STRUCTURAL CHARACTERIZATION OF ACETALDEHYDE ADDUCTS FORMED BY A SYNTHETIC PEPTIDE MIMICKING THE N-TERMINUS OF THE HEMOGLOBIN BETA-CHAIN UNDER REDUCING AND NONREDUCING CONDITIONS

This work was carried out to elucidate the structures resulting from a cetaldehyde,induced modifications at the haemoglobin beta-chain N-term inal residues under different experimental conditions. A synthetic pep tide (Val-His-Leu-Thr-Pro-Glu-Cys) of m/z 798, which represents the si x N-terminal residues of the haemoglobin beta-chain N-terminus with an additional C-terminal cysteine, was used as a model compound. Peptide - acetaldehyde adducts were separated by reverse-phase HPLC. Fast-ato m-bombardment MS and linked-scan (B/E) spectra were used to study addu ct structures. Under nonreducing conditions at pH 7.4 (1:10 peptide/ac etaldehyde molar ratio), two types of adducts of m/z 824 were formed, both with modifications at the N-terminal valine. These two adducts we re shown to be a Schiff base, and a cyclic 2-methyl-imidazolidine-4-on e. The 2-methyl-imidazolidine-4-one adduct was demonstrated to be form ed via the Schiff base and to undergo dissociation gradually after 24 h. Reducing conditions at pH 7.4 (peptide/acetaldehyde molar ratio of 1:1 with 20 mM NaCNBH3) resulted in the formation of an adduct of m/z 826, which was shown to be an N-ethylated adduct of valine. A small am ount of nonreduced adducts were also formed under these conditions. Re ducing conditions at pH 9.0 (1:10 peptide/acetaldehyde molar ratio wit h 20 mM NaCNBH3) yielded two secondary, i.e. diethylated (m/z 854), pr oducts very rapidly. The cysteine residue of tile peptide was not foun d to form an adduct with acetaldehyde under physiological pH.