Guanidination of the lysine residues present in fibrous elastin and in soluble elastin peptides.

We have found that only 20 to 30% of the lysine residues measured after acid hydrolysis of insoluble elastin and alpha-elastin were blocked by prior guanidination of elastin with o-methylisourea. In contrast free lysine was guanidinated almost 100% and a purified, highly crosslinked peptide fraction of low molecular weight, 85%. The non-reactivity of lysine does not seem to be due to steric hindrance in the fibrous protein or in the large peptides derived therefrom but rather to the presence of epsilon [delta-adipic acid]-lysine, the oxidized form of lysinonorleucine described by Bailey et al. Since photolysis of desmosine and isodesmosine leads also to the formation of new lysine residues, methods will have to be devised to distinguish lysine-containing peptides derived from photolyzed elastin.