MEASUREMENT OF MUSCLE PROTEIN FRACTIONAL SYNTHETIC RATE BY CAPILLARY GAS-CHROMATOGRAPHY COMBUSTION ISOTOPE RATIO MASS-SPECTROMETRY

The measurement of skeletal muscle protein fractional synthetic rate using an infusion of (1-C-13)leucine and measuring the isotopic abundance of the tracer in skeletal muscle protein by preparative ps chromatography (GC)/ninhydrin isotope ratio mass spectrometry (IRMS) is laborious and subject to errors owing to contamination by C-12. The purpose of this study was to compare muscle (C-13)leucine enrichment measured with the conventional preparative GC/ninhydrin IRMS approach to a new, continuous-flow technique using capillary GC/combustion IRMS. Quadriceps muscles were removed from four Sprague-Dawley rats after each was infused at a different rate with (1-C-13)leucine for 6-8 h. Muscle leucine enrichment (at.% excess) measured by both methods differed by less than 4%, except at low (C-13)leucine enrichments (< 0.03 at.% excess). In addition, capillary GC/combustion IRNIS was used to assess muscle (C-13)leucine enrichment and fractional muscle protein synthesis rate in ten normal young men and women infused with (1,2-C-13(2))leucine for 12-14 h. This approach reduced the variability of the isotope abundance measure and gave estimates of muscle protein synthesis rate (0.050 +/- 0.011% h-1 (mean +/- SEM); range = 0.023-0.147% h-1) that agree with published values determined using the standard analytical approach. The measurement of (C-13)leucine enrichment from skeletal muscle protein by capillary GC/combustion IRMS provides a simple, acceptable and practical alternative to preparative GC/ninhydrin IRMS.