Compartmentation of 14CO2 in the perfused rat liver.

The specific activity of the mitochondrial CO2 + bicarbonate system has been measured in perfused livers using the specific activities of urea and acetoacetate derived from 2-ketoisocaproate catabolism. Label was supplied either as NaH14CO3, 2-keto[1-14C]isocaproate, [1-14C]pyruvate, [1-14C]glutamine, or [14C]formate. With labeled bicarbonate, pyruvate, or 2-ketoisocaproate, the specific activities of effluent bicarbonate, urea, and acetoacetate were equal (acetoacetate was labeled only on C-1). In the presence of [14C]formate, the specific activity of acetoacetate was double that of urea. Acetazolamide (0.2 mM), an inhibitor of carbonic anhydrase, decreased the specific activities of urea and acetoacetate labeled from NaH14CO3 and increased the specific activities of urea and acetoacetate labeled from the other tracers. We conclude that: acetoacetate derived from 2-ketoisocaproate is, like urea, an index of the specific activity of mitochondrial CO2 in liver, carbonic anhydrase activity equalizes the specific activities of the CO2 + bicarbonate system on both sides of the mitochondrial membrane, and a fraction of [14C] formate-derived 14CO2 appears to be generated in a mitochondrial compartment, in the close vicinity of methylcrotonyl-CoA carboxylase.