Characterization of a novel collagen chain in human placenta and its relation to AB collagen

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Abstract
A novel collagen chain, termed .alpha.C, was isolated from human placenta by limited pepsin digestion. The collagen containing the .alpha.C chain copurifies with placental AB collagen during selective salt precipitation but is virtually absent from fetal birth membranes, which contain relatively larger amounts of AB. Both native AB and .alpha.C-containing collagens are resistant to human skin collagenase under conditions that support cleavage of type I by greater than 90%. The .alpha.C chain was separated from .alpha.B by phosphocellulose chromatography and subsequently from .alpha.A by chro-matography on carboxymethyl-cellulose. Its amino acid composition is distinct from .alpha.A and .alpha.B although all 3 chains possess compositional features in common; the carbohydrate content of the .alpha.C chain was intermediate between .alpha.A and .alpha.B. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of peptides produced by CNBr cleavage and by limited digestion with the enzyme mast cell protease indicated different and unique products for the .alpha.A, .alpha.B and .alpha.C chains. Evidently another collagen chain exists which is related to the .alpha.A and .alpha.B chains but is structurally unique. The proteins containing these chains may in turn comprise a subfamily of collagen isotypes which represents a divergence from and/or specialization of the type IV basement membrane collagens.