Oxidative Enzymes of the Liver in Protein Depletion

Abstract

The activities of the following oxidative enzymes or enzyme systems of rat liver have been studied after the experimental animals spent 49 to 53 days on a protein-free diet accompanied by sub-acute food restriction: cytochrome oxidase, succinoxidase, succinic dehydrogenase, D-amino acid oxidase, DPN-cytochrome c reductase, uricase, xanthine oxidase and pyruvate oxidation. Pair-fed animals and ad libitum-fed animals served as controls. The results have been expressed on the basis of the activity per milligram of nitrogen (the unit activity) and the activity per liver (the total activity). A comparison of the depleted animals and the pair-fed controls on the basis of unit activity reveals that the activities of the enzyme systems decrease under the experimental conditions employed, except that the activity of cytochrome oxidase increases, and xanthine oxidase and pyruvate oxidation by the residue preparation show no change. The xanthine oxidase activities were markedly reduced in the livers of both groups. The general decrease in activity per unit of nitrogen indicates that the enzyme proteins were lost more rapidly than was the total protein. Cytochrome oxidase protein was conserved. Sub-acute food restriction (a comparison of pair-fed controls and ad libitum-fed controls) had no effect on unit activities, except that in the case of xanthine oxidase the unit activity was markedly reduced. Protein depletion decreases the total liver activity of 5 of these systems. The total cytochrome oxidase activity is not affected, and the total xanthine oxidase activity is so markedly reduced by the small restriction in the food intake that the further omission of all of the dietary protein has no apparent significant effect. The food restriction suffered by the pair-fed animals, with the ad libitum-fed animals as controls, decreased the total activities of 4 enzymes (cytochrome oxidase, DPN-cytochrome c reductase, uricase and xanthine oxidase). The total succinoxidase activity was unchanged. The differences for succinic dehydrogenase and D-amino acid oxidase are not considered significant in spite of the fact that the livers of the restricted rats contained significantly less protein than those of the controls.