HEPATIC PURINE ENZYMES AND URIC-ACID EXCRETION AS INDICATORS OF PROTEIN-QUALITY IN CHICKS FED GRADED L-LYSINE DIETS

Indicators of protein metabolism may be used in protein quality determ ination since they respond very rapidly to the quantity and quality of the protein consumed. Accordingly, in this study, the uric acid excre tion and the hepatic activity of the purine enzymes nucleoside phospho rylase (EC 2.4.2.1) and xanthine dehydrogenase (EC 1.2.3.7) measured i n chickens were correlated with the results obtained with two traditio nal methods used in protein quality evaluation such as protein efficie ncy ratio (PER) and net protein ratio (NPR). Data were obtained using diets prepared with single amino acid mixtures which provided 0, 5, 10 , 25, 50 and 100% of the chicken lysine requirements. These mixtures p rovided a gradual increase in protein quality and were also tested in rats. The high correlation coefficients obtained when the results of P ER or NPR were compared with the uric acid excretion or with the activ ities of both enzymes, suggest a practical use of these indicators in protein quality evaluation. This possibility was supported by the fact that the PER and NPR values of the mixtures measured in the chickens were similar to those obtained with rats with correlation coefficients between both species higher than 0.9.