title: The alternative pathway C3/C5 convertase: chemical basis of factor B activation.
abstract: The structural basis of activation of the alternative pathway C3 convertase was explored. For this purpose a modified isolation procedure of the activating enzyme, Factor D, was elaborated. The procedure affords a 70,000-fold purification of the enzyme with a 20% yield. A simple assay was designed for the quantitation of both Factor D and Factor B activity. On the basis of activity measurements and amino acid analysis, Factor D concentration in plasma was estimated to be 1 microgram/ml. Highly purified Factor D was used to activate Factor B in the presence of C3b and Mg++. The resulting fragments, Ba and Bb, were characterized with respect to their circular dichroism spectra, amino acid compositions, reactive sulfhydryl groups, and partial amino- and carboxy-terminal sequences. The results indicate that the Ba fragment constitutes the amino-terminal region and the Bb fragment the carboxy-terminal region of Factor B. The bond in Factor B that is cleaved by Factor D is proposed to be an arginyl-lysine bond.
mesh_terms: D002634:Chemotaxis, Leukocyte; D003166:Complement Activating Enzymes / Q000276:immunology*; D003167:Complement Activation; D050577:Complement C3-C5 Convertases / Q000276:immunology*; D011415:Complement Factor B / Q000494:pharmacology*; D011416:Complement Factor D / Q000494:pharmacology; D003170:Complement Pathway, Alternative; D004306:Dose-Response Relationship, Immunologic; D004591:Electrophoresis, Polyacrylamide Gel; D004792:Enzyme Precursors / Q000494:pharmacology*; D006461:Hemolysis; D006801:Humans
chemical_list: D004792:Enzyme Precursors; D003166:Complement Activating Enzymes; D050577:Complement C3-C5 Convertases; C493488:CFD protein, human; D011416:Complement Factor D; D011415:Complement Factor B