The action of C3bINA and β1H on cell-bound C3b is described in this paper. The α-polypeptide of C3b that binds covalently to cell surfaces is cleaved by the C3bINA and β1H into two fragments: one of 60,000 (C3bα-60) and another of 40,000 (C3bα-40) daltons. The β-chain of C3b is unaffected by the C3bINA and β1H. The three polypeptides, C3bα-60, C3bα-40, and C3β, are held together as a single unit by disulfide bonds. This unit, referred to as C3b', is covalently bound to cell surfaces via the C3bα-60 polypeptide. The conversion of C3b to C3b' by C3bINA and β1H abolishes the ability of the C3b-bearing cells to adhere to human erythrocytes as well as the ability to form, on the cell surface, the B, &D, and properdin-dependent amplification C3-convertase. However, the agglutinability of the cells with either anti-C3c or anti-C3d is not affected.

Treatment of the C3b'-bearing cells with trypsin releases fragments of C3b' into solution, leaving a polypeptide of 32,000 daltons covalently linked to the membrane. Since the trypsinized cells are agglutinable by anti-C3d but not by anti-C3c, the 32,000 dalton polypeptide appears to correspond antigenically to C3d.