Studies on the Turnover Rates of Cytosolic and Mitochondrial Fumarases in Rat Liver1

The turnover rates of mitochondrial and cytosolic fumarase in the rat liver were determined by injecting L-[U-14C]leucine and following the decay of specific radioactivity incorporated into immunoprecipitates from the partially purified enzymes. The half-life of mitochondrial fumarase (t 1/2 = 9.7 days) was significantly different from that of the cytosolic enzyme (t 1/2 = 4.8 days). Studies on the incorporation of radioactive leucine into fumarase in the liver under steady-state conditions showed that the rate of synthesis of this enzyme in cytosol was about 2 times higher than that in the mitochondrial enzyme. The results showed that the mitochondrial fumarase turns over considerably more slowly than the cytosolic enzyme in the rat liver. These results suggest that the turnover of two fumarases with different localizations may be under different and independent control systems. In the case of the mitochondrial fumarase, the decay curve of its specific radioactivity obtained by single injection of L-[U-14C]leucine was quite unusual. No change was observed in the specific radioactivity of the mitochondrial fumarase for about 7 days after pulse labeling, then the specific radioactivity decreased exponentially with a half-life of 9.7 days.