METABOLISM OF ASPARAGINE AND GLUTAMINE IN GROWING RATS AT VARIOUS DIETARY-PROTEIN LEVELS

The metabolic fate of the carbon skeletons of L-[U-C-14]asparagine and L-[U-C-14]glutamine was investigated in growing rats fed with diets containing different percentages of protein calories (0, 5, 10, 15 and 30 PC%) at 4100 kcal of metabolizable energy per kg of diet. The incorporation of C-14 into the body protein 12 hr after injecting C-14-labeled asparagine was high in the lower PC% groups, about 60% of the dose being recovered, but the value decreased in the higher PC% groups. Most of the radioactivity incorporated into the tissue protein was present in the aspartic acid and/or asparagine fraction. The pattern of expired (CO2)-C-14 production was in inverse relation to that of C-14 incorporation into the body protein. The incorporation of C-14-glutamine into the body protein was markedly less in all the dietary groups, but the expired (CO2)-C-14 production was extremely high, more than 70% of the dose being oxidatively degraded. These results show that the metabolic response of glutamine to dietary protein well resembles that of glutamic acid, aspartic acid or alanine, but markedly differs from that of asparagine, which is preferentially used for body protein synthesis, especially during protein depletion.