Reversible Activation of Proactivator (Factor B) of the Alternative Pathway Without Cleavage of the Molecule

The serum of a patient (M.C.) with chronic glomerulonephritis and renal deposits of IgA, C3, and properdin converted C3 on overnight exposure to 0°C. The cold reaction was dependent on immunoglobulin, initiating factor, Factors B and D, and magnesium but not on properdin. Factor B, the C3‐cleaving enzyme in this reaction, was used in zymogen form. After participation in this reaction, Factor B zymogen in M.C. serum could be fully activated by cobra venom factor (CVF) at 37°C. That activation without fragmentation was not due to an abnormal form of Factor B was shown by its typical cleavage on incubation of M.C. serum with CVF or C3b or after depletion of C3b inactivator. The evidence indicates that in the cold reaction only the initial C3 convertase of the alternative pathway is formed and that this enzyme is responsible for the observed C3 consumption.