718 EYRE, PAZ & GALLOP elastin. Over the years a variety of cross-linking amino acids have been identified in both proteins. Desmosine and isodesmosine, the first dis covered, are the main cross-linking residues of mature elastin. The main cross-linking residues of mature collagen, however, have proved elusive despite research over fifteen years that has revealed an assortment of borohydride-reducible intermediates that disappear as coHagen fibrils mature. Lately, research has centered on the mature, nonreducible cross links.

Two pathways of cross-linking can be defined in the fibrillar collagens, one based on allysine, the lysine-derived aldehyde, the other on hydroxy allysine, the hydroxylysine-derived aldehyde. There is now strong evidence that trifunctional, 3-hydroxypyridinium residues are the adult cross-links on the hydroxyallysine route, which predominates in most connective tissues except skin. Two forms of 3-hydroxypyridinium cross link have been identified. The major one, hydroxylysyl pyridinoline (HP), embodies three hydroxylysines, and a less abundant form, lysyl pyridi noline (LP) embodies two hydroxylysines and one lysine. Sensitive methods have been developed for quantifying these naturally fluorescent amino acids in hydrolysates of whole connective tissue. The mature cross-linking residues on the ally sine pathway are still unknown, although complex structures capable of linking three or more molecules are suspected. Histidine is probably a component. on this pathway, although there is not yet general agreement on this point.