Soluble and Insoluble Collagen measurement Collagen is the most abundant protein found in animals. During a healthy life span the insoluble, covalent, cross-linked collagen fibers retain their biophysical functions and shapes. The collagen fibers remain isolated from most of the biochemical activities of the cells. Trauma caused by metabolic internal events, external chemical agents or physical injuries however, quickly lead to dramatic activity, resulting in rapid collagen removal followed by a wound healing response (collagen regeneration and associated remodeling). Newly formed soluble collagen production and the residual insoluble collagen fibers can be monitored using the Sircol Soluble Collagen and Sircol Insoluble Collagen Assays. Collagen proteins contain one or more domains with a triple helical structure. The three chains are described as alpha chains and should not be confused with the alpha helixes found in other proteins. The fibrillar collagens (Types I, II, III, V & XI) have most of their alpha chain structure composed of a continuous repeating tri-peptide sequence made up of glycine in every third amino acid residue [(gly-X-Y)n ]. It is to this sequence that the Sircol Dye binds. Proline is frequently an occupant in the ‘Y’ location of the tri-peptide [(gly-X-Y)n]. Many of these residues are converted, post translation, into hydroxyproline residues prior to triple helix formation and the release of the tropocollagen into the ECM. Until now, an investigator seeking to measure covalent cross-linked insoluble collagen required measurements based on free hydroxyproline content – a procedure that requires strong acid (6.0 M HCl), high temperatures (+ 95 °C) and overnight cooking (18 to 24 hrs).