Microsomal desaturation-elongation of linoleic acid following parenteral feeding with lipid emulsions in the rat.

The effect of total parenteral nutrition (TPN) with lipid emulsion to supply either 27.5% or 2% total calories on in vitro desaturation-elongation of 18:2(n-6) by liver microsomes was studied in the rat. The emulsion lipid contained ca. 50% 18:2(n-6) plus 8% 18:3(n-3) or 77% 18:2(n-6) plus 0.5% 18:3(n-3). The reaction rate was influenced by the in vitro substrate concentration and inhibited above 50 mumol 18:2(n-6)/mg microsomal protein. At maximum rates of desaturation-elongation, the formation of triene and tetraene (n-6) fatty acids was reduced in rats given either of the two emulsions. The rate of (n-6) pentaene formation was increased in rats given the emulsion with low 18:3(n-3) but not in rats given the emulsion with 8% 18:3(n-3). Analyses of the microsomal lipid indicated increased free cholesterol in all rats that received TPN. Esterified cholesterol was increased only in rats given 27.5% TPN calories as lipid. Microsomal total phospholipids and phospholipid class distributions were not altered by TPN. The data are consistent with reports of reduced levels of long-chain desaturation-elongation metabolites of 18:2(n-6) in tissue phospholipids following infusion of parenteral lipid. The data suggest that the mechanism may include alterations in other metabolic pathways such as cholesterol, in addition to desaturase enzyme activities, and may be influenced by both the quantity and fatty acid composition of the lipid infused.