The nature of the collagen synthesized by cultured human fibroblasts.

The hydroxyproline-containing proteins (hyproproteins) synthesized by cultured human fibroblasts have been partially characterized. The hyproprotein extracted from the cell layer was found to be similar to the collagen extracted from skin in the ratio of hydroxyproline to proline, chain composition, solubility, and resistance to proteolytic digestion.The hyproproteins isolated from the medium were different. About 20% of the peptide-bound hydroxyproline was found in randomly coiled chains. The alpha2 chains were present in considerable excess over the alpha1 chains, suggesting that the alpha2 chain may be synthesized in quantities greater than required to form a collagen molecule with a chain composition (alpha1)(2)alpha2. The remaining medium hyproprotein appeared to be an unusual form of native collagen which, unlike typical native collagen, was soluble under physiological conditions. This hyproprotein did not yield alpha chains when denatured and contained material that had a molecular weight greater than alpha chains. A similar size distribution was observed in the protein synthesized in the presence of beta-aminopropionitrile, a specific inhibitor of collagen cross-linking. After treatment with pepsin, typical alpha1 and alpha2 chains were obtained from the protein in a 2:1 ratio. Since the medium protein is soluble and has properties different from the typical collagen molecule, it may represent a modified form that functions in the transport of collagen from the cell to the fiber.