The carboxyterminal propeptide of type I procollagen (PICP) and the aminoterminal propeptide of type I procollagen (PINP) are enzymatically removed from the procollagen molecule by specific proteases.

PICP and PINP are indicators of the synthesis of type I collagen. Cleavage of PICP is required for the initiation of fibril formation, whereas PINP may be retained in the collagen molecule resulting in type I pN-collagen. The retention of PINP is transient, unlike the presence of IIIpN-collagen, and has been considered to regulate fibril diameter (Fleischmajer et al. 1985).

PICP is a trimeric, globular protein consisting of three polypeptide chains: two proα1(I) and one proα2(I) chains. The molecular mass of PICP is 100 000 and it is cleared by mannose receptors in liver endothelial cells (Smedsrød et al. 1990).