Cross Linked C-Telopeptide Of Type I Collagen (CTXI)

The carboxyterminal telopeptide of type I collagen (ICTP) is an indicator of degradation of type I collagen. ICTP antigen consists of a trivalent collagen cross-link joining three polypeptide chains of which two are α1 chains of one collagen molecule while the third is derived from either an α1 or an α2 chain of the helical region of another molecule (Fig. 1) (Risteli et al. 1993). The major determinant for antigeneity must contain two phenylalanine-rich regions, which is only possible if the cross-link is trivalent in nature (Sassi et al. 2000). In osteoclasts, catepsin K cleaves the trivalently cross-linked ICTP structure at two sites between the phenylalanine-rich region and the cross-link, thereby destroying any reactivity with ICTP antibodies (Sassi et al. 2000). Therefore degradation of type I collagen in bone is not measurable by the present ICTP-test. ICTP is cleared from circulation by the kidneys and has a molecular mass of about 12 000–20 000 (Risteli et al. 1993).