Effects of carbohydrate availability on lipogenesis in sheep

Abstract

1. Lipogenesis in sheep liver and adipose tissue was investigated by incorporation studies in vitro with radioactive glucose and acetate and by assays of key enzymes. 2. Carbohydrate availability to sheep was increased by feeding on a diet containing 70% soluble carbohydrate, by infusing glucose into the abomasum or by direct intravenous infusion of glucose. 3. Under these conditions lipogenesis from glucose and acetate was increased from very low values in lìver and adipose tissue, especially in those animals where rumen fermentation was by-passed by glucose infusion. 4. Large increases in the activities of ATP citrate lyase (EC 4.1.3.8) and NADP–malate dehydrogenase (EC 1.1.1.40) occurred in both tissues when lipogenesis was increased. 5. No adaptations were found in the activities of pyruvate carboxylase (EC 6.4.1.1) in adipose tissue, glucokinase (EC 2.7.1.2) in liver or 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30) in liver. It is proposed that the absence of these enzymes is not related to glucose availability. 6. The effect of glucose on liver lipogenesis was to increase conversion of acetate into lipid. 7. This effect also occurred in adipose tissue, but in this tissue glucose also became a quantitatively important precursor of triglyceride fatty acid.