Zonation of acetate labeling across the liver: implications for studies of lipogenesis by MIDA

Measurement of fractional lipogenesis by mass isotopomer distribution analysis (MIDA) of fatty acids or cholesterol labeled from [C-13]acetate assumes constant enrichment of lipogenic acetyl-CoA in all hepatocytes. This would not be the case if uptake and release of acetate by the liver resulted in transhepatic gradients of acetyl-CoA enrichment. Conscious dogs, prefitted with transhepatic catheters, were infused with glucose and [1,2-C-13(2)]acetate. Stable concentrations and enrichments of acetate were measured in artery (17 mu M, 36%), portal vein (61 mu M, 5.4%), and hepatic vein (17 mu M, 1.0%) and were computed for mixed blood entering the liver (53 mu M, 7.4%). We also measured balances of propionate and butyrate across gut and liver. All gut release of propionate and butyrate is taken up, by the liver. The threefold decrease in acetate concentration and the sevenfold decrease in acetate enrichment across the Liver strongly suggest that the enrichment of lipogenic acetyl-CoA decreases across the liver. Thus fractional hepatic lipogenesis measured in vivo by MIDA may be underestimated.