A Cysteine‐Rich Collagenous Protein from Bovine Placenta

A high‐molecular‐weight collagenous protein was isolated, after limited pepsin digestion, from the endometrial villi of bovine placenta; it resembled the protein isolated from human aortas by Chung et al. [Chung, E., Rhodes, R. K. and Miller, E. J. (1976) Biochem. Biophys. Res. Commun. 71 , 1167–1174] and from human placenta by Furuto and Miller [Furuto, D. K. and Miller, E. J. (1980) J. Biol. Chem. 255 , 290–295]. After denaturation and reductive cleavage of disulfide bridges three chains with M r of 40000–55000 were liberated. Two of the chains could be separated by ion‐exchange chromatography; the third chain aggregated under non‐denaturing conditions and could be isolated by sequential molecular‐sieve chromatography in denaturing and non‐denaturing buffers. Their amino acid composition resembled that of basement membrane collagens with respect to the high content of completely glycosylated hydroxylysine but differed with respect to their relatively low hydroxyproline content and high values of cysteine, tyrosine and glucosamine. A relatively low glycine content indicated regions of non‐collagenous structure. Reduction under non‐denaturing conditions and another pepsin treatment removed non‐collagenous regions containing mainly hydrophobic amino acid residues. The denaturation temperature of the component was considerably reduced by this treatment, indicating stabilization of triple‐helical structures by disulfide bridges in the native molecule. Fibrillar aggregates were precipitated with ATP which showed in the electron microscope a typical crossstriation pattern different to those of segment‐long‐spacing crystallites obtained from other collagen types. Immunofluorescence tests demonstrated the protein in blood vessels and interstitial areas of the renal medulla but not in basement membranes of glomeruli and tubuli.