TENDON HYPERTROPHY IS ASSOCIATED WITH INCREASED HYDROXYLATION OF NONHELICAL LYSINE RESIDUES AT 2 SPECIFIC CROSS-LINKING SITES IN TYPE-I COLLAGEN

This study was designed to investigate whether the changes in lysine h ydroxylation known to occur in hypertrophic tendon occur randomly or a t specific lysine residues in the type I collagen molecule. Peptides c orresponding to the two known major cross-linking sites of type I coll agen (a lysine (or hydroxylysine) at position 9N cross-linked to a hyd roxylysine at 930 and a lysine (or hydroxylysine) at position 16C cros s-linked to a hydroxylysine at position 87) were prepared by collagena se digestion, size fractionation, and separation by high performance l iquid chromatography from normal chicken tendon and from chicken tendo n subjected to increased tensile load as a result of muscle hypertroph y. The ratio of the difunctional cross-links dihydroxylysinonorleucine to hydroxylysinonorleucine in normal tendon is 0.75:1; this ratio is increased to 6:1 in hypertrophic tendon. The dihydroxylysinonorleucine to hydroxylysinonorleucine ratio is increased to the same extent in s amples of the purified cross-linked peptides derived from both the N-t erminal and C-terminal lysine aldehyde residues. On the other hand, th e relative hydroxylysine content of preparations of the pooled larger helical peptides obtained by cyanogen bromide digestion of normal and hypertrophic tendons was essentially identical. These results demonstr ate that there is a specific increase in hydroxylation of only the N- and C-terminal non-hellical lysine residues that participate in the fo rmation of the reducible difunctional cross-links of type I collagen i n hypertrophic tendon, while the extent of hydroxylation of lysine res idues in the helical regions is not affected. The specific mechanism b y which the enzyme lysyl hydroxylase acting on its substrate can disti nguish between lysine residues destined to be in non-helical versus he lical regions in a nascent collagenous peptide that has not yet attain ed its final secondary structure remains to be defined.