Formation of N-formylkynurenine suggests the involvement of apolipoprotein B-100 centered tryptophan radicals in the initiation of LDL lipid peroxidation

Tryptophan oxidation products were determined in pronase E digests of apo B of delipidated, native and Cu 2+ oxidized LDL using a sensitive and specific HPLC method. Oxidized LDL contained N-formylkynurenine, kynurenine and tryptamine but no oxindolylalanine and 5-hydroxytryptophan. N-Formylkynurenine increased from an initial value of 0.21 to 1.67 mollmol apo B within 5 h. Apo B of native LDL also contained kynurenine (0.80 mollmol) and tryptamine (0.13 mol/mol). The results support the assumption that oxidation of Trp residues in apo B is an early event and possibly an elementary reaction involved in initiating LDL oxidation.