Structural studies on human type IV collagen.

Type IV collagen was isolated from limited pepsin digests of human placenta by selective salt precipitation at acidic and neutral pH. The native protein was resistant to human skin collagenase but was cleaved by a rat mast cell protease. Molecular sieve chromatography of the reduced and alkylated material separated relatively homogeneous components of molecular weight 140,000 (140K) and 100,000 (100K) and a third component of molecular weight 70,000 which was further fractionated on CM-cellulose into 70K-I and 7OK-II components. Amino acid compositions and peptide maps produced by digestion of the denatured chains with cyanogen bromide and mast cell protease indicated that the 1OOK and 7OK-I fragments were derived from the larger 140K fragment, but that the more basic 70K-II fragment represented a different sequence which was most probably derived from a related but distinct collagen chain. The data are consistent with the presence of two genetically distinct type IV-like collagen chains in placenta. The existence of a pepsin-resistant collagenous fragment that contains approximately one-third glycine and has a molecular weight of 140,000 by the criteria of molecular sieve chromatography and acrylamide gel electrophoresis, and 145,000 by sedimentation equilibrium, argues for the presence of a triple-helical region in type IV collagen that is longer than an a chain of type I collagen, even when the hydroxylysine-linked carbohydrate content of this chain is considered.