Modification of Lysine Residues to Alkyl-substituted Pyridiniums on Exposure of Proteins to Vaporized Hexanal

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Abstract
We examined the structure of lysine residues which had been impaired due to exposure of proteins to hexanal, using a free lysine/hexanal system (I), a Gly-His-Lys/hexanal system (II) and a lysozyme/hexanal system (III). The most abundant ninhydrin positive compound (N-50) was isolated from the products with system I and identified as l-(5-carboxy-5-aminopentyl)-2-pentyl-3,5-dibutylpyridinium betaine, which was formed through condensation of ε-amino groups of lysine with three molecules of hexanal. A new ninhydrin positive compound (N-51) was isolated from hydrolysates of the products with systems II and III, and identified as the same compound as N-50 from the UV and mass spectra and thin layer chromatogram. These results indicated that the e-amino groups of lysine residues of proteins are modified to alkyl-substituted pyridinium rings on exposure to hexanal.