THIYL RADICAL-INDUCED CIS TRANS-ISOMERIZATION OF METHYL LINOLEATE IN METHANOL AND OF LINOLEIC-ACID RESIDUES IN LIPOSOMES/

Purpose: To investigate the role of a thiol-containing biologically ac tive compound in lipid peroxidation of membranes. Materials and method s: Thiyl radicals were generated from 3-(2-mercaptoethyl)quinazoline-2 ,4(1H,3H)-dione (MECH) using pulse radiolysis and gamma-radiolysis in aqueous and alcoholic solutions saturated with N2O. The products were analysed by H-1 NMR and by HPLC. Results: The thiyl radicals abstract bisallylic hydrogens from [cis-9, cis-12]-methyl linoleate, yielding a pentadienyl radical. In the absence of oxygen, a thiyl radical-induce d cis/trans-isomerization leads to linoleic-type isomers. These chain- type isomerization reactions can occur with the long living pentadieny l radical, followed by a 'repair' reaction of the attached thiol, and with the thiyl radical adduct with a double bond of the fatty acid res idue. Conclusions: The results show that the mechanism of cis/trans-is omerization is an integral part of the thiyl radical attack on polyuns aturated fatty acids in homogeneous solutions and in bilayers.