Jiann-Jiu Wu

A Novel 3-Hydroxyproline (3Hyp)-rich Motif Marks the Triple-helical C Terminus of Tendon Type I Collagen

Because of its unique physical and chemical properties, rat tail tendon collagen has long been favored for crystallographic and biochemical studies of fibril structure. In studies of the distribution of 3-hydroxyproline in type I collagen of rat bone, skin, and tail tendon by mass spectrometry, the repeating sequences of Gly-Pro-Pro (GPP) triplets at the C terminus of α1(I) and α2(I) chains were shown to be heavily 3-hydroxylated in tendon but not in skin and bone. By isolating the tryptic peptides and subjecting them to Edman sequence analysis, the presence of repeating 3-hydroxyprolines in consecutive GPP triplets adjacent to 4-hydroxyproline was confirmed as a unique feature of the tendon collagen. A 1960s study by Piez et al. (Piez, K. A., Eigner, E. A., and Lewis, M. S. (1963) Biochemistry 2, 58–66) in which they compared the amino acid compositions of rat skin and tail tendon type I collagen chains indeed showed 3–4 residues of 3Hyp in tendon α1(I) and α2(I) chains but only one 3Hyp residue in skin α1(I) and none in α2(I). The present work therefore confirms this difference and localizes the additional 3Hyp to the GPP repeat at the C terminus of the triple-helix. We speculate on the significance in terms of a potential function in contributing to the unique assembly mechanism and molecular packing in tendon collagen fibrils and on mechanisms that could regulate 3-hydroxylation at this novel substrate site in a tissue-specific manner.

Collagen cross-links as markers of bone and cartilage degradation

An early study showed that desmosine and isodesmosine from degraded elastin are not metabolized, but are quantitatively excreted in urine [1].The structurally related pyridinoline cross-links of collagen were later also found in urine [2].Robins and colleagues introduced methods for measuring urinary pyridinoline and deoxypyridinoline residues as markers of systemic collagen degradation, and in particular of bone resorption [3].Finding that most of the urinary pyridinolines were in a narrow size-range of small peptides, we identified the amino acid sequences containing the cross-linking residues for immunoassay on the premise that this would provide greater specificity to their originating tissues and collagen types [4].First, we targeted cross-linked N-telopeptide fragments of type I collagen, most of which must have originated from bone based on their HP:LP ratio [5].Commercial versions of this assay (NTx) for urine and serum have seen widespread use in clinical studies, for example of anti-osteoporotic drugs [6,7].Immunoassays for the cross-linked Ctelopeptides of type I collagen (CTx), also identified in urine [8],have been introduced as comparable bone resorption markers [9].