Lipofuscin-like pigments by autoxidation of polyunsaturated fatty acids in the presence of amine neurotransmitters: The role of malondialdehyde

Iron-promoted autoxidation of arachidonic acid in acetate buffer (pH 6.0) at 37 degrees C in the presence of glycine led to the formation after 24 h of a complex pattern of fluorescent products, one of which was identified as 1-carboxymethyl-4-methyl-1,4-dihydropyridine-3,5-dicarboxaldehyde (1), arising by reaction of glycine with malondialdehyde. When glutamic acid was used as the amine compound, the corresponding dihydropyridine 2 was detected in the oxidation mixture. Formation of adducts 1 and 2 was markedly decreased at pH 7.4, or in the presence of a large excess of the amino acid, and was suppressed by glutathione or ascorbic acid. In the presence of dopamine, norepinephrine or serotonin, autoxidation of arachidonic acid led Likewise to complex patterns of fluorescent products, but no evidence of the dihydropyridine adducts 3-5, nor of their oxidation products 6-8 was obtained. No malondialdehyde-derived product could be detected when linoleic acid was used as the fatty acid. These and other results that are presented suggest that malondialdehyde is a possible contributor to, but not the primary determinant of fluorescent pigment formation by peroxidative degradation of polyunsaturated fatty acids in the presence of amine compounds.