Influence of Varying Levels of Different Dietary Proteins on Growth Rate, Liver Xanthine Oxidase and Succinic Dehydrogenase of Young Rats

Abstract

Growing rats were fed diets containing varying levels of egg albumen, fish meal, soybean protein, wheat gluten and casein supplemented with or without methionine for two weeks. The influence of these variations of dietary protein on growth rate, liver weight, liver nitrogen, the activity of liver xanthine oxidase and that of liver succinic dehydrogenase was observed. When rats received the different dietary proteins studied, except soybean protein, it was shown that the response curve of liver xanthine oxidase or succinic dehydrogenase plotted against the protein levels in the diet was similar to those of growth rates, liver weight and liver nitrogen plotted in a similar way. The correlations of the enzyme activity with the growth rate were higher in xanthine oxidase than in succinic dehydrogenase. With soybean protein, there was no such correlation between the growth rate and the activities of these enzymes as compared with that observed with other proteins examined. When rats were fed casein diets supplemented with methionine, the minimal protein level for the maximal growth and the maximal activity of liver xanthine oxidase was shifted from the 20% casein level to the 12% one. Liver xanthine oxidase could be utilized as a criteria for the quantity of dietary protein required for the maximal growth of young rats. This procedure, however, has a limitation, because other factors besides protein level and protein adequacy can influence the enzyme activity, as in the case of soybean protein.