Quantification of cytosol and membrane proteins in rumen epithelium of sheep with low or high CH(4) emission phenotype

BACKGROUND: Ruminant livestock are a major contributor to Australian agricultural sector carbon emissions. Variation in methane (CH(4)) produced from enteric microbial fermentation of feed in the reticulo-rumen of sheep differs with different digestive functions. METHOD: We isolated rumen epithelium enzymatically to extract membrane and cytosol proteins from sheep with high (H) and low (L) CH(4) emission. Protein abundance was quantified using SWATH-mass spectrometry. RESULTS: The research found differences related to the metabolism of glucose, lactate and processes of cell defence against microbes in sheep from each phenotype. Enzymes in the methylglyoxal pathway, a side path of glycolysis, resulting in D-lactate production, differed in abundance. In the H CH(4) rumen epithelium the enzyme hydroxyacylglutathione hydrolase (HAGH) was 2.56 fold higher in abundance, whereas in the L CH(4) epithelium lactate dehydrogenase D (LDHD) was 1.93 fold higher. Malic enzyme 1 which converts D-lactate to pyruvate via the tricarboxylic cycle was 1.57 fold higher in the L CH(4) phenotype. Other proteins that are known to regulate cell defence against microbes had differential abundance in the epithelium of each phenotype. CONCLUSION: Differences in the abundance of enzymes involved in the metabolism of glucose were associated with H and L CH(4) phenotype sheep. Potentially this represents an opportunity to use protein markers in the rumen epithelium to select low CH(4) emitting sheep.