CHARACTERIZATION OF ALPHA CHAINS OF CHICK SKIN COLLAGEN AND NATURE OF NH2-TERMINAL CROSS-LINK REGION

Carboxymethylcellulose chromatography of neutral salt-extracted collagen from normal and lathyritic chicks indicates that chick skin collagen contains two identical a chains (αl) and one α chain (α2) which is different from the other two. Acid-extracted collagen, however, contains another species of α chain (α1′) which is chromatographically separable from the α1 chain but has a similar amino acid composition and gel electrophoretic mobility. Studies of the peptides produced by cyanogen bromide digestion of α1 and α1′ show that both yield the same peptides except that α1-CBO and α1-CB1 (and α1-CBlAld) are lacking in digests of α1′. Since the missing peptides represent the NH2-terminal portion of the al chain, it appears that α1′ is a form of α1 from which a small part of the NH2-terminal sequence is absent. Whether the absence of the short sequence in the older (acid-extracted) collagen but not in more newly synthesized (salt-extracted) collagen reflects the presence in vivo of a physiologic proteolytic mechanism or is an artifact arising from limited proteolysis of the protein during extraction and purification is not known. These findings may explain, in part, observations of hetero-geneity in the chain structure of other collagens. From cyanogen bromide digests of isolated a chains, a septadecapeptide (α1-CB1) was isolated from α1 and a pentadecapeptide (α2-CB1) from α2 which predominate in the α chains of recently synthesized (salt-extracted) collagen and lathyritic collagen in which the formation of interchain cross-links had been blocked while in the digests of the α chains of more highly cross-linked (acid-extracted) collagen these peptides are largely replaced by aldehyde-containing peptides (α1-CB1Ald and α2-CBlAld) of identical amino acid composition except for the absence of a lysyl residue and the presence of a residue of allysine (α-aminoadipic δ-semialdehyde). These peptides are absent from CNBr digests of βI2 but a new peptide is present which is a dimer of α1-CB1Ald and α2-CB1Ald and contains a functional group with the properties of an α,β-unsaturated aldehyde presumably arising by an aldol condensation of two residues of allysine. These findings suggest that intramolecular cross-link formation by aldol condensation previously shown to occur in rat skin collagen is common to many vertebrate collagens. © 1969, American Chemical Society. All rights reserved.