALYZED BY RAT-LIVER CYTOSOLIC AND MITOCHONDRIAL INORGANIC PYROPHOSPHATASES IN BOTH DIRECTIONS

We have investigated the kinetics of pyrophosphate synthesis catalyzed by rat liver cytosolic and mitochondrial pyrophosphatases in the pres ence of Mg2+ as cofactor. A common kinetic model derived for these rea ctions implies that the process proceeds through two parallel pathways , either using two magnesium phosphate molecules (pathway I) or using one magnesium phosphate and one free phosphate (pathway II). Pyrophosp hate formation is greatly facilitated in the active sites of the pyrop hosphatases ([E . PPi]/[E . 2P(i)] = 0.11 - 0.24). Both enzymes are hi ghly efficient in pyrophosphate synthesis: the catalytic constants for synthesis are 14 and 9.3 sec(-1) for the cytosolic and mitochondrial enzymes, respectively (9 and 16% of the hydrolysis catalytic constants ). The results demonstrate that the enzyme-catalyzed synthesis of pyro phosphate, the simplest high-energy polyphosphate, can proceed at a hi gh rate in the absence of an external energy input such as that provid ed by proton-motive force in membranous systems.