title: Mechanism of action of factor D of the alternative complement pathway.
abstract: Factor D (C3 proactivator convertase) of human serum has been shown to be absolutely necessary for alternative pathway function, for activation of the C3/C5 convertase of that pathway and not to be a subunit of this enzyme. Factor D was found to be present in human plasma in active form only, at a concentration of 2 microgram/ml, and not to be controlled by plasma protease inhibitors or by spontaneous decay. Unlike trypsin, factor D cleaves and activates factor B only when it is in Mg++-dependent complex with C3b, has no esterolytic activity, and is unable to cleave the B chain of insulin. The alleged functional and antigenic relationship of factor D to alpha-thrombin could not be verified. The results of this study led to the description of the mechanism of action of factor D in terms of the cryptic site hypothesis.
mesh_terms: D003166:Complement Activating Enzymes / Q000378:metabolism*; D003167:Complement Activation; D003176:Complement C3 / Q000378:metabolism; D011416:Complement Factor D / Q000032:analysis / Q000276:immunology / Q000378:metabolism*; D003170:Complement Pathway, Alternative; D000939:Epitopes; D006801:Humans; D008274:Magnesium / Q000378:metabolism; D013917:Thrombin / Q000276:immunology
chemical_list: D003176:Complement C3; D000939:Epitopes; D003166:Complement Activating Enzymes; C493488:CFD protein, human; D011416:Complement Factor D; D013917:Thrombin; D008274:Magnesium