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1. Sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (B807–27 or POCA) inhibits ketogenesis from endogenous and exogenous long-chain fatty acids and 14CO2 production from [U-14C]palmitate, but not from [U-14C]palmitoylcarnitine or octanoate, and inhibits gluconeogenesis from lactate and pyruvate in perfused livers of starved rats. 2. Inhibition of ketogenesis, 14CO2 production and gluconeogenesis was complete at concentrations of 10 μmol/l POCA, but onset was more rapid for inhibition of ketogenesis and CO2 production than for gluconeogenesis. Infusion of octanoate abolished inhibition of all three processes. 3. Experiments with isolated rat liver mitochondria showed that carnitine palmitoyltransferase I (EC 2.3.1.21) is inhibited by POCA-CoA. The inhibitory process is dependent on time and concentration, and more pronounced in mitochondria from fed than from fasted rats. Concentrations required for 50% inhibition after 20 min preincubation with POCA- CoA are 0.02, 0.06 and 0.1 μmol/l in liver mitochondria from fed, 24-h-fasted and 48-h-fasted rats, respectively. The inhibitor appears to be tightly bound to the enzyme. 4. The extent of inhibition of carnitine palmitoyltransferase I correlates well with the hypoglycaemic and hypoketonaemic effects of the compounds in fasted rats. We conclude that specific inhibition of the enzyme leads, due to inhibition of long-chain fatty acid utilization, to depressed ketogenesis and gluconeogenesis and, in consequence, to hypoglycaemic and hypoketonaemia in vivo under gluconeogenic and ketogenic conditions. [ABSTRACT FROM AUTHOR]