Ca2+-induced accumulation of pyrophosphate in mitochondria during acetate metabolism.

The mechanism of pyrophosphate (PPi) accumulation in rat liver during acetate metabolism was investigated. Perfusion of the liver with acetate in the presence of noradrenaline and glucagon induced marked accumulation of PPi (2 mumol/g of liver, 200 times that of control). In contrast, perfusion with glutamine, which generates PPi only in the cytosol, caused little accumulation of PPi, even in the presence of the two hormones. The site of PPi accumulation was shown to be the mitochondria by the finding that isolated mitochondria from the liver perfused with acetate and the hormones contained 50 nmol of PPi/mg of protein. The addition of an uncoupler to mitochondria with accumulated PPi caused gradual decrease in their PPi content, with concomitant release of a stoichiometric amount of Ca2+. Similar accumulation of PPi was observed when isolated mitochondria were incubated with acetate and Ca2+. These results show that an increase in cytosolic Ca2+ caused by the co-administration of the two hormones induced uptake of the ion into mitochondria, and that PPi accumulated in mitochondria only when it was generated in the organelles with an elevated concentration of Ca2+. High mitochondrial concentrations of Ca2+ are considered to inhibit inorganic pyrophosphatase through the formation of a stable complex, CaPPi-. Mitochondria with accumulated PPi had normal respiratory activities, and their adenine nucleotide concentrations were increased 2-fold rather than being decreased, the increases also being considered to be caused by their high concentration of Ca2+.