Characterization of urinary degradation products derived from type I collagen - Identification of a beta-isomerized Asp-Gly sequence within the C-terminal telopeptide (alpha 1) region

The heterogeneity of urinary degradation products of C-terminal telopeptides derived from the alpha 1 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 (alpha 1) of type I collagen. Fragments containing the sequence EXAH-DGGR, with a DG site being either nonisomerized (Asp-Gly) or beta-isomerized (beta Asp-Gly), were identified, Pyr-idinoline was detected among the pyridinium crosslinks, 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 (alpha CTX) and/or beta Asp-Gly (beta CTX) 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, alpha CTX/beta CTX, 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 alpha 1 chain of type I collagen contains an Asp-Gly site prone to undergo beta-isomerization and that the degree of beta-isomerization of this linkage apparently increases with increasing age of bone. These findings indicate that the ratio alpha CTX/beta CTX might be clinically important in diagnosing metabolic bone diseases.