Pro-C4 of human plasma: isolation and description of chemical and antigenic properties

The fourth component of complement (C4) occurs in human plasma as a protein with a three polypeptide chain structure. Lately a single-chain form of C4 has been purified from human plasma by sequential steps of immunoadsorbent chromatography, gel filtration in 6 M guanidine hydrochloride, and gel filtration in 6 M guanidine hydrochloride and 0.015 M dithiothreitol. Single-chain C4, referred to as pro-C4, behaved as a protein of approximately 200,000 daltons upon gel filtration, and was separated from the three chains of C4 on the basis of molecular size. Pro-C4 could be isolated from each of five human plasmas examined and was found to constitute approximately 2% of total, immunoreactive C4. Replicate amino acid analyses of pro-C4 and C4 agreed within 0.5 mol % for all residues except glycine and threonine, which agreed within 0.7 mol percent. Polyacrylamide gel electrophoresis of pro-C4 in the presence of sodium dodecyl sufate in 5% gels under reducing conditions indicated an approximate molecular weight of 202,000. The sum of the molecular weights of the alpha-, beta-, and gamma-chain of C4 is 205,000. After gel electrophoresis, pro-C4 stained positively with periodic acid-Schiff reagent, suggesting the presence of covalently-bound carbohydrate. Competitive inhibition radioimmune assays with pro-C4, purified alpha-, beta-, or gamma-chain, and chain-specific antisera demonstrated the existence of antigenic sites on pro-C4 that are assignable to common determinants on each of the alpha-, beta-, and gamma-chains of C4.