Characterization of Urinary Degradation Products Derived from Type I Collagen IDENTIFICATION OF A β-ISOMERIZED ASP-GLY SEQUENCE WITHIN THE C-TERMINAL TELOPEPTIDE (α1) REGION

Abstract The heterogeneity of urinary degradation products of C-terminal telopeptides derived from the α1 chain of human type I collagen was investigated and characterized. The urinary fragments characterized in this study consisted of two cross-linked (X) amino acid sequences derived from the C-terminal telopeptide (α1) of type I collagen. Fragments containing the sequence EXAHDGGR, with a DG site being either nonisomerized (Asp-Gly) or β-isomerized (βAsp-Gly), were identified. Pyridinoline was detected among the pyridinium cross-links, but there was a dominance of deoxypyridinoline and a cross-link containing pyridinoline having a molecular weight identical with that of galactosyl pyridinoline. A nonfluorescent cross-link was also found. The concentration of fragments derived from the C-terminal telopeptide region of type I collagen containing the sequence Asp-Gly (αCTX) and/or βAsp-Gly (βCTX) was measured by enzyme-linked immunosorbent assays in urine and in collagenase digests of trabecular and cortical bone of young and old origin. It was shown that the urinary ratio between such fragments, αCTX/βCTX, was higher in children compared with adults and that the ratio decreased with increasing age of bone. The results indicated that the C-terminal telopeptide fragments derived from type I collagen excreted into urine originated mainly from bone. In conclusion, it is demonstrated for the first time that the C-terminal telopeptide α1 chain of type I collagen contains an Asp-Gly site prone to undergo β-isomerization and that the degree of β-isomerization of this linkage apparently increases with increasing age of bone. These findings indicate that the ratio αCTX/βCTX might be clinically important in diagnosing metabolic bone diseases.