Regulation of ketone body production from (14C)palmitate in rat liver mitochondria: effects of cyclic nucleotides and unlabeled fatty acids

Importance of albumin binding in the assay for carnitine palmitoyltransferase

Alterations in the long-chain acyl-CoA binding to albumin in the carnitine palmitoyltransferase (CPT) assay appreciably affect the reaction at commonly used substrate concentrations. Since in the CPT assay the latter are typically well below saturation or Vmax. values, the measured enzyme activity depends on both the absolute quantity of albumin in the CPT assay and any biochemical modification of its binding. The present study verifies the striking dependence of the K0.5 for palmitoyl-CoA on albumin and the misleading 'activation' of the enzyme by compounds that also avidly bind to albumin. In assessing the intracellular physiological relevance of any modifier of CPT, the effects of protein binding in the assay assume particular importance. Indeed, any compound that alters CPT activity may do so, not directly, but as an assay artifact changing the free or unbound substrate concentrations.

Effects of dibutyryl adenosine 3',5'-monophosphate on hepatic metabolism of free fatty acids

The effects of dibutyryl cyclic adenosine 3',5'-monophosphate (Bu2cAMP) on metabolism of free fatty acids by perfused livers from normal fed male rats were investigated. In one group of experiments, Bu2cAMP was added to the medium and infused at a constant rate to maintain concentrations of 0, 0.4, 1.0, 4.0, or 10. 0 X 10(-5) M nucleotide in the perfusate plasma, assuming the nucleotide was not metabolized by the liver. Oleic acid was infused as the complex with albumin at the rate of 124.3 mumoles/hr. Uptake of free fatty acid by the liver was identical in all groups. Production of ketone bodies, however, increased, and output of triglyceride decreased with increasing concentration of Bu2cAMP. The nucleotide also stimulated output of glucose. Maximal effects were observed when the concentration of Bu2cAMP was approximately 2-3 X 10(-5) M. The output of very low density lipoproteins, as judged by flotation in the zonal ultracentrifuge, was also diminshed by the nucleotide. In other experiments, 1-14C-oleate was infused (120.8 mumoles/hr) along with 2 X 10(-5) M Bu2cAMP, and the disposition of 14C into CO2, ketone bodies, and esterified lipids was evaluated. Bu2cAMP depressed the proportion of 1-14C-oleate converted to triglyceride and increased the fraction converted to ketone bodies and CO2. Not only was ketogenesis stimulated, but a larger proportion of the ketone bodies was derived from exogenous fatty acid.

Inhibition of carbon dioxide fixation by lead acetate in rat liver mitochondria

These studies were undertaken to determine the mechanism by which intravenously administered lead salts inhibit hepatic gluconeogenesis. Within 1 h after the intravenous administration of lead acetate (10 mg), there is 97% inhibition of CO2 fixation in isolated rat liver mitochondria. This effect is concentration-dependent. The induction of phosphoenolpyruvate carboxykinase activity observed with starvation was also inhibited by intravenously administered lead acetate, but the activities of pyruvate kinase, glucose 6-phosphate dehydrogenase and pyruvate carboxylase were unaffected, as was the oxidation of palmitate and palmitoyl-CoA by mitochondria from Pb2+-treated animals. The addition of reduced glutathione to mitochondria from Pb2+-treated animals had no effect on the inhibited CO2 fixation. ATP concentrations in mitochondria from Pb2+-treated animals are decreased and the dose-response relationships for the effect of Pb2+ on CO2 fixation and ATP concentrations correspond. We conclude that the decrease in mitochondrial ATP in Pb2+-treated animals is probably responsible for the marked inhibition ov CO2 fixation, and hence the impairment of gluconeogenesis from alanine, lactate and pyruvate observed by others.