Characteristics of Fluorescence Formed by the Reaction of Proteins with Unsaturated Aldehydes, Possible Degradation Products of Lipid Radicals.

Blue fluorescence with maxima at 350-370/420-440 nm produced by the non-radical reaction of amino groups of proteins with lipid peroxy (or alkoxy) radicals has been suggested to be derived from aldehydes other than malonaldehyde. Characteristics of fluorescence produced in the reaction of amino acids with aldehydes depended on the α-or ε-amino groups and on the unsaturation of the aldehydes. Unsaturated aldehydes, 2-hexenal and 2-octenal, produced fluorescence on ε-amino groups whose maxima at 360-380/430-460 nm were close to those produced by lipid radicals. Intensities of the fluorescence from lipid radicals and unsaturated aldehydes were similarly decreased in an alkaline solution and on borohydride treatment. The fluorescence formation from unsaturated aldehydes was enhanced by organic hydroperoxides without affecting the chromatographic profiles of the fluorescence. In the reaction of lipid radicals, it is suggested that lipid hydroperoxides from the radicals enhance the fluorescence formation from their degradation products, unsaturated aldehydes. Lipid radicals and 2-octenal formed cross-links in protein accompanying a similar decrease in lysyl and histidyl residues. Unsaturated aldehydes may be major causative molecules for protein damage by lipid radicals.