The Control of Phosphoenolpyruvate Formation by Rabbit Liver Mitochondria
THE RELATIONSHIP BETWEEN PHOSPHOENOLPYRUVATE, MALATE, AND ASPARTATE FORMATION AND THE REGULATION OF INTRAMITOCHONDRIAL OXALACETATE
Alan J. Garber 1 and R. W. Hanson 1
From the
1 From the Fels Research Institute and Department of Biochemistry, Temple University Medical School, Philadelphia, Pennsylvania 19140
Previous studies on the mechanism of gluconeogenesis in rabbit liver have considered P-enolpyruvate formation to occur only in the mitochondria. In this study, a small but significant activity of P-enolpyruvate carboxykinase was noted in the cytosolic fraction of liver from fed rabbits. This activity is induced approximately 6-fold by prolonged fasting, thus implying the possibility of cytosolic P-enolpyruvate formation for gluconeogenesis.
The GTP requirement for mitochondrial P-enolpyruvate formation may be met by substrate level phosphorylation or by transphosphorylation via nucleoside diphosphokinase. Neither process alone appears sufficient to support maximal rates of P-enolpyruvate formation by isolated mitochondria from rabbit liver. An analysis of the in vitro alterations in the freeze clamped livers of fasted rabbits as compared with fed rabbits demonstrates a shift toward oxidation of the intramitochondrial nicotinamide coenzymes. If this shift in oxidation-reduction potential is reproduced with isolated rabbit liver mitochondria, P-enolpyruvate formation is favored over malate and aspartate synthesis. It is concluded that the synthesis of P-enolpyruvate for gluconeogenesis in rabbit liver occurs in both the cytosol and mitochondria, and that differing control mechanisms may therefore apply to these two enzyme systems.
Submitted on March 19, 1971
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