Biosynthesis of phospholipid molecular species in isolated liver cells studied by combining fatty acid substrates esterified in the sn-1 and sn-2 positions

Generation and remodeling of highly polyunsaturated molecular species of rat hepatocyte phospholipids

Freshly isolated rat hepatocytes were incubated for 20 min with [U-14C]glycerol in the presence or absence of unlabeled linoleic (18:2n-6), arachidonic (20:4n-6), or docosahexaenoic (22:6n-3) acid, added as albumin complex in 10% ethanol. Most of the radioactivity (approximately 95%) recovered in hepatocyte lipids was present in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and triacylglycerol (TAG). The presence of exogenous fatty acids resulted in (i) higher incorporation of [U-14C]glycerol, (ii) higher percentage of label in TAG, and (iii) enhanced formation of PC and PE molecular species bearing the exogenous fatty acid at both the sn-1 and sn-2 positions of glycerol. In each case, these molecular species contained 60 to 70% of the label in that lipid class. Further incubation of the cells for 40 and 80 min in the absence of labeled substrate and exogenous fatty acids resulted in a redistribution of label among PC and PE molecular species due to deacylation-reacylation at the sn-1 position of glycerol.

The metabolism of 22:5(-6) and of docosahexaenoic acid [22:6(-3)] compared in rat hepatocytes

Elevated levels of 22:5(-6), which is the elongated and desaturated product of arachidonic acid, is induced by selective n-3 fatty acid deficiency, especially in brain cortex. Less elongation and desaturation of 20:4(-6) than of 20:5(-3) has been found in intact rat liver cells in previous studies and is probably the main reason why so little 22:5(-6) is found under adequate nutritional conditions. The present study compares the metabolism of 22:5(-6) with the metabolism of 22:6(-3), the main n-3 fatty acid in mammals. Freshly isolated rat liver cells were incubated with [1-14C]22:5(-6) and [1-14C]22:6(-3). Oxidation and esterification in triacylglycerols, diacylglycerols and phospholipids were studied. The phospholipid classes were separated and the different molecular species identified. Rats with essential fatty acid deficiency were compared with control rats. 22:5(-6) was found to be a good substrate for membrane phospholipid biosynthesis and was conserved well in the phospholipid fraction of the rat liver cells for more than 3 h of incubation. More 22:5(-6) was esterified in the total phospholipid fraction and less was incorporated in triacylglycerols than observed with 22:6(-3) in hepatocytes from control animals. This was not the case in animals with essential fatty acid deficiency. 22:5(-6) was esterified to a greater extent in phosphatidylcholine than 22:6(-3) in control cells but not in essential fatty acid deficiency cells. More 22:5(-6) was coupled with 18.0 in the sn-1 position of the phospholipid molecular species than 22:6(-3) was in control cells.

Studies on the metabolism of [1-14C]5.8.11-eicosatrienoic (Mead) acid in rat hepatocytes

The oxidation, esterification and formation of chain elongated and desaturated products of [1-14C]5,8,11-eicosatrienoic (Mead) acid was studied. Liver cells from essentially fatty acid deficient (EFAD) and control rats were used. The metabolism of [1-14C]20:4, n-6 and [1-14C]20:5, n-3 were studied under the same experimental conditions. More 20:3, n-9 than 20:4, n-6 and 20:5, n-3 was oxidised both in EFAD and control cells. 20:3, n-9 was elongated to [14C]22:3, n-9 in both cell types and significant amounts of [14C]22:4, n-9 were formed in EFAD cells. Less 20:3, n-9 was esterified in phospholipids and more in triacylglycerol than observed with 20:4, n-6 and 20:5, n-3 in both cell types. 20:3, n-9 was mainly esterified in phosphatidylcholine and little was esterified in phosphatidylethanolamine compared to 20:4, n-6 and 20:5, n-3. In comparison, 20:3, n-9 was rather efficiently esterified in phosphatidylinositol as 18:0-20:3. [14C]22:4, n-9 formed from 20:3, n-9 in EFAD hepatocytes was esterified in triacylglycerol, not in phospholipids, unlike [14C]22:5, n-6 and [14C]22:6, n-3 which were mainly esterified in phospholipids.