Lipogenesis in the Ruminant: in vitro Study of Tissue Sites, Carbon Source and Reducing Equivalent Generation for Fatty Acid Synthesis

Abstract

The lipogenic capacity of liver and adipose tissue was measured in vitro using tissue from ovine and bovine of different ages and physiological states. Adipose tissue had the greatest rate of fatty acid synthesis in all groups of nonlactating animals with lipogenesis in liver representing about 1% of the adipose rate. In lactating animals, mammary tissue was the most active. A comparison of the lipogenic capacity of adipose tissue obtained from different body sites indicated that the internal fat depots were most active in younger animals (lambs and calves) while the subcutaneous fat sites tended to have the greatest activity in mature ruminants (sheep and steers). Substrate studies indicated that acetate was the predominant carbon source for lipogenesis in ruminant adipose tissue from both internal and subcutaneous fat depots. The negligible glucose incorporation into fatty acids, even in the absence of acetate, was found to coincide with the low activities of ATP-citrate lyase and NADP-malate dehydrogenase. The activities of NADPH-generating enzymes were determined in adipose tissue samples from lambs, sheep, and calves. A high NADP-isocitrate dehydrogenase activity relative to the pentose pathway dehydrogenases was found. It is suggested that acetate may be furnishing a portion of the NADPH needed for lipogenesis via NADP-isocitrate dehydrogenase in ruminant adipose tissue.