Dietary Influences on Fatty Acid Metabolism in the Liver of the Neonatal Lamb

Abstract

A ‘protected’ polyunsaturated fatty acid supplement was fed to ewes during the last 8 weeks of pregnancy. The effect of this dietary supplement on the essential fatty acid composition of the liver phospholipids and associated changes in the liver Δ9- and Δ6-desaturase activities were investigated and compared with the effects of directly dosing the newborn lambs for 8 days with an unprotected polyunsaturated oil, maize oil. The maternal dietary supplement produced significant changes in the essential fatty acid composition of the neonatal lamb liver phospholipids. Although the liver Δ6-desaturase activity in these animals was also significantly higher than in the lambs from control ewes, the Δ9-desaturase was unaffected by maternal dietary supplementation. Direct dosing of the newborn lambs with maize oil failed to produce any significant changes either in the liver phospholipid fatty acid composition or in the Δ9- and Δ6-desaturase activities. These results are discussed in terms of the dietary control of liver desaturase enzyme systems.

Δ9- and Δ6-Desaturase Activities of the Ovine Placenta and Their Role in the Supply of Fatty Acids to the Fetus

“…The A9-desaturase activity of the sheep pla cental tissue studied in the present investiga tions was significantly higher per weight of tissue than the level of desaturase activity pre viously observed in neonatal lamb liver prepara tions (13) and, as reported here and elsewere (17), is also higher than the level of A9-desaturation in adult sheep liver. It is possible that even higher levels of incorporation of the (1-,4C)18:0 substrate into 18:1 (n-9) would have been obtained had not the substrates and cofac tors necessary for chain elongation been in cluded in the incubation medium allowing the further metabolism of the synthesised 18:1 (n -9 ).…”

“…In the sheep, the overall A6-desaturation and chain elongation of 18:2 (n -6 ) to 20:4 (n -6 ) shown to occur in the placental homogenates was also significantly higher than the A6-desaturase activity observed in homogenates of liver from adult animals and from early neonatal lambs (13) and a correspondingly higher pro portion of the (1-14C) 18:2 (n -6) substrate was incorporated into the intermediates of this fatty acid conversion by the placental tissue than was observed previously with neonatal lamb liver. Preliminary results (unpublished work) have shown that of the 39% of the radioactivity incorporated into these inter mediates, a high proportion (about 28%) cor responded chromatographically to 7-linolenic acid -18:3 (n-6) -suggesting that the rate determining step in the conversion of 18:2 (n -6 ) to 20:4 (n -6 ) in the placenta of the sheep is the elongation of 18:3 (n -6 ) and not the initial desaturation of 18:2 (n -6 ) as has been observed in rat liver (5).…”

“…Substrates and Assay Procedure for Desaturase Activities 1 ml of placental or liver homogenate was added to 1 ml of the following assay mixture: ATP (3.5 mAf); coenzyme A (0.1mA/); NADH (0.5 mA/); NADPH (0.5 mA/); acetyl coenzyme A (100 mW : malonyl co enzyme A (100 mA/); l-14C-stearic or linoleic acids (0.5 iiCi) as their complexes with bovine serum al bumin (13), and MgCl: (2.4 mA/).…”

“…It must be concluded, therefore, that a secondary source of 20:4 (n -6 ), via the A6-desaturation and elongation of 18:2 (n -6 ) in the placenta or fetal tissues, supplies the fetal lamb with its requirements for 20:4 (n-6). However, recent work has shown that the pla cental transfer of 18:2 (n -6 ) from mother to fetus in the sheep is very low (10) and in vitro experiments have failed to show any substantial desaturation of 18:2 (n -6 ) in the livers of newborn lambs (13). By elimination, therefore, it seems likely that the placenta is the active site for the synthesis of 20:4 (n -6 ) from maternally available 18:2(n-6).…”

Incorporation of Linoleic and Arachidonic Acids into Ovine Placental Phospholipids in vitro

“…However, it is also clear that linoleic acid and its longer-chain metab olites must cross the placenta since signifi cant proportions of linoleic and arachidonic acids are found in the lipids of the plasma and tissues of the fetal lamb, and the levels of these acids can be increased by feeding a ■protected' polyunsaturated fatty acid diet to the ewe during the last 8 weeks of pregnancy [3][4][5], These studies have also shown that the main source of pre-formed fatty acids for the ovine fetus, the unesterified fatty acids [6], contained negligible amounts of arachidonic acid. This finding has led to investigations into the role of the placenta in the metabo lism of linoleic acid.…”