Collagen‐stimulating factor from embryonic brain has ascorbate‐like activity and stimulates prolyl hydroxylation in cultured muscle cells

Embryonic rat‐brain extract contains a collagen‐stimulating factor which enhances the production of collagen types I, III, IV and V by cultured rat muscle cells. Here we report on the partial characterization and possible mechanism of action of a low‐molecular‐mass fraction with ascorbate‐like activity isolated from embryonic rat brain extracts. This activity eluted very close to ascorbate when filtered through Bio‐Gel P‐2 and Sephadex G‐10. The peak of biological activity showed properties of a reducing agent. Both the biological and reducing activities were lost when the fraction was treated with the enzyme ascorbate oxidase. This factor enhanced in a time‐dependent manner, the secretion of procollagen, pulse‐labeled with [³H]proline. Incubation of the muscle cultures with the factor increased by 15‐fold the ratio of hydroxyproline to proline residues in secreted macromolecules over controls. A fourfold increase in the above ratio was obtained for the celluar proteins. Crude homogenates from control and factor‐stimulated cultures were tested for prolyl hydroxylase activity using [³H](Pro‐Gly‐Pro)_n as a substrate. Cultures treated with the collagen‐stimulating factor showed a 5–50‐fold increase in prolyl hydroxylation activity compared to controls. No effect on prolyl hydroxylation was found when the factor was added in vitro to either control or stimulated enzyme preparations. Our results suggest that the collagen‐stimulating factor contains ascorbate‐like activity which promotes the secretion of collagenous proteins by increasing hydroxylation of proline residues in their polypeptide backbone.