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

Abstract

1. Whole-body protein synthetic rates in non-pregnant ewes were determined by the continuous infusion of L-[ 15 N]- and [1- 13 C]leucine and measuring the plasma enrichment of leucine, α-ketoisocaproate (α-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 [ 13 C]leucine flux. Deamination of leucine was 0·55 (SE 0·035) mmol/kg per d and accounted for approximately 17% of plasma [ 15 N]leucine flux. 4. The rate of α-KIC reamination to leucine, calculated by subtracting 13 C flux from 15 N 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.