Biochemical aspects of energy utilisation in ruminants
Scollan, Nigel David
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The activities of acetyl-CoA hydrolase and acetyl-CoA synthetase and the influence of diet and feeding level on them were investigated in various ovine tissues and used to determine both the potential rate of substrate cycling between acetate and acetyl-CoA and the contribution of this cycle to energy expenditure in the ruminant. Two experiments, using lambs, were conducted in an attempt to further understand biochemical pathways which may influence the efficiency of utilisation of ME and explain why this is lower for fibre (sugarbeet pulp) when compared to starch (barley) based diets. The influence of carbohydrate source, feeding level and protein level on plasma concentrations of acetate, glucose and insulin and the rates of acetate incorporation into CC>2 and lipid were studied.ATP-stimulated acetyl-CoA hydrolase is present in rumen epithelium, muscle and the cytoplasm of ovine liver but not in perirenal adipose tissue, and it is not inactivated by cold. "Mitochondrial" acetyl-CoA hydrolase was detected in all tissues investigated. The activities of acetyl-CoA hydrolase and acetyl-CoA synthetase tended to be higher in perirenal adipose tissue of lambs fed on sugarbeet diets and their activities decreased with increasing level of feeding (P<0.05)Variation in plasma characteristics were more easily observed in lambs fed semi- continuously compared to lambs which received their ration once daily. Higher levels of crude protein in the diet resulted in lower plasma acetate concentrations (P<0.05). The rates of acetate incorporation into CC>2 and lipid were influenced by acetate (P<0.01), glucose (P<0.0001) and insulin (P<0.01). Higher levels of crude protein in the diet resulted in greater rates of acetate incorporation into lipid (P<0.05) whereas feeding sugarbeet pulp resulted in increased rates of acetate incorporation into CC>2 (P<0.05).A technique involving open column ion exchange chromatography, freeze drying and HPLC was developed for the concentration and separation of plasma organic acids. Organic acid recoveries were 43-69%.An experiment was conducted to investigate the activity of the substrate cycle between acetate and acetyl-CoA in calf liver in vivo. Several problems were encountered, notably huge variation in blood flows (including negative rates). This introduced large variation into the calculated fluxes and no meaningful conclusions were made.It was calculated from enzyme measurements made in vitro, that the substrate cycle in ovine liver may potentially account for 2.5% of basal heat production. It is suggested that the efficiency of utilisation of ME is related to glucose homeostasis, involving VFA and protein metabolism.