Role of hepatic carbonic anhydrase in de nova lipogenesis

The role of carbonic anhydrase in de novo lipid synthesis was examined by measuring (l-¹⁴C]acetate incorporation into total lipids, fatty acids and non-saponifiable lipids in freshly isolated rat hepatocytes. Two carbonic anhydrase inhibitors, trifluoromethylsulphonamide (TFMS) and ethoxozolamide (ETZ) decreased incorporation of ¹⁴C into total lipids. Both fatty acid and non-saponifiable lipid components of the total lipid were inhibited to approximately the same extent by 100 μM TFMS (29 ± 0.3% and 35 ± 0.3% of control respectively in replicate studies). However, neither drug significantly affected ATP concentrations or the transport activity of Na⁺/K⁺-ATPase, two measures of cell viability. To establish the site of this inhibition, water-soluble ¹⁴C-labelled metabolites from perchloric acid extracts of the radiolabelled cells were separated by ion-exchange chromatography. TFMS inhibited ¹⁴C incorporation into citrate, malate, α-oxoglutarate and fumarate, but had no effect on incorporation of ¹⁴C into acetoacetate. Since ATP citrate-lyase, the cytosolic enzyme that catalyses the conversion of citrate into acetyl-CoA, catalyses an early rate-limiting step in fatty acid synthesis, levels of cytosolic citrate may be rate controlling for de novo fatty acid and sterol synthesis. Indeed citrate concentrations were significantly reduced to 37 ± 6% of control in hepatocytes incubated with 100 μM TFMS for 30 min. TFMS also inhibited the incorporation of ¹⁴C from [l-¹⁴C]pyruvate into malate, citrate and glutamate, but not into lactate. This supports the hypothesis that TFMS inhibits pyruvate carboxylation, i.e. since all of the ¹⁴C from [l-¹⁴C]pyruvate converted into citric acid cycle intermediates must come via pyruvate carboxylase (i.e. rather than pyruvate dehydrogenase). Our findings indicate a role for carbonic anhydrase in hepatic de novo lipogenesis at the level of pyruvate carboxylation.