Covalent interactions of type IX collagen in cartilage

The cross-linking of type IX collagen in fetal bovine cartilage was investigated. The main cross-link was dihydroxy-lysinonorleucine (borohydride-reduced) with a lesser amount of the mature cross-link, pyridinoline. Dihydroxylysinonorleucine was present in all three chains of the COL2 domain of the type IX molecule, but only two of them contained pyridinoline even in mature cartilage. Amino acid sequence analysis of individual tryptic peptides that contained 3H-labeled cross-links showed that they had derived from sites of covalent interaction between type IX collagen and the telopeptide sequences of type II collagen. One two-chained peptide was a helical sequence of alpha 2 (IX) COL2 linked to an alpha 1 (II) N-telopeptide. A second peptide was a different helical sequence from another type IX chain linked to an alpha 1(II) c-telopeptide. This latter helical sequence was also the principal site of pyridinoline cross-linking in type IX collagen.