Differently cross-linked and uncross-linked carboxy-terminal telopeptides of type I collagen in human mineralised bone.

In bone matrix, type I collagen is stabilised by covalent cross-links formed between adjacent collagen molecules; the majority of which is believed to be immature, divalent bonds. For studying these immature forms in detail, we have developed an immunoassay for a synthetic peptide SP 4 that is analogous with and detects a linear epitope within the C-telopeptide of alpha1-chain of type I collagen. The SP 4 assay, together with the ICTP assay, which is specific for the trivalently cross-linked C-telopeptide, was used for the isolation of the differently cross-linked C-telopeptide structures of the alpha1-chain of type I collagen present in mineralised human bone. Amino acid analysis, peptide sequencing and MALDI-TOF mass spectrometry were used to identify and characterise each of the isolated structures. The cross-link content of each isolated peptide was identified. In the trivalent ICTP peptide, only 40% was cross-linked with pyridinoline, the remainder of the cross-links being currently uncharacterized. The divalent peptides contained only previously characterised cross-linking structures. Most of the divalent cross-links were dihydroxylysinonorleucine (DHLNL), with minor amounts of hydroxylysinonorleucine (HLNL). The relative proportion of the HLNL cross-link was slightly higher in the divalent alpha1Calpha2H peptide. A substantial amount of uncross-linked telopeptide structures was also found. Previous studies, where direct chemical cross-link analyses have been used to assess the maturity of cross-linking, have inferred that bone contains more divalently than trivalently cross-linked C-telopeptides. The immunochemical peptide approach used in this study may help to detect presently uncharacterized, trivalent cross-links, the presence of which is strongly suggested in this study. It also provides additional information regarding the extent and maturity of tissue type I collagen cross-linking in health and disease.

Assay for cross-linked carboxyterminal telopeptide of type I collagen (ICTP) unlike CrossLaps assay reflects increased pathological degradation of type I collagen in rheumatoid arthritis.

We compared the ability of assay for cross-linked carboxyterminal telopeptide of type I collagen (ICTP) and CrossLaps assay to reflect increased pathological degradation of type I collagen in serum and synovial fluid samples of patients with rheumatoid arthritis (RA; n = 40). ICTP and CrossLaps concentrations were correlated with each other and with markers of collagen synthesis (PINP and PIIINP, amino terminal propeptides of type I and type III procollagens, respectively) and with markers of inflammation, i.e., C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR). Serum ICTP was increased in half of the RA patients, whereas CrossLaps assays were increased only occasionally. Serum ICTP correlated with the other markers of collagen metabolism as well as with CRP and ESR. Serum CrossLaps correlated only with PINP and ICTP, but not with serum PIIINP, CRP or ESR. Two patients had false-positive reactions in the CrossLaps assay due to the rheumatoid factor. The ICTP and CrossLaps antigens were clearly separate peaks in gel filtration analysis. The CrossLaps assay is able to detect the same ICTP antigen, but not vice versa. The ICTP assay reflects increased matrix metalloproteinase-mediated collagen degradation in joints in RA. In contrast, the physiological cathepsin K-mediated bone resorption measured by the CrossLaps assay was only occasionally increased.