MEMBRANE PEROXIDATION - INHIBITING EFFECTS OF WATER-SOLUBLE ANTIOXIDANTS ON PHOSPHOLIPIDS OF DIFFERENT CHARGE TYPES

Quantitative kinetic methods of autoxidation are used to determine the antioxidant activities of two water-soluble antioxidatants of the chr omanol type, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (T rolox) and -2,5,7,8-tetramethyl-2-N,N,N-trimethylethanaminium methylbe nzene-sulfonate (MDL 73404), during free radical peroxidation of phosp holipid membranes of different charge types. The stoichiometric factor (n) for peroxyl radical trapping for both Trolox and MDL 73404 was fo und to be 2. Trolox was found to partition partially, approximately 20 %, into the lipid phase of liposomes. The antioxidant activity of Trol ox during peroxidation of membranes determined by measurements of the absolute rate constant for inhibition of oxygen uptake, k(inh), was fo und to vary with the membrane surface charge that is controlled by var iation in pH. When peroxidation is initiated in the lipid phase by azo -bis-2,4-dimethylvaleronitrile (ADVN), using a typical zwitterionic li posome, dilinoleoylphosphatidyl choline (DLPC), the k(inh) was found t o be 2.98 x 10(3) M(-1)s(-1). The k(inh) of Trolox increased approxima tely 2-fold for membranes that have a positive surface, including DLPC at pH 4, DLPC containing stearylamine at pH 7, and for a membrane of dimyristoylphosphatidic acid containing linoleic acid (DMPA/LA). Conve rsely, Trolox does not inhibit peroxidation of negatively charged dili noleoylphosphatidyl glycerol (DLPG) at pH 7-11. Studies made of the po sitively charged MDL 73404 show that its antioxidant activity using DL PC and DLPG is pH dependent. Trolox inhibits the peroxidations of DLPC initiated in the aqueous phase by azo-bis-(2-amidinopropane.HCl)(ABAP ) at pH 4 or 7. However, Trolox does not inhibit the peroxidation of D LPG at pH 7. The different antioxidant activities of Trolox and MDL 73 404 are rationalized in terms of a peroxyl-radical diffusion model and specific charge interactions between antioxidants and membrane surfac e.