Synthesis of a phosphatidyl derivative of vitamin E and its antioxidant activity in phospholipid bilayers

A novel phospholipid containing a chromanol structure at its polar head group was synthesized from egg yolk phosphatidylcholine and 2,5,7,8-tetramethyl-6-hydroxy-2-(hydroxyethyl)chroman by transphosphatidylation catalyzed by phospholipase D from Streptomyces lydicus. The structure of the product synthesized was shown by spectral analysis to be 1,2-diacyl-sn-glycero-3-phospho-2'-hydroxyethyl- 2',5',7',8'-tetramethyl-6'-hydroxychroman. The phosphatidylchromanol (PCh) showed antioxidant activity against radical chain oxidation of methyl linoleate in solution in a manner similar to that of d-alpha-tocopherol (alpha-Toc) and 2,2,5,7,8-pentamethyl-6-chromanol. However, PCh was less effective as a chain-breaking antioxidant than was alpha-Toc when unilamellar egg yolk phosphatidylcholine liposomes were exposed to either a water-soluble or a lipid-soluble radical initiator. It is likely that the phospholipid nature of PCh affects the location and the mobility of the chromanol moiety in the membrane bilayer resulting in a decrease in antioxidant activity. On the other hand, the antioxidant activity of PCh was little different from that of alpha-Toc in unilamellar liposomes when exposed to a lipid-soluble radical initiator in the presence of ascorbic acid. It appears that PCh in phospholipid bilayers can be regenerated by ascorbic acid in aqueous phase as can be alpha-Toc. The new phospholipid, phosphatidylchromanol, should prove useful as a chain-breaking antioxidant in phospholipid membranes.