Distributions of hydroperoxide positional isomers generated by oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine in liposomes and in methanol solution.

The differences in distribution of geometric isomers of unsaturated PC hydroperoxides generated by free radical oxidation were compared, as corresponding hydroxy analogs, in heterogeneous liposomes and in a homogeneous methanol solution by using HPLC with UV detection due to the presence of conjugated dienes. Identification of fractionated peak components was carried out by GC-MS. When the oxidation of 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine, PC(16:0/18:2), was initiated in liposomes by a hydrophilic azo radical initiator, and in a methanol solution by a hydrophobic azo radical initiator, there was no significant difference in the relative percentages of 1-palmitoyl-2-(9-hydroxy-trans-10,trans-12-octadecadienoyl)-sn-glycero-3-phosphocholine (9-t,t-OH PC) and 1-palmitoyl-2-(13-hydroxy-trans-9,trans-11-octadecadienoyl)-sn-glycero-3-phosphocholine (13-t,t-OH PC) between the PC oxidized in liposomes and in the methanol solution. For the oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine, PC(16:0/20:4), the relative percentage of 1-palmitoyl-2-(5-hydroxy-trans-6,cis-8,11,14-eicosatetraenoyl)-sn-glycero-3-phosphocholine (5-OH PC) was significantly higher (P < 0.01) than that of 1-palmitoyl-2-(15-hydroxy-cis-5,8,11,trans-13-eicosatetraenoyl)-sn-glycero-3-phosphocholine (15-OH PC) in liposomes. For the homogeneous methanol solution of PC(16:0/20:4), the relative percentage of 5-OH PC was close to that of 15-OH PC. For the PC(16:0/20:4) oxidized in bulk with added pentamethylchromanol, the individual amount of 15-OH PC, 1-palmitoyl-2-(11-hydroxy-cis-5,8,trans-12,cis-14-eicosatetraenoyl)-sn-glycero-3-phosphocholine (11-OH PC), 1-palmitoyl-2-(12-hydroxy-cis-5,8,trans-10,cis-14-eicosatetraenoyl)-sn-glycero-3-phosphocholine (12-OH PC), 1-palmitoyl-2-(8-hydroxy-cis-5,trans-9,cis-11,14-eicosatetraenoyl)-sn-glycero-3-phosphocholine (8-OH PC), 1-palmitoyl-2-(9-hydroxy-cis-5,trans-7,cis-11,14-eicosatetraenoyl)-sn-glycero-3-phosphocholine (9-OH PC), and 5-OH PC were close to each other compared to the corresponding values in liposomes and in methanol solution. The results obtained by gel permeation chromatography of the PC liposomes containing hydrophilic 2,2'-azobis-2-amidinopropane) dihydrochloride (AAPH) suggest that the AAPH added to the liposomes of PC(16:0/20:4) was partitioned into the water phase and out of the hydrophobic region of the fatty acyl moieties of the PC. These results confirm that the distance that exists in the bis-allylic carbons of the unsaturated fatty acyl moieties of PC from the interface between the hydrophilic region of PC and the water phases played an important role in influencing hydrogen abstraction to form a symmetrical distribution of hydroperoxide isomers in both the heterogeneous liposomes and the homogeneous methanol solution.