Evidence for natural existence of pyridinoline crosslink in collagen

Pyridinoline is a fluorescent crosslinking amino acid isolated from collagen. Recently it was claimed that this material is an artefact produced from contaminating proteins during acid hydrolysis. However, in our hands, bovine tendon collagen could not be depleted of pyridinoline by the suggested treatments. A peptide which had the same fluorescence properties as those of pyridinoline could be isolated from enzymic digests of collagen. After acid hydrolysis, presence of pyridinoline in the peptide could be demonstrated on amino acid analysis. The composition of the peptide suggests that it originates from the specific regions of collagen molecule. These results clearly indicate the existence of pyridinoline in collagen $\text{in}\text{vivo}$.