Enzymatic Synthesis of Mixed Acid Phospholipids

Abstract: Mixed acid phospholipids are valuable compounds, which are used mainly in membrane- and lipoprotein research and in liposome technology. The chemical synthesis of these special lipids is rather difficult and involves both toxic chemicals and harsh reaction conditions, which makes the products less attractive for medical and biological applications. Enzymes in contrast catalyze chemical reactions under mild conditions, providing a promising alternative to the traditional chemical approach. In the thesis 1,3 specific lipases and phospholipase A2 have been applied in order to incorporate fatty acids specifically into the sn-1 and sn-2 position respectively. The phospholipase A2-catalyzed esterification of 1-acyl lysophosphatidylcholine with oleic acid to produce phosphatidylcholine with oleic acid in the sn-2 position was studied in toluene under water activity controlled conditions. The aim of the study was to find the conditions most favorable for the synthesis reaction. To do this the impact of various reaction parameters such as water activity, substrate concentration and temperature on enzyme activity and equilibrium yield was determined. The phosphatidylcholine to lysophosphatidylcholine ratio at equilibrium increased with decreasing water activity and increasing fatty acid concentration, as can be expected from the law of mass action of an esterification reaction. The enzyme activity on the other hand decreased under conditions that favor the esterification. The best yield in the synthesis reaction was 60% at a water activity of 0.11 and an oleic acid concentration of 1.8 M. 1,3 specific lipases were employed to introduce caproic acid into the sn-1 position of phosphatidylcholine. The reaction was carried out in either one step (transesterification reaction) or in two steps (ethanolysis and re-esterification). A comparison between these two reaction modes was made with regard to product yield, product purity, reaction time, and by-product formation as a consequence of acyl migration. Esterification and transesterification reactions were studied in toluene under water activity controlled conditions. The yield in both types of reactions was the same under identical conditions. The highest yield (78%) was obtained at a water activity of 0.11 and a caproic acid concentration of 0.8 M. The reaction time was shorter in the esterification than in the transesterification reaction. The difference in reaction time was especially pronounced at low water activities and high fatty acid concentrations. The loss due to acyl migration was around 16% under a wide range of conditions. The incorporation of a fatty acid into the sn-1 position proved to be thermodynamically more favorable than the incorporation of a fatty acid into the sn-2 position. By a combination of lipase and phospholipase A2 action it was possible to convert egg yolk phosphatidylcholine, containing various fatty acids into phosphatidylcholine with capric acid in the sn-1 position and caproic acid in the sn-2 position. Two egg yolks with a weight of 16 g were required in order to obtain 160 mg of the desired product.