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

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Abstract
The serum of patient with chronic glomerulonephritis and renal deposits of IgA [immunoglobulin A], C3 [the 3rd complement component] and properdin converted C3 on overnight exposure to 0.degree. C. The cold reaction was dependent on IG, initiating factor, Factors B and D, and Mg, 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 serum could be fully activated by cobra venom factor (CVF) at 37.degree. C. That activation without fragmentation was not due to an abnormal form of Factor B was shown by its typical cleavage on incubation of 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.