Isolation of a unique collagenous fraction from limited pepsin digests of human placental tissue. Characterization of one of the constituent polypeptide chains.

Fractionation of the highly soluble collagens released during limited pepsin digestion of whole human placenta has resulted in the isolation of a unique collagenous fraction comprised exclusively of high molecular weight aggregates. On reduction and alkylation, the aggregates dissociated yielding collagen-like polypeptides with an apparent Mr = 40,000 as well as a heterogeneous mixture of much smaller noncollagenous peptides. Ion exchange chromatography of the collagen-like, 40,000-dalton subunits results in recovery of a single relatively acidic component and a mixture of at least two relatively basic components. Both the basic and acidic components contain large amounts of cysteine, hydroxylysine glycosides, and glucosamine, and exhibit compositional features indicative of the presence of collagenous and noncollagenous domains. These unusual features have been verified for the more acidic component through isolation of its major cyanogen bromide cleavage products. Characterization of the cyanogen bromide fragments further indicated that this particular subunit is characterized by alternating collagenous and noncollagenous domains as opposed to a lengthy collagenous domain which is either preceded or followed by noncollagenous sequences. The molecular organization and possible derivation of these unique pepsin-resistant collagenous components is discussed.