Assay of Biologic Value of Milk Proteins by Liver Xanthine Oxidase Determination

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Abstract
Because the biologic value of milk proteins in diets prepared for infant feeding that resemble human milk in composition is difficult to evaluate by nitrogen balance methods in the rat, an indirect approach to the resolution of this problem was sought in the measurement of liver xanthine oxidase activity. Whole egg protein and milk proteins, as processed for powdered infant foods, fed to rats either protein-depleted or transferred from a stock diet, give liver xanthine oxidase values in keeping with known biologic values reported for these two types of protein. Liver xanthine oxidase activity reflects the biologic value of dietary proteins. A decrease in liver xanthine oxidase is paralleled by a decrease in the content of total liver nitrogen. Liver xanthine oxidase activity permits study of protein biologic value of diets that normally produce diarrhea in the rat. Prior protein depletion of the adult rat does not increase the sensitivity of liver xanthine oxidase activity as an index of protein quality.