Significance of Adipose Tissue and Liver as Sites of Fatty Acid Synthesis in the Pig and the Efficiency of Utilization of Various Substrates for Lipogenesis

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Abstract
A combination of in vivo and in vitro techniques as well as the assay of the activity of several enzymes presumably involved in lipogenesis have been employed to study the relative importance of liver and adipose tissue in overall fatty acid synthesis in the pig. Both the in vivo and in vitro results indicate that when glucose-U-14C is used as substrate, virtually all the newly synthesized fatty acids are formed in the adipose tissue. The incorporation of acetate-1-14C into liver fatty acids was, however, much greater than that of glucose-U-14C, suggesting that, if acetate was freely available in vivo, the contribution of liver to overall lipogenesis may be appreciable. Data on the activity of citrate cleavage enzyme and of three NADPH-generating dehydrogenase enzymes in liver and adipose tissue complement the results obtained in the lipogenic studies. The hepatic capacity for the production of cytoplasmic acetyl CoA from mitochondrially derived citrate is insignificant, as is its ability to generate NADPH required in the reductive synthesis of fatty acids. Collectively the results indicate that the adipose tissue plays a major, if not a nearly exclusive role in fatty acid synthesis in the pig. Nonsaponifiable lipid synthesis in the liver requires acetate rather than glucose as a starting substrate.