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 heart (H) was assessed in six adult miniature swine using L-[1-13C]leucine and L-[ring-2H5]phenylalanine as tracers. L leucyl-tRNA enrichment was higher than arterial plasma leucine and ketoisocaproate (KIC) enrichments, and L phenylalanyl-tRNA enrichment was higher than arterial phenylalanine enrichment (P < 0.05). No such differences were noted in SM and H. Leucyl- and 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 linear regression analysis to determine the accuracy of the prediction, whereas plasma phenylalanine reasonably predicted phenylalanyl-tRNA (artery: 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 leucine (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. Use of KIC in lieu of leucyl-tRNA for calculating muscle protein synthesis is supported by this study.

High-performance liquid chromatographic technique for non-derivatized leucine purification: evidence for carbon isotope fractionation.

A high-performance liquid chromatographic (HPLC) method, utilizing an ion-pairing agent with a reversed-phase column, isocratic elution and ultraviolet detection, was developed for the separation of non-derivatized leucine from a mixture of amino acids. Fractionation of leucine isotopes during HPLC separation was observed, creating potential problems for collection of leucine plasma or tissue protein hydrolyzates for subsequent isotopic determinations. It was demonstrated that [13C]leucine and [12C]leucine have different retention times (p less than 0.01). It is concluded from this study that partial collection of leucine eluted from an HPLC column will result in erroneous estimation of the isotopic enrichment of leucine.