Reaction of ozone with glycophorin in solution and in lipid vesicles

Glycophorin from human red blood cells was exposed to ozone in aqueous solution. Amino acid analysis of glycophorin exposed to a 10-fold molar excess of ozone showed that the only residue affected was methionine. Both methionine residues of the protein were oxidized to methionine sulfoxide. Exposure of the oxidized protein to cyanogen bromide caused no cleavage of the polypeptide chain. Glycophorin was incorporated into unilamellar lipid vesicles made from phosphatidylcholine. The protein containing vesicles were exposed to ozone in a 10-fold molar excess to the glycophorin. Gas chromatography of the methyl esters showed negligible change in the fatty acid composition. Amino acid analysis of the ozone-treated protein showed the oxidation of only one methionine residue per polypeptide chain to methionine sulfoxide. Ghosts of human erythrocytes were exposed to ozone. Cyanogen bromide treatment of the oxidized glycophorin yielded fragments showing that the only methionine residue oxidized by ozone was residue 8. These results indicate that in this membrane model (a) amino acid is more susceptible to ozone than is the lipid, and (b) amino acids external to the membrane are more susceptible than those in the polypeptide chain spanning the membrane.