Increased adenine nucleotides in liver mitochondria after mannoheptulose injection in vivo

Stimulation of the electron transport chain in mitochondria isolated from rats treated with mannoheptulose or glucagon

We investigated the kinetics of the mitochondrial respiratory chain, proton leak, and phosphorylating subsystems of liver mitochondria from mannoheptulose-treated and control rats. Mannoheptulose treatment raises glucagon and lowers insulin; it had no effect on the kinetics of the mitochondrial proton leak or phosphorylating subsystems, but the respiratory chain from succinate to oxygen was stimulated. Previous attempts to detect any stimulation of cytochrome c oxidase by glucagon are shown by flux control analysis to have used inappropriate assay conditions. To investigate the site of stimulation of the respiratory chain we measured the relationship between the thermodynamic driving force and respiration rate for the span succinate to coenzyme Q, the cytochrome bc1 complex and cytochrome c oxidase. Hormone treatment of rats altered the kinetics of electron transport from succinate to coenzyme Q in subsequently isolated mitochondria and activated succinate dehydrogenase. The kinetics of electron transport through the cytochrome bc1 complex were not affected. Effects on cytochrome c oxidase were small or nonexistent.

Treatment of rats with glucagon or mannoheptulose increases mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase activity and decreases succinyl-CoA content in liver

1. The activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase (EC 4.1.3.5) in extracts of rapidly frozen rat livers was doubled in animals treated in various ways to increase ketogenic flux. 2. Some 90% of the activity measured was mitochondrial, and changes in mitochondrial activity dominated changes in total enzyme activity. 3. The elevated HMG-CoA synthase activities persisted throughout the isolation of liver mitochondria. 4. Intramitochondrial succinyl-CoA content was lower in whole liver homogenates and in mitochondria isolated from animals treated with glucagon or mannoheptulose. 5. HMG-CoA synthase activity in mitochondria from both ox and rat liver was negatively correlated with intramitochondrial succinyl-CoA levels when these were manipulated artificially. Under these conditions, the differences between mitochondria from control and hormone-treated rats were abolished. 6. These findings show that glucagon can decrease intramitochondrial succinyl-CoA concentration, and that this in turn can regulate mitochondrial HMG-CoA synthase. They support the hypothesis that the formation of ketone bodies from acetyl-CoA may be regulated by the extent of succinylation of mitochondrial HMG-CoA synthase.

Mitochondrial function after acute alteration of the endogenous insulin-to-glucagon ratio

Mannoheptulose (2g/kg i.p.) increases serum glucagon and decreases serum insulin via its effect on pancreatic islet cells. These changes in endogenous hormone status had effects on rat liver mitochondria that were comparable to the effects of injecting porcine glucagon (0.5 mg/kg i.p.). Mitochondrial adenine nucleotide content was increased 38 or 39% by mannoheptulose or glucagon respectively, citrulline synthesis by 165 or 193%, pyruvate carboxylation by 113 or 135%, coupled respiration by 34 or 42%, and uncoupled respiration by 40 or 54%. We conclude that the reciprocal changes in endogenous insulin and glucagon brought about by mannoheptulose offer a useful and interesting alternative to glucagon injection for studying the effects of these pancreatic hormones on liver mitochondria.