Procollagen I N-Terminal Propeptide (PINP)

The PINP molecule is similar to PIIINP consisting of three distinct structural domains: Col 1 is on the aminoterminal side of the molecule, while Col 2 and Col 3 are situated on the middle of the helically structured molecule (Kühn et al. 1982). The PINP molecule has a molecular mass of 35 000 and is cleared by scavenger receptors in liver endothelial cells (Melkko et al. 1994). PINP often occurs in circulation in two forms of different molecular sizes. One is identical to the trimeric authentic antigen (intact PINP) whereas the other consists of smaller forms of PINP, resembling a single domain of the proα1(I) chain of PINP and is probably a degradation product of type I procollagen or I pN-collagen. Thus, an assay of intact PINP rather than total PINP appears to be more sensitive in detecting changes in the rate of type I collagen synthesis (Melkko et al. 1996, Risteli & Risteli 1999).