Abstract
A study is made of the effect of GSH as a co-antioxidant with vitamin E during free radical chain autoxidation inhibition studies of dilinoleoylphosphatidylcholine (DLPC) liposomes. Oxidations are initiated in the aqueous phase with azobis(2-amidinopropane hydrochloride) and in the bilayer phase of DLPC with azobis(2,4-dimethylvaleronitrile) under known conditions of the rate of free radical chain initiation (Ri). In reactions initiated in the aqueous phase, GSH is not an efficient antioxidant when acting alone; however, in cooperation with vitamin E in the bilayers, it does effect significant extensions of the efficient induction period of vitamin E. Quantitative studies show that GSH "spares" 0.4 molecules of vitamin E in the bilayer/molecule of GSH and therefore terminates approximately 0.8 peroxyl radical chains as a co-antioxidant with vitamin E. In contrast, GSH is not an effective co-antioxidant with an efficient water-soluble antioxidant, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylate (Trolox). GSH spares only 0.08 molecules of Trolox/molecule of GSH during autoxidation initiated in the aqueous phase with azobis(2-amidinopropane hydrochloride). The inhibition rate constant for GSH in trapping aqueous phase peroxyls is at least an order of magnitude less than that of Trolox. When peroxidation is initiated in the bilayer phase of DLPC with azobis(2,4-dimethylvaleronitrile), GSH is not an effective co-antioxidant with either vitamin E in the bilayer or Trolox in the water. Comparatively higher ratios of GSH to E (GSH/E = 50) or Trolox (GSH/Trolox = 30) are required to give significant extensions of the E or Trolox induction periods. GSH is estimated to preserve only approximately one vitamin E or Trolox molecule for a hundred GSH for peroxidations initiated in the DLPC bilayers. From the kinetic studies and GSH decay studies during inhibition periods, it is concluded that GSH does not act synergistically by regenerating ArOH from the phenoxyl, ArO, radical of vitamin E or Trolox. The mode of antioxidant action of GSH is concluded to be that of trapping peroxyl radicals in the aqueous phase and thereby indirectly sparing vitamin E in the bilayer.
Highlights
The Cooperative Antioxidant Roloef Glutathione with a Lipid-soluble and a Water-soluble Antioxidant duringPeroxidation of Liposomes - Initiated in the Aqueous Phase and in the Lipid Phase*
When peroxidation is initiated in thebilayer phaseof DLPC with azobis(2,4-dimethylvaleronitrile),GSH is not an effective co-antioxidant with either vitamEiinn the bilayer iments indicate that GSH can regenerate vitamin E from the a-tocopheroxyl radical in solution [19] andinteract with vitamin E during autoxidation [20],but it is not established if such interaction is applicable to biphasic membrane systems
Period on the azobis(2-amidinopropane hydrochloride) (ABAP)-initiated autoxidation of DLPC liposomes (Fig. 1, 2), and the rate of oxygen uptake is slow to return to theuninhibited rate.The antioxidant action of GSH differs significantly from vitamin C which is known to be an effective inhibitor of such autoxidations initiated by ABAP 12)
Summary
Period on the ABAP-initiated autoxidation of DLPC liposomes (Fig. 1, 2), and the rate of oxygen uptake is slow to return to theuninhibited rate.The antioxidant action of GSH differs significantly from vitamin C which is known to be an effective inhibitor of such autoxidations initiated by ABAP 12). On the other hand, when a-tocopherol was sequestered in the DLPC liposomes and GSH was present in the water, the combination gives a significant extension of the efficient inhibition period typical of vitamin E alone (Fig. 1,compare traces 3 and 4 ) ; again therate isslow to return tothe uninhibited ratein trace 4. Using the slopes of these plots and the kp for DLPC under thesecondi-
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