Several peptides were isolated from tryptic digests of insoluble calf aorta matrix by chromatography. Reductive pyridylethylation of a tryptic 15 kDa pool released fragments deriving from the C-terminus of type III collagen. A 50-residue peptide TC(III) was shown by sequence analysis to be the C-terminal peptide from the α1(III)-chain, containing a helical and non-helical region of equal sizes. The peptide was further digested with collagenase to give ColC(III), comprising the complete C-terminal non-helical region of α1(III) including a hydroxylysine in position 16C. The peptide TC(III)×TN(III) was isolated, demonstrating covalent cross-linking between the C-terminal non-helical region of one type III molecule and the N-terminal helical cross-linking region of another. Its digestion with cyanogen bromide yielded the small fragments α1(III)CB3B* and α1(III)CB3C, confirming TN(III) as an N-terminal helical cross-link site. Sequence analysis of both TC(III)×TN(III) and its collagenase-derived cross-linked peptide ColC(III)×TN(III) established the 4D-staggered alignment of adjacent collagen III molecules. The cross-link structure of both peptides was mainly dihydroxylysinonorleucine with a small amount of hydroxylysinonorleucine, indicating that the lysine residues involved in formation of the cross-links are both hydroxylated. No pyridinoline or histidinohydroxylysinonorleucine cross-links were found within the non-reduced C-telopeptide region of type III collagen.

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