Isodesmosine is a lysine derivative found in elastin. Isodesmosine is an isomeric pyridinium-based amino acid resulting from the condensation of four lysine residues between elastin proteins by lysyl-oxidase. These represent ideal biomarkers for monitoring elastin turnover because these special cross-links are only found in mature elastin in mammals.[1]
^Miao, M.; Bruce, A.E.E.; Bhanji, T.; Davis, E.C.; Keeley, F.W. (2007). "Differential expression of two tropoelastin genes in zebrafish". Matrix Biology. 26 (2): 115–124. doi:10.1016/j.matbio.2006.09.011. PMID17112714.