1992 Syntex Award Lecture Model biomembranes: quantitative studies of peroxidation, antioxidant action, partitioning, and oxidative stress

Abstract

This review outlines a decade of research that employs quantitative kinetic methods of autooxidation to phospholipids aggregated into membranes. The classical rate law for autoxidation:[Formula: see text]was found to apply to heterogeneous systems of bilayers and micelles, where kp and 2kt. are the rate constants for chain propagation and termination, respectively, and Ri, the rate of chain initiation, is controlled by thermal initiators. The oxidizability of a typical lipid chain, linoleate 18:2, at 30 °C was similar (0.02–0.04 M−1/2 s−1/2) in different media (solution, micelles, bilayers) and for different procedures using water-soluble or lipid-soluble initiators and inhibitors. A reduction in the absolute rate constant for termination, 2kt by a hundredfold in bilayers of dilinoleoylphosphatidyl choline (DLPC) compared to that in tert-butyl alcohol solution, and a drop in the kp by a factor of five are interpreted in terms of diffusion of polarized peroxyl radicals from the hydrophobic bilayer phase to the aqueous surface, where peroxyls are strongly solvated by water. Such phenomena may also account for significant changes as observed by 31P NMR spectra in bilayer lamellar structure accompanying extensive peroxidation. Analysis of the hydroperoxides formed by peroxidation of mixed bilayers of DLPC + DPPC (16:0) initiated by a water-soluble initiator, azobis(2-amidinopropane•HCl) (ABAP), showed a linear trend between the ratio of cis, trans to trans, trans geometrical isomeric hydroperoxides and [DPLC] consistent with a peroxidation mechanism proposed in homogeneous solution.The antioxidant activities, kinh, of three classes of antioxidants: (a) polyalkyl-6-hydroxychromans (e.g., vitamin E), (b) polyalkyl-4-methoxyphenols, and (c) trialkylphenols, were measured in DLPC membranes. The results show an overall leveling and depression of kinh values in DLPC membranes in the series (a) (by several orders of magnitude), (b), and (c) compared to known values in solution in chlorobenzene. In aqueous bilayers it is proposed that kinh values are attenuated by hydrogen bonding by water at both the para ether oxygen and phenolic groups. Restricted diffusion (e.g., of α-tocopherol) may also reduce antioxidant activities in membranes. A synergistic effect between ascorbic acid and α-tocopherol was discovered under conditions of inhibited peroxidation of linoleate in SDS micelles. The natural peptide glutathione, GSH, however, acts as a co-antioxidant with vitamin E by trapping peroxyls in the aqueous phase.Solid cholesterol was found to partition directly into PC lipsomes by shaking, above or below the phase transition temperature, and membrane-bound cholesterol, unlike the solid, undergoes facile peroxidation. A water-soluble form of α-tocopherol complexed with bovine serum albumin (α-toc:BSA) is an effective antioxidant for autoxidations of linoleate in SDS micelles. In contrast, α-toc:BSA required a long equilibration time (e.g., 12 h) with liposomes (DLPC) before the α-tocopherol was transferred to the liposomes to provide effective antioxidant action.