Phospholipids are oxidized a t 25 'C and a t 60 "C under the formation of brown pigments significantly more rapidly in mixtures with cellulose than in mixtures with casein of egg albumin. The extent of browning and the difference of reactivities are more pronounced in case of phosphatidylethanolamine, lysophosphatidylethanolamine and phosphatidylserine in mixtures with a low water content than in those containing a higher percentage of water. Oxidized phospholipids arc partially combined with protein in unextractable insoluble lipoprotein complexes.Phospholipids, particularly phosphatidylethanolamine and phosphatidylserine, are easily oxidized by air oxygen, even a t room temperature or in course of cold storage [I]. The reason is a high content of polyunsaturated fatty acids in the phosphatidyl- Hydroperoxides formed as primary oxidation products of phospholipids are rapidly decomposed to brown pigments [3]. Therefore, the determination of the extent of browning is more suitable as a criterion of autoxidation degree of phospholipids than the determination of peroxide value. The production of brown pigments is due to the reaction of secondary carbonylic oxidation products or their free radical precursors with the primary amino group of phosphatidylethanolamine or phosphati-Phospholipids are usually present in food materials in close contact with protein, frequently combined with protein in lipoproteins. Therefore the oxidation products of phospholipids could easily react with the &-amino group of lysine bound in protein instead of reacting with the primary amino group of phosphatidylethanolamine. The mechanism of interactions of oxidized phospholipids is the subject of this study.
MaterialEgg yolk phospholipids were isolated after LEA et al. [5]. Phosphatidylethanolaniine and phosphatidylcholine were separated by column chromatography [I]. Lysophosphatidylethanolamine (prepared from egg yolk by enzymic hydrolysis) and phosphatidyl-L-serine (from bovinc brain) were products of Koch-Light Laboratories Ltd., Colnbrook, England. Casein after Hammarsten and microcrystalline cellulose (products of Lachema, n. p., Brno, CSSR) were used as non-lipidic mstriceq.Egg albumin (purity DAB, Erg. Bd. 6) was purified by chromatography on Sephadex G-75.
ProcedureThe 10% solution of a phospholipid was mixed with cellulose or protein and the solvent (dicthyl ether or petroleum ether) was removed from the mixture in a vacuum rotating evaporator. The mixture was then stored a t constant temperature in dark and at free access of oxygen. The sample was extracted three times, each time by :shaking I g of the mixture for 20 min with 2 -5 nil of chloroform-methanol ( I : I v/v). The solvent was evaporated from the combined extracts under reduced pressure, the residue was dissolved in chloroform, dried with anhydrous sodium sulphatc, and the colour intensity of the filtrate was measured at 400-4480 nm (Spekol, V E B Carl ZeiB, Jena). I h e wcight of extract was deterinincd after having dried it a t 80 "C in a stream of nitrogen to a const...
