PRECURSOR POOLS OF PROTEIN-SYNTHESIS - A STABLE-ISOTOPE STUDY IN A SWINE MODEL

The accuracy of using other free pools in lieu of tRNA for calculation of tissue protein synthesis in liver (L), skeletal muscle (SM), and h eart (H) was assessed in six adult miniature swine using L-[1-C-13]leu cine and L-[ring-H-2(5)]phenylalanine as tracers. L leucyl-tRNA enrich ment was higher than arterial plasma leucine and ketoisocaproate (KIC) enrichments, and L phenylalanyl-tRNA enrichment was higher than arter ial phenylalanine enrichment (P < 0.05). No such differences were note d in SM and H. Leucyland phenylalanyl-tRNA enrichments in L were best predicted by the respective amino acid enrichments in tissue fluid [TF ; Leu: slope (m) = 0.954 +/- 0.035; Phe: m = 1.011 +/- 0.032] using li near regression analysis to determine the accuracy of the prediction, whereas plasma phenylalanine reasonably predicted phenylalanyl-tRNA (a rtery: m = 0.821 +/- 0.032; vein: m = 0.947 +/- 0.135). In SM, plasma KIC (artery: m = 0.846 +/- 0.046; vein: m = 0.881 +/- 0.043) and TF le ucine (m = 0.788 +/- 0.034) predicted leucyl-tRNA with high accuracy. In H tissue, TF (m = 0.991 +/- 0.044) was the best predictor of leucyl -tRNA enrichment, whereas arterial phenylalanine (m = 0.912 +/- 0.015) was the most reliable predictor of phenylalanyl-tRNA enrichment. The relationships between aminoacyl-tRNA and other free pools in the same species under the same study conditions differ in different tissues. U se of KIC in lieu of leucyl-tRNA for calculating muscle protein synthe sis is supported by this study.