Determination of leucine metabolism and protein turnover in sheep, using gas-liquid chromatography-mass spectrometry

1. Whole-body protein synthetic rates in non-pregnant ewes were determined by the continuous infusion of L-[15N]- and [1-13C]leucine and measuring the plasma enrichment of leucine, alpha-ketoisocaproate (alpha-KIC) and expired carbon dioxide by gas-liquid chromatography-mass spectrometry. 2. The mean whole-body protein synthesis estimated from plasma leucine flux corrected for oxidation was 5.38 (SE 0.54) g/kg per d. 3. Under the conditions of the present study leucine oxidation was 0.323 (SE 0.067) mmol/kg per d and accounted for 10.71 (SE 2.26) % of plasma [13C]leucine flux. Deamination of leucine was 0.55 (SE 0.035) mmol/kg per d and accounted for approximately 17% of plasma [15N]leucine flux. 4. The rate of alpha-KIC reamination to leucine, calculated by subtracting 13C flux from 15N flux, was 0.228 (SE 0.101) mmol/kg per d. 5. The rate of whole-body protein degradation was 4.49 (SE 0.54) g/kg per d and there was a net protein gain of 0.89 (SE 0.21) g/kg per d.