1. The administration of CoCl2 to rats caused a decrease in hepatic catalase activity as well as a decrease in the amount of catalase protein as measured by immunological assay. The mitochondrial enzyme decreased progressively over 2 days, whereas the cytosol enzyme decreased over 12h and then remained essentially unchanged for 2 days after a single injection of cobalt. 2. Incorporation of [14C]glycine into catalase haem was dramatically decreased by a single injection of cobalt, but that into catalase protein remained essentially unaltered. 3. Incorporation of [3H]leucine into liver protein increased in rats in a steady state receiving a daily injection of cobalt, which was in contrast with a marked inhibition observed in 5-amino[3H]laevulinate incorporation. 4. The initial rate of [3H]leucine incorporation into mitochondrial and cytosol catalase did not alter or was slightly depressed in the cobalt-treated animals, whereas the incorporation of 5-amino[3H]laevulinate into mitochondrial and cytosol catalase was conspicuously decreased, indicating that haem synthesis was limiting catalase formation. 5. The degradation rate of catalase protein, as measured by a double-labelling method, was not changed by the cobalt treatment.

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