Comparisons of lipogenesis and glucose metabolism between ovine and bovine adipose tissues

Studies were initiated to compare glucose and lipid metabolism in vitro in subcutaneous adipose tissue of mature sheep and cattle. Mean adipocyte volume was significantly less in subcutaneous adipose tissue of sheep than in adipose tissue from cattle. The presence of acetate and lactate in the incubation medium increased total glucose utilization two- to three-fold in ovine adipose tissue, but had no effect on total glucose utilization in adipose tissue from cattle. Acetate provided 72-82% of the acetyl units to lipogenesis, depending on species and substrate concentration. There were no significant (P greater than 0.05) differences in the contribution of the pentose cycle to the provision of reducing equivalents to fatty acid biosynthesis, based on the incorporation of label from [3-3H]glucose into fatty acids. In ovine adipose tissue, acetyl-CoA carboxylase appeared to be rate-limiting to lipogenesis, while in bovine subcutaneous adipose tissue, the activity of fatty acid synthetase may have been the limiting step in lipogenesis. In addition, the low activity of ATP-citrate lyase, especially relative to aconitate hydratase, probably limited the conversion of lactate to fatty acids in ovine adipose tissue. It is unlikely that ATP-citrate lyase activity was rate-limiting to lipogenesis from lactate in bovine adipose tissue. The data indicate that extending the results obtained from adipose tissue from one species to lipid metabolism in ruminants in general may not be valid.