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Purified catalase and peroxidase were denatured by heat, acid and urea. Denaturation resulted in up to 22-fold increase in nonenzymatic lipid oxidation activity concomitant with loss of enzymatic activity. It is proposed that the increased nonenzymatic activity is due to increased exposure of the heme group. Acid-splitting of the hemoproteins into apoprotein and heroin had the greatest influence on both of the catalytic activities and recombination reversed the effect. Urea-denatured hemoprotein possessed increased nonenzymatic activity due to increased exposure of the protein-bound heine, however, peroxidase increased less than catalase which is consistent with the fact that peroxidase is the more heat stable enzyme. Nonenzymatic activity of the heat denatured hemoproteins was maximum when catalase was treated at 90 C for 2 min and peroxidase at 100 to 125 C for 5 to 30 min.

The hemoproteins catalase and peroxidase, after heat treatment which decreased their enzyme activities, became more efficient as heme catalysts of linoleic acid oxidation. These effects might be of importance for preservation and storage of food.

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