12. Ascorbic Acid Inhibits the Oxidation of Linoleic Acid By Coaaer (II) Complexes
Jun-ichi Ueda and Toshihiko Ozawa
Keywords: ascorbic acid, linoleic acid, copper(II) complexes, lipid peroxidation, HPLC
The oxidation of linoleic acid by Cu(II) (HGG) Cu(II)(en)2, or Fe(III)EDTA in the absence and presence of ascorbic acid was investigated by absorption spectroscopy and high performance liquid chromatography (HPLC). The absorbance at 234 nm due to the conjugated diene formed during Cu (II)or Fe(III)-catalyzed oxidation of linoleic acid increased with incubation time. However the addition of ascorbic acid to Cu(II) (HGG) or Cu(II) (en)2 suppressed the increase in absorbance at 234 nm, depending on the increase in the concentration of ascorbic acid. By contrast, Fe(III)EDTA-mediated oxidation of linoleic acid was further accelerated by the addition of ascorbic acid.
A probable mechanism of Cu (II) -dependent oxidation of linoleic acid may involve the cleavage of preformed linoleic acid hydroperoxide (LOOH) in linoleic acid. Therefore, the amount of LOOH may be important to enhance the oxidation of linoleicacid. Then, to ascertain how ascorbic acid can influence Cu (II) -mediated oxidation of linoleic acid having a large amount of preformed LOOH, ascorbic acid was added to the reaction mixture after linoletc acid was incubated with Cu(II) (HGG) for 12 h. As a result, the absorbance at 234 nm at an ascorbic acid/ Cu (II) (HGG) ratio of 1 increased a little as compared with that in the absence of ascorbic acid. When the molar ratio of ascorbic acid to Cu(II) (HGG) was 2:1, however, ascorbic acid suppressed the increase in absorbance at 234 nm. Furthermore, the absorbance at 234 nm due to LOOH decreased with the further addition of ascorbic acid, suggesting that LOOH was decomposed by Cu (I) (HGG) generated by the reduction of Cu(II) (HGG) with ascorbic acid. This was supported from HPLC results showing the disappearance of LOOH and the appearance of oxodiene compound. On the other hand, the addition of ascorbic acid to Fe(III)EDTA led to the promotion of Fe(III)-mediated oxidation of linoleic acid depending on the increase in the concentration of ascorbic acid.
Further the oxidation of linoleic acid by Cu(II) (HGG) or Cu(II) (en)2 in the absence and presence of Trolox (a water soluble derivative of a -tocopherol) was investigated by absorption spectroscopy and HPLC. The addition of Trolox to Cu(II) (HGG) or Cu(II) (en)2 suppressed the increase in absorbance at 234 nm, depending on the increase in the concentration of Trolox. By contrast, Fe(III)EDTA-mediated oxidation of linoleic acid was completely inhibited by the addition of Trolox.