Autoxidation of model membranes. The kinetics and mechanism of autoxidation of mixed phospholipid bilayers

Abstract

Kinetics of autoxidation of bilayer mixtures of oxidizable dilinoleoylphosphatidylcholine (DLPC) with saturated phospholipids as solvents (dimyristoyl-, DMPC, and dipalmitoyl-phosphatidylcholine, DPPC) follow the classical rate law of autoxidation: -d[O2]/dt = kp/2kt1/2[RH]Ri1/2. The kinetic order in substrate [RH] was found to be unity for peroxidation initiated by a lipid-soluble initiator, azobis-2,4-dimethylvaleronitrile (ADVN), and by the water-soluble azobis(2-amidino-propane·HCl) (ABAP). The kinetic order in rate of chain initiation, Ri, was found to be one-half for both initiation by ADVN, photochemically decomposed, and by ABAP. The oxidizability of unilamellar DLPC liposomes (kp/2kt1/2 = 0.232 M-1/2s-1/2) is twice that of multilamellar DLPC (kp/2kt1/2 = 0.116 M-1/2s-1/2). Analysis of the hydroperoxides formed during ABAP-initiated autoxidations of mixed DLPC + DPPC liposomes showed a linear trend between the ratio of cis,trans to trans,trans geometrical isomeric hydroperoxides and [DLPC], consistent with the peroxidation mechanism proposed for homogeneous systems. 31P nmr spectra of mixed bilayers were used to distinguish between heterogeneous and homogeneous mixtures of DLPC + DPPC. Such spectra taken at various stages of oxidation indicate that the bilayer structure of DLPC is preserved at least to the 10% extent of oxidation used in kinetic studies. At much higher oxidative conversion, the spectra indicate changes in lamellar structures.