Lipogenesis in the Ruminant: Effect of Fasting and Refeeding on Fatty Acid Synthesis and Enzymatic Activity of Sheep Adipose Tissue

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Abstract
The metabolic control of lipogenesis was investigated in growing and fattening lambs by fasting and refeeding studies. Fasting state was monitored by indirect calorimetry measurements of respiratory quotient and methane production. At various time periods, rump adipose tissue was obtained by biopsy and used for determination of lipogenic capacity and the activities of relevant enzymes. The activity of acetyl CoA carboxylase very closely paralleled the progressive decrease in fatty acid synthesis by adipose tissue slices during starvation and the increase upon refeeding. Acetyl CoA synthetase and fatty acid synthetase activities also decreased substantially during fasting, but in fattening lambs, this decrease did not occur until after 48 hours of fasting. Additionally, the activities of acetyl CoA synthetase and fatty acid synthetase did not increase significantly with refeeding. In both groups of lambs the activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and NADP-isocitrate dehydrogenase decreased only moderately during starvation and had not increased significantly at the time of biopsy after refeeding. The results of these investigations indicate that acetyl CoA carboxylase is the rate limiting enzyme in fatty acid synthesis by ruminant adipose tissue. The correlation between in vitro lipogenic capacity and acetyl CoA carboxylase activity during fasting and refeeding was 0.95 and 0.93 with growing and fattening lambs, respectively.