Enzymes in Protein Depletion

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Abstract
The cytochrome oxidase, succinate-cytochrome c reductase, and DPNH-cytochrome c reductase activities of brain, kidney, skeletal muscle and spleen, and cholinesterase of brain, D-amino acid oxidase of kidney, and aldolase of skeletal muscle and heart ventricle were assayed after rats had been fed a protein-free diet for 49 days. The effects of sub-acute food restriction were interpreted with the aid of pair-fed animals which were given a diet containing 18% casein. Animals fed the 18% casein diet ad libitum served as further controls. Food restriction alone had little or no effect on the unit activities of the enzymes. Some unit activities rose slightly, others fell. Food restriction did, however, slightly decrease the total enzyme activities, since the organs of the pair-fed animals (brain excepted) were somewhat smaller than the organs of the animals fed ad libitum. Protein depletion with its attendant food restriction had no effect on the unit activities of the enzymes of brain, but it did lower the activities of the enzymes of kidney, skeletal muscle and spleen by about 10 to 20%. Since the weights of the organs were also reduced by protein depletion, brain excepted, the total enzyme activities of kidney, skeletal muscle and spleen were reduced by about 50 to 60%. The total enzyme activities of brain were reduced by about 5 to 10%. A comparison of these results with those presented in the first two papers of this series reveals that the total protein and enzymes of liver are the most labile in protein depletion (cytochrome oxidase excepted), while the total protein and cholinesterase of brain are very resistant. The total protein and enzymes of heart ventricle are more resistant than those of kidney, skeletal muscle and spleen, and almost as resistant as those of brain.