Characterization of type III procollagen from chick embryo blood vessels.

The precursors of collagen types I and III were extracted from radioactively labeled chick embryo blood vessels. Procollagen III was found to consist of three identical proa III chains, each containing a large amino and a large carboxyl propeptide. Interchain disulfide bridges occur at three locations: between the amino propeptides, the carboxyl propeptides, and within the vertebrate collagenase B fragment of the collagen helix. The precursor chains proa (III) and proal (I), and their propeptides, are similar. While denatured trimers of procollagen I chains are linked at only one end, those of procollagen III are held together at both ends. This causes a significant difference of sedimentation coefficient of the denatured procollagens and was used to separate them. Biosynthesis of procollagen III in the presence of a,&-dipyridyl, which interferes with collagen helix formation by abolishing hydroxylation of proline, gave molecules which were linked at only one end. Subsequent hydroxylation of these molecules permitted complete disulfide bridge formation. The blood vessels synthesized both procollagen III and procollagen I. While procollagen I was nearly completely converted to collagen I, corresponding formation of collagen III was not observed. Instead some of the procollagen III was converted to a molecule which consisted of three chains; the size of these chains was intermediate between proa and a chains and they were mutually disulfide-linked near both ends. We suggest that all or part of the amino propeptides remained in these intermediate molecules.