The development of antibodies able to recognize some sequences of the non-helical terminal portions of type I collagen has allowed the development of new analytical methods for evaluating biomarkers of bone resorption. In particular, a monoclonal antibody has been developed that recognizes a specific amino acid sequence (QYDGKGVG) of the amino-terminal portion of collagen I, which contains an essential region of intermolecular cross-links. This sequence, known as the amino-terminal telopeptide of type I collagen (NTX), is released due to the hydrolytic activity of cathepsin K produced by osteoclasts. In addition, a second peptide sequence consisting of an octapeptide (EKAHDGGR) located in the carboxy-terminal portion of the α1 chains of type I collagen, termed the carboxy-terminal telopeptide of type I collagen (CTX), which contains 40% of the deoxypyridinoline cross-links, has been identified. This octapeptide represents the result of non-enzymatic post-translational isomerization and racemization at the level of the aspartic acid residue contained in the sequence. Thus, in addition to the native αL-type form present in newly synthesized collagen, there are potentially three isomers present at more advanced stages of maturation: the isomerized form containing β-aspartate (βL), the racemized form containing the D-aspartate residue (αD), and the isomerized/racemized βD form. Theoretically, the ratio of the native αL form to each of the other three forms could be an index of the age of the bone tissue from which they are released.

Both NTX and CTX fragments, small, are filtered through the renal glomerulus. Their measurement can be performed on both serum and urine samples.

A larger fragment than the two previously described has been measured in serum. This is a hydrophobic fragment, relatively rich in phenylalanine, site of intermolecular link ages between α1 chains, located on the carboxy-terminal portion of type I collagen, known as ICTP and better defined by the abbreviation CTX-MMP. CTX-MMP and CTX, although derived from the carboxy-terminal fragment of collagen, are released, during the resorption phase, by different enzymatic processes. In fact, the action of cathepsin K, the major proteinase produced by osteoclasts, leads to the formation of CTX, while the action of several metalloproteinases produces CTX-MMP.

Some immunometric methods are available to determine NTX and CTX epitopes in serum and urine. For the measurement of CTX, these methods use antibodies specific for the octapeptide containing a β-isomerized aspartic acid residue. The presence of the β-isomer makes this marker more specific for bone tissue, correlating significantly with the age of the tissue itself. Serum CTX assay methods require the presence of a pair of β-D-isomerized octapeptides, making serum determination more specific than urine determination which requires the presence of only one octapeptide.

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