Influence of fasting and diet on lipogenic enzymes in the american eel, Anguilla rostrata LeSueur.

To determine the potential site(s) of fatty acid synthesis and source(s) of reducing equivalents, the activities of the cytoplasmic NADPH producing enzymes--isocitrate dehydrogenase (IDH), malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH), and of aconitase, ATP-citrate lyase (CCE) and malate dehydrogenase (MDH) were measured in homogenates of liver, intestine, visceral fat, red muscle and white muscle of eels (Anguilla rostrata) fed beef liver or worms, or fasted for 2 to 6 months. There were no differences in enzyme activities between eels fed beef liver or fasted for 2 months. Eels fed worms had significantly greater G6PDH activity than fasted eels. Liver size and hepatosomatic index decreased in fasted eels, but lipid content per gram of liver or muscle increased. Based on the total activities of the NADPH producing enzymes, the liver appeared to be the primary organ for lipogenesis, although the intestine contained active lipogenic enzymes as well. In the liver, IDH had the lowest Km (NADP) and highest activity of the NADP-dehydrogenases. In the liver cytoplasm, the low activities of CCE and ME and the presence of an active aconitase, with a 20-fold greater affinity than CCE for citrate, suggest tha citrate cleavage is unimportant and that IDH is a major source of reducing equivalents. The source of carbon for fatty acid synthesis is discussed in relation to these conclusions.