2.4. Procollagen and collagen synthesis

Collagens are the major fibrillar components of most connective tissues. At least nineteen different subtypes of collagens have been identified (Prockop & Kivirikko 1995). The collagens I, III, IV, V and VI are known to predominate in normal and fibrotic lungs (Madri & Furthmayr 1980, Raghu et al. 1985, Specks et al. 1995). Collagens form a family of proteins, and each type has its own characteristic amino acid sequence. Each collagen molecule consists of three individual polypeptides known as α-chains. Each α-chain has a left-handed helical secondary structure around its axis. Three α-chains are then coiled around each other into a right-handed superhelix. This triple-helical conformation is unique to collagen. (Uitto et al. 1981).

Interstitial collagens are synthesised and secreted out of cells as procollagens. During the secretion, carboxy- and aminoterminal propeptides are cleaved off from the parent molecule by specific proteases (Prockop et al. 1979). This releases the collagen molecule, together with its respective procollagen propeptides, in stoichiometric amounts into the extracellular space (Fig. 1). The propeptide molecules are then further degraded by non-specific proteases to smaller peptides, which retain the antigenicity of the parent procollagen propeptide molecule (Rohde et al. 1983). The function of the carboxyterminal propeptide is to direct the assembly of the three polypeptide chains and to initiate their winding into a triple-helical conformation. The cleavage of carboxyterminal propeptide seems to be a prerequisite for the assembly of collagen fibres. (Miyahara et al. 1982). The cleavage of the aminoterminal propeptide can be incomplete, resulting in the formation of a collagen molecule with attached aminoterminal propeptide (pN-collagen). This is typical of type III collagen, and such molecules have been observed in vivo on the surface of type III collagen fibrils (Karttunen et al. 1989). It is known that when these fibrils are degraded, type III pN-collagen is released into the extracellular space (Risteli & Risteli 1986). Antigenicity related to PIIINP is found in human serum with different molecular sizes. The largest form may represent intact type III pN-collagen or procollagen, the second form is larger than the authentic propeptide, the third form is similar in size to the authentic propeptide, and the smallest form is obviously derived from further degradation of the others and it predominates in human serum (Niemelä 1985, Niemelä et al. 1985). The commercial radioimmunoassays employing bovine antigen also detect the procollagen or pN-collagen antigenicity, whereas the assay based on human antigen detects the second and third forms as described above, being more sensitive to the synthesis of type III collagen. (Niemelä 1985, Risteli & Risteli 1990). Cleaved procollagen propeptides disappear during the processing of histological specimens. The intracellular immuno­reactivity of aminoterminal propeptides detects the intracellular aminoterminal propeptide domain of the corresponding procollagen, and extracellularly they react with the aminoterminal domain still attached to the corresponding pN-collagen molecule.