No abstract

A radioactive peptide has been isolated from cyanogen bromide digests of sodium b~r o [~H ] -liydride-reduced collagen from rabbit bone, tendon and skin, It was identified as a crosslinked peptide linking the short C-terminal cyanogen bromide peptide al-CB6B (1 7 amino acid residues) to a1 -CB5 (37 residues) from the helical part of the chain of an adjacent molecule. Both peptides could be separated after cleaving the crosslink with periodate. Thus the crosslinked peptide al-CB5 x a1 -CB6B originates from an intermolecular crosslink between quarter-staggered molecules within collagen fibrils previously assigned as 'head-to-tail' link.The chemical nature of the reduced crosslinking component was identified and was shown to differ between peptides derived from different tissues: a1-CB6B x al-CB5 from bone contains hydroxylysinohydroxynorleucine [05 Lys(05wNle)] whereas the skin peptide contains hydroxylysinonorleucine [05 Lys(oNle)]. The peptide derived from tendon contains both components.The relation of o'Lys(wN1e) to o5Lys(o5oN1e) in the peptides corresponds to that of the original tissue. On the other hand, histidino-hydroxymerodesmosine which is a major reduced crosslinking component in skin and tendon, is completely absent in the isolated peptides.The crosslinking component in the skin peptide is completely glycosylated, mainly by glucosylgalactosyl residues and to a smaller extent by galactosyl residues. 05Lys(05wNle) from the bone peptide is only partly glycosylated, containing equal amounts of the disaccharide and monosaccharide. Only slight glycosylation was found in the tendon peptide.Formation of intermolecular crosslinks, which covalently link single molecules within the fibril, may be regarded as the final step in the biosynthesis of collagen (for review see 111).Aldehydes derived from lysyl or hydroxylysyl residues are mainly involved in this process. Condensation products of the Schiff base or aldol type are considered to be the primary step in crosslinking (reviewed in [2]). More complex reducible compounds and one nonreducible crosslinking compound, in which histidine and two or three lysine or hydroxylysine residues participate, have been identified by Tanzer et al. [3] and Housley and Tanzer [4].Whereas the chemical structure of crosslinking compounds is well known, only a few data are available on the precise location of intermolecular cross-. ~-Ahbreviutions. 05Lys(05cuNle), S-hydroxylysino-S-hydroxynorleucine; 05Lys(cuNle), 5-hydroxylysinonorleucine.links. The non-helical regions of the molecule certainly have an important function in crosslink formation; both the N-terminal and the C-terminal regions of collagen molecules contain oxidisable lysine residues [5,6]. Due to the quarter-staggered order of collagen molecules in fibrils the non-helical regions can form crosslinks with only a few distinct regions within the helical portions of a1 and a2 chains as has been discussed in detail by Zimmermann et al. 171. Evidences for the existence of crosslinks between nonhelical and helical regions...