Noncatalytic subunits of human blood plasma coagulation factor XIII. Preparation and partial characterization of modified forms.

Human blood plasma coagulation factor XIII is a zymogen of subnit structure a2b2. Several modified forms of the isolated noncatalytic b chains from this zymogen have been prepared and tested for their ability to complex noncovalently with preparations of factor XIII that contain only catalytic a subunits. A single CNBr fragment of b chain and a reduced and alkylated form of b chain display this capacity to complex with a subunit-containing zymogens. The molecular weight of the CNBr fragment, b', was estimated as 48,000 to 50,000, i.e. about one-half that of the monomeric form of unmodified b chain. Close agreement of values obtained by exclusion chromatography and by sodium dodecyl sulfate-gel electrophoresis suggests a monomeric structure for b'; native b chain appears to exist as a dimer. Evidence that b' is composed of more than a single polypeptide chain and that these chains are linked by disulfide bonds was obtained by sodium dodecyl sulfate-gel electrophoresis after reduction and by NH2-terminal analysis. whereas native b chain when complexed with human placental factor XIII, an a2 zymogen, protects a chains against a specific reaction with iodoacetamide, b' does not. Uncertainties concerning the subunit stoichiometry of a2 zymogen . b' complexes arise as a result of the anomalous chromatographic behavior of the a2 zymogens. The b chain in which one disulfide bond is reduced and in which the two sulfhydryl groups formed are alkylated with iodoacetamide binds with a2 zymogen to give a complex that is indistinguishable by exclusion chromatography from plasma zymogen. Guanidine-denatured b chain and denatured, reduced, and alkylated b chain fail to bind to a2 zymogen, although they appear to retain a dimeric structure.