Effects of Calcium and Sucrose on Pyruvate Carboxylase Activity in Intact Rat Liver Mitochondria

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      In intact rat liver mitochondria, pyruvate carboxylase activity and the previously described inhibition of pyruvate carboxylation by low concentrations of calcium (Kimmich, G. A., and Rasmussen, H. (1969) J. Biol. Chem. 244, 190) were shown to be dependent on the sucrose concentration in the incubation medium. Pyruvate carboxylation was maximal at sucrose concentrations below 60 mm and was progressively inhibited at higher sucrose concentrations. Pyruvate oxidation was inhibited by sucrose to about the same extent as pyruvate carboxylation.
      At low sucrose concentrations, 100 µm calcium was found to have little influence on pyruvate carboxylation (<10%). When the enzymatic activity was depressed by sucrose, a relative inhibition of up to 74% was produced by calcium. However, the absolute decrease of pyruvate carboxylase activity in the presence of calcium never exceeded 15% of the enzyme activity measured under optimal conditions.
      In mitochondria preloaded with calcium, pyruvate carboxylation was strongly depressed. The content of several intramitochondrial components had changed during preloading and the enzyme activity appeared to be mainly regulated by the intramitochondrial ATP:ADP ratio.

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