Determination of malonaldehyde in oxidized lipids by the Hantzsch fluorometric method.

Malonaldehyde is a secondary product formed during lipid oxidation. We developed a sensitive and reliable Hantzsch fluorometric method for determination of malonaldehyde in oxidized lipids. The principle of the method is based on the formation of highly fluorescent 1,4-dimethyl-1,4-dihydropyridine-3,5-dicarbaldehyde MI by reaction of malonaldehyde, methylamine, and acetaldehyde under neutral conditions. Compound MI formed could be estimated by high-performance liquid chromatography. Free malonaldehyde, that liberated under neutral conditions (labile forms) and that liberated by acid pretreatment (acid labile forms), could be determined by use of the calibration curves of MI versus malonaldehyde sodium salt. Oxidized methyl linoleate with a peroxide value of 1600 neq/mg contained 0.95 (free and labile) and 1.3 nmol (acid labile) malonaldehyde/mg, oxidized sardine oil with a peroxide value of 640 neq/mg contained 1.1 (free and labile) and 3.0 nmol (acid labile) malonaldehyde/mg, and the lipid fraction of oxidized rat liver microsomes contained less than 0.2 (free and labile) and 0.8 nmol (acid labile) malonaldehyde/mg. The malonaldehyde contents were much lower than those obtained by traditional 2-thiobarbituric acid test. It appears likely that the malonaldehyde contents, both free and labile, and acid labile forms, in oxidized lipids are too low to be taken into account.