Stimulation of mitochondrial pyruvate carboxylation by Mn²⁺ and the distribution of the products between mitochondrion and medium

Role of bivalent cations in the control of enzymes involved in gluconeogenesis

“…Pyruvate carboxylase was assayed spectrophotometrically in extracts from mitochondria prepared as previously described [3] and was resuspended at room temperature in the minimum volume of 50% sucrose [7]. Protein concentration wag measured by the method of Lowry et al [8].…”

“…A mixture of both ions at optimal concentrations -Mg 2+ at 8 mM and Mn 2+ at 1 or 2 mMproduced no greater activity than either alone. At optimal concentrations of either ion, Ca 2+ was inhibitory -1 mM caused 60% inhibition with Mg 2+ but only 25% with Mn 2+ [3]. In contrast with pyruvate carboxylase however, addition of Mn 2+ in the presence of Mg 2+ produced a marked increase in the activity over that in the presence of either ion alone ( [11] and fig.…”

“…The function of Ca 2+ is uncertain but may be to prevent the breakdown of phosphoenolpyruvate by pyruvate kinase. Since the three key enzymes required for the reverse of the glycolytic sequence -pyruvate carboxylase, phosphoenolpyruvate carboxykinase and fructose-1,6-diphosphataseare all activated by bivalent cations such as Mg 2+, it is likely that their action is also inhibited by Ca 2+ [2,3]. Thus the control of the relative rates of glycolysis and gluconeogenesis could result from translocation of Ca 2+ between mitochondrion and cytoplasm, differentially altering the extent of inhibition of pyruvate kinase and pyruvate carboxylase [4].…”

“…Mn 2+ can offset the inhibitory effect of Ca 2+ on mitoehondrial carboxylation of pyruvate [3] ; this may result partly because both Mn 2+ and Mg 2+ can activate carboxylase [3], which when activated by Mn 2+ is much less sensitive to inhibition by Ca 2+. In the present study, a comparison of the ability of Mn 2+ and Mg 2+, separately or together, to activate the three key gluconeogenic enzymes has been made and the effect of these ions on Ca 2+ inhibition has been investigated.…”

The relation between mitochondrial anion accumulation and the pH gradient with special reference to phosphate distribution

“…Rat liver mitochondria were prepared as before [1,2]. Ratios between inside content of malate or citrate and the external concentration were measured using the 14C-labelled compounds with centrifugal separation of the mitochondria through silicone exactly as before [3 ].…”