Effect of clofibrate on CO2 fixation into glycogen and fatty acids via the leucine catabolism pathway in Tetrahymena.

Tetrahymena pyriformis were grown to stationary phase and then incubated for 17 h with 0.21 mM clofibrate, a concentration that causes considerable growth inhibition when added to exponentially growing cells. After the clofibrate treatment, the cells were resuspended in a salt solution and the incorporation of label from [1-(14)C]leucine, [1-(14)C]tyrosine, [1-(14)C]pyruvate, and [14C]bicarbonate into glycogen and into the fatty acid and glycerol moieties of lipids was measured. Each of these substrates yields 14CO2 at an early step of its catabolism, so that incorporation of label into these products is a measure of CO2 fixation. Clofibrate-treated cells incorporated a 2- or more-fold label from leucine, tyrosine, and bicarbonate into the fatty acid moieties of the lipids than did control cells, but only slightly more into the glycerol moiety. Because the only pathway for CO2 fixation into fatty acids in Tetrahymena is via leucine degradation, these results demonstrate that clofibrate increases CO2 fixation via the leucine degradative pathway. Clofibrate treatment reduced 14CO2 formation from [1-(14)C]-labeled glucose, ribose, and glycerol by about 30--40%, but not from [1-(14)C]-labeled glyoxylate, acetate, hexanoate, or octanoate. Incorporation of label from each of these substrates (and from tyrosine and leucine) into glycogen was increased (1.2-fold for glucose, up to 3.2-fold for octanoate) by clofibrate treatment. In addition to the increase in 14CO2 fixation via the leucine catabolic pathway, these results show that clofibrate does not appreciably alter flux through the Krebs cycle or the glyoxylate bypass, but increases glyconeogenic capacity and inhibits glycolytic capacity.