Effect of Adipose Tissue Site, Animal Weight, and Long-term Fasting on Lipogenesis in the Bovine

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Abstract
The in vitro rate of acetate incorporation into long-chain fatty acids was used to measure the lipogenic capacity of adipose tissue taken from several sites of 278-, 385-, and 528-kg Holstein steers fed ad libitum and from 368-kg Holstein steers that had been fasted for 3 weeks. Effects of site, animal weight, and fasting on the rate of glucose oxidation and activities of glucose-6-phosphate (G-6-P) dehydrogenase and of NADP-isocitrate dehydrogenase also were determined. Subcutaneous adipose tissue (backfat) had a significantly higher lipogenic capacity than did internal (perirenal and omental) and intermuscular tissues which were similar. The lipogenic capacity, expressed on a tissue-weight or cytosol-protein basis, increased linearly with increasing animal weight. Rate of fatty acid synthesis in fasted steers was less than 1% of that in the controls (385-kg steers). The rate of glucose oxidation to CO3 was highly correlated with the rate of fatty acid synthesis. Activity of G-6-P dehydrogenase varied with adipose tissue site, but did not differ significantly with animal weight and was not significantly correlated with either glucose oxidation or fatty acid synthesis. Activity of G-6-P dehydrogenase in fasted steers was about 50% of that in the controls. NADP-isocitrate dehydrogenase activity was reduced 32% by fasting. These data indicate that activities of these NADPH-generating enzymes do not seem rate-limiting for fatty acid synthesis in bovine adipose.