Abstract
The effect of phospholipids on the oxidative decomposition of d-γ-tocopherol (γ-Toc) and the formation of its dimers during the thermal oxidation of trilaurin (Tri-La) and of trilinolein (Tri-Li) was examined.
After triglycerides, in which γ-Toc and additives (phospholipids or their model compounds) dissolved, were oxidized with air at 180 for 10 h, residual γ-Toc and formed dimers were determined by HPLC.
On the oxidation in Tri-La, little change of γ-Toc consumption was observed and the amount of formed 5- (γ-tocopheroxy) -γ-tocopherol (γ-TED) decreased by the addition of phospholipids (Table-1). When a small amount of phosphatidyl choline was added to Tri-Li, γ-Toc consumption greatly decreased, and a similar effect was observed also by the addition of phosphatidyl ethanolamine (Fig.-1). From this fact, it was assumed that nitrogenous base of phospholipid molecule is independent of the inhibitory effect of phospholipids on the thermal oxidation of γ-Toc. For clarifying the mechanism of the suppresssion of the thermal oxidation of γ-Toc with phospholipids, the following experiments were carried out : the oxidation of γ-Toc in Tri-Li containing tri-n-octylamine (TOA) as a model compound of nitrogenous base, or tri-n-butyl phosphate (TBP) as a model compound of phosphate ester. A large amount of TOA depressed the oxidation, but a small amount has little influence. On the other hand, γ-Toc consumption was remarkably reduced by the addition of a small amount of TBP (Fig.-4). The action of TBP is very similar to that of phospholipids. From the above results, it is considered that the functional group which suppresses the thermal oxidation of γ-Toc in Tri-Li is phosphate ester linkage.
The dimers which formed during the thermal oxidation of γ-Toc in the presence of phospholipids were qualitatively the same as those in the absence of phospholipids. The amount of γ-TED increased to some extent, and the amounts of 5- (γ-tocopherol-5-yl) -γ-tocopherol and its atropisomer [γ-TBD (H) and γ-TBD (L)] were not influenced by the addition of phospholipids (Figs.-2 and 3). When TOA or TBP was added to Tri-Li, a little increase of the amount of γ-TED formed was observed, but the change of the amounts of γ-TBD (H) and γ-TBD (L) was less than that of γ-TED (Figs.-5 and 6).
All of the examined additives depressed the thermal oxidation of Tri-Li (Fig.-1).
