Isolation of Alternative Pathway C3 Convertase Containing Uncleaved B and Formed in the Presence of C3 Nephritic Factor (C3NeF)

Abstract

The capacity of serum from patients with membranoproliferative glomerulonephritis to interact with normal human serum to inactivate C3 was ascribed to the presence of C3 nephritic factor ³ (C3NeF) ⁴ (1). The action of C3NeF was magnesium ion dependent, operative in C4 deficient serum (2), and impaired in serum selectively deficient in B and D̄ (3) indicating that C3 cleavage occurred by the alternative pathway. Fluid phase interaction of C3, B, and D̄ in the presence of C3NeF permits recovery of C3 convertase with an estimated sedimentation rate of 10S containing at least C3̄ and B̄ (4). The present study shows that interaction of larger amounts of C3NeF with C3 and B in the absence of D̄ allows isolation of a 10S complex containing C3 and B̄, an uncleaved form of B, and manifesting C3-cleaving activity. Materials and Methods. B (5), D̄ (6, 7), D̄ (7), and C3 (8, 9) were purified to homogeneity and quantitated as described.