The primary functional role of collagen is as a supporting tissue and it is now well established that the aggregated forms of the collagen monomers are stabilized to provide mechanical strength by a series of intermolecular crosslinks. These links are formed by oxidative deamination of the ε-amino group of the single lysine in the amino and carboxy-telopeptides by lysyl oxidase. The aldehyde thus formed reacts with an ε-amino group of a lysine at a specific point in the triple helix because of the quarter-staggered end-overlap alignment of the molecules in the fibers. The chemistry of these crosslinks is dependent on both the nature and age of the collagenous tissue (1 ,2 ). Differences in the crosslinks are because of the degree of hydroxylation of both the telopeptide and the specific lysine in the triple helix. Thus, the amounts of intermediate crosslinks present in immature tissue, dehydro-hydroxylysinonorleucine (A-HLNL), and hydroxylysino-keto-norleucine (HLKNL) may vary considerably between tissues, e.g., rat tail tendon and skin contain A-HLNL whereas cartilage and bone contain predominantly HLKNL.
