Collagen cross-linking. Isolation of cross-linked peptides from collagen of chicken bone

Abstract

Cross-linked peptides were isolated from chicken bone collagen that had been digested with CNBr or with bacterial collagenase. Analyses of ³H radioactivity in disc electrophoretic profiles of the CNBr peptides from bone collagens that had been treated with NaB³H indicated that a major site of intermolecular cross-linking in chicken bone collagen is located between the carboxy-terminal region of an α1 chain and a small CNBr peptide, probably situated near the amino-terminus of an α1 or α2 chain in an adjacent collagen molecule. A small amount of this cross-linked CNBr peptide was isolated from a CNBr digest of chicken bone collagen by column chromatography. Amino acid analysis showed that the CNBr peptide, α1CB6B, the carboxy-terminal peptide of the α1 chain, was the major CNBr peptide in the preparation, and the reduced cross-linking components were identified as hydroxylysinohydroxynorleucine (HylOHNle), with a smaller amount of hydroxylysinonorleucine (HylNle). However, the composition and the low recovery of the cross-linking amino acids suggested that the preparation was a mixture of CNBr peptides α1CB6B and α1CB6B cross-linked to a small CNBr peptide whose identity could not be determined. A small cross-linked peptide was isolated from chicken bone collagen that had been reduced with NaB³H₄ and digested with bacterial collagenase. This peptide was the major cross-linked peptide in the digest and contained a stoicheiometric amount of the reduced cross-linking compounds. A peptide which had the same amino acid composition, but contained the cross-linking compounds in their reducible forms, was isolated from a collagenase digest of chicken bone collagen that had not been treated with NaBH₄. The absence of the reduced cross-links from this peptide indicates that, at least for the cross-linking site from which the peptide derives, natural reduction is not a significant pathway for biosynthesis of stable cross-links. However, most of the reducible cross-linking component in the peptide appeared to stabilize in the bone collagen by rearrangement from aldimine to ketoamine form.