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 of 0 degrees 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 rection, Factor B zymogen in M.C. serum could be fully activated by cobra venom factor (CVF) at 37 degrees C. That activation without fragmentation was not due to an abnormal form of Factor B was shown by its typical cleavage on incubation of MC. 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 parhway is formed and that this enzyme is responsible for the observed C3 consumption.