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The activity of properdin factor D was measured by the generation of the hemolytically active cellular intermediate, EAC43B(D), bearing the C3b-dependent alternate pathway C3 convertase. Treatment of factor D with DFP prevented formation of EAC43B(D); thus, a serine esterase is essential for the generation of the alternate pathway C3 convertase, a situation analogous to the role of C1 in the formation of the classical C3 convertase, C42. The definition of factor D as a serine esterase prompted a search for its proenzyme form, and resulted in the chromatographic isolation from plasma of a single peak of trypsin-inducible factor D activity, distinct from activated factor D. Analytical gel filtration indicated an apparent mol wt of 25,000. This protein from which trypsin elaborated factor D activity, as assessed by the formation of EAC43B(D), the generation of the CoVF-dependent C3 convertase, and the cleavage of factor B in the presence of C3b, was designated "precursor factor D." The DFP resistance of precursor factor D, and the susceptibility of its trypsin-activated form to inactivation by DFP is analogous to the behavior of other plasma serine esterases, including C1.

The protein in the properdin pathway responsible for conversion of precursor factor D to D has been isolated and found to be identical with properdin. Sequential ion exchange and gel filtration chromatography demonstrated identity between properdin protein, measured by radial immunodiffusion, and the capacity to activate D to D, assessed by formation of the intermediate, EAC43B(D). Properdin, purified in this manner, was homogeneous on acid polyacrylamide disc gel electrophoretic analysis, with the band of protein corresponding to the position of eluates in the replicate gel capable of activating highly purified D. Demonstration of the homogeneity of purified D by alkaline disc gel electrophoresis, coupled with the linear stoichiometric hemolytic titrations of each factor, indicates that direct interaction between properdin and D generates D. Thus, activation of D by properdin represents a mechanism in the properdin pathway by which D becomes available for formation of the C3b-dependent C3 convertase.

Evidence has accumulated indicating the existence of a second complement activation mechanism which is functionally analogous to C1, C2, and C4. The noncomplement protein C3PA, previously recognized through its ability to form a complex enzyme with a protein from cobra venom, appears to be the precursor of the C4,2 analogue. It is a thermolabile ß-globulin with a molecular weight of 80,000. When serum is treated with naturally occurring plant or bacterial polysaccharides, the C3PA is cleaved into at least two fragments, one having the electrophoretic mobility of a γ-globulin and a molecular weight of 60,000, and the other being an acidic peptide with a molecular weight of 20,000. The larger fragment has the ability to cleave C3 into C3a and C3b and is therefore called C3 activator. It arises from the action of an as yet unidentified serum enzyme upon the C3PA, which is tentatively called C3PA convertase. In addition to endotoxins, yeast cell walls, inulin, and agar, aggregates of immunoglobulins were found to be activating substances, including human IgA, guinea pig γ1, and duck antibody. Serum depleted of C3PA had reduced E. coli bactericidal and increased hemolytic activity. The relationship of the C3-activator system to experimental and clinical observations has been discussed.

RICKETS, attended by a reduction in gain of weight, has been frequently reported in weaned lambs grazing on green oat plants and other green feeds during the winter in the South Island of New Zealand1,2. The effect is not confined to green feeds grown in this country, since Ewer has demonstrated a rachitogenic effect of green oats grown in England3.