Covalent Crosslinking between Molecules of Type I and Type III Collagen

Several peptides containing intermolecular crosslinks were isolated from tryptic digests of the insoluble matrix of human leiomyoma and calf aorta. From leiomyoma type III collagen, two crosslinked peptides were isolated. The peptide T N (III) 2 , is a dimer which contains the 73 amino‐terminal residues of two, zero‐D staggered, crosslinked α1(III) chains (D being the distance by which molecules or chains are staggered relative to each other). After de‐blocking with pyroglutamate aminopeptidase, its amino‐terminal amino acid sequence was determined to be < Glu‐Tyr‐Asp‐Ser‐Tyr‐Asp‐Val‐Xaa‐Ser‐Gly‐Val‐Ala‐Val‐Gly‐Gly‐Leu‐Ala‐Gly‐Tyr‐Hyp‐Gly‐. The involvement of T N (III) 2 in the formation of 4D‐staggered intermolecular bonds was demonstrated by sequence analysis of T N (III) 2 ×T(III). For this three‐chained peptide, a double sequence which corresponded to T N (III) and T(III), a nine‐residue peptide from the carboxy‐terminal helical crosslinking site of type III collagen, was found. Crosslinks between type I and type III collagens were identified by isolating a peptide T N (III)×T N (I) from leiomyoma. This peptide was characterized, first by digesting it with collagenase giving rise to Col N (III)×Col N (I) and then chymotrypsin giving C(III) and C(I). C(I) and C(III) are peptides characteristic for type I and type III collagens, respectively. Further, three‐component crosslink‐containing peptides, from both type I and type III collagen, were isolated from aorta and leiomyoma. The peptides Col N (III)× Col N (I)× T(III) from leiomyoma and Col ×N (III) Col N (I)× T(I) from aorta are crosslinked amino‐terminal regions of 0D‐staggered type I and type III molecules, joined either to an adjacent 4D‐staggered type III or type I molecule. Crosslinked peptides involving only type III collagen molecules were also isolated from calf aorta col N (III) 2 and CoI N (III) 2 ×T(III)]. As intramolecular bonds between type I and III collagens can be excluded the isolation of these crosslinked peptides demonstrated that type I and type III collagen molecules are present within the same collagen fibrit with O D stagger, bound by intermolecular crosslinks. The significance of these findings with respect to the proposed models for collagen fibres and the physiological properties of the extracellular matrix, are discussed.