Gluconeogenesis in vitro. Formation of glucose 6-phosphate from malate by a cell-free rat-liver system consisting of cytosol and mitochondria.

A cell-free system prepared from rat liver containing cytosol and mitochondria as well as a number of cofactors at near physiological concentrations was shown to form glucose 6-phosphate from malate + 3-phosphoglycerate at a rate of 1.11 +/- 0.09 mumol . min-1 . g liver-1 (mean +/- SEM, n = 9, 30 degrees C). At least 70% of glucose 6-phosphate formed was derived from malate as calculated from experiments with [U-14C]malate. The indicated rates were measured between 10 min and 30 min incubation time when the system was near steady state with respect to the lactate/pyruvate ratio and to most of the gluconeogenic intermediates. In the absence of mitochondria, the rate of formation of glucose 6-phosphate from malate was about seven times lower than in their presence. A comparison between incubations carried out in presence or absence of mitochondria revealed that mitochondria decreased the lactate/pyruvate ratio and increased the ratio of (ATP + ITP)/(ADP + IDP). It could be shown that under the present incubation conditions, formation of glucose 6-phosphate was closely linked to the ratio of (ATP + ITP)/(ADP + IDP) whereas changing redox ratios had little influence on the gluconeogenic rate.