Production of endogenous acetate by the liver in lactating ewes

The production of endogenous acetate by the liver has been investigated in lactating ewes using animals with indwelling arterial, and portal and hepatic venous cannulae. The capacity of the liver to produce acetate from acetyl-CoA in vitro has also been examined using homogenates prepared from liver biopsy samples. Mean arterial, portal and hepatic venous blood acetate concentrations in four ewes at 4 weeks lactation were 0.40, 1.00 and 1.46 mM respectively. The mean exogenous and endogenous acetate production rates were 56 and 54 mmol/h respectively, giving a total of 110 MMOL/h. The mean portal-hepatic venous difference in free fatty acid concentration was 81 muM. Converting this uptake of free fatty acids by the liver (based on palmitate as a standard) to 2-carbon equivalents, the acetate produced accounted for 70% of the fatty acids taken up. The correlation coefficient (r2) between uptake of free fatty acids and production of acetate by the liver was 0.83 (P less than 0.01). Calculation of the net acetate production in vivo gave a mean value for the production of acetate of 0.75 nmol/min. Calculation of the in vitro enzymic capacity of the liver to produce acetate from acetyl-CoA gave a mean of 0.94 mmol/min. These results indicate that enzymic production of acetate from acetyl-CoA, via carnitine acetyltransferase and acetylcarnitine hydrolase (see Costa and Snoswell 1975a), can adequately account for the substantial production of acetate by the liver in lactating ewes.