title: Properdin factor D: characterization of its active site and isolation of the precursor form.
abstract: 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.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002713:Chlorine / Q000494:pharmacology; D003165:Complement System Proteins; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000302:isolation & purification*; D004950:Esterases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D005916:Globulins / Q000037:antagonists & inhibitors / Q000302:isolation & purification; D006461:Hemolysis; D007531:Isoflurophate / Q000494:pharmacology; D008239:Lysine / Q000494:pharmacology; D012911:Snakes; D014105:Tosyl Compounds / Q000494:pharmacology; D014357:Trypsin / Q000494:pharmacology; D014688:Venoms
chemical_list: D004792:Enzyme Precursors; D005916:Globulins; D014105:Tosyl Compounds; D014688:Venoms; D007531:Isoflurophate; D002713:Chlorine; D003165:Complement System Proteins; D004950:Esterases; D014357:Trypsin; D008239:Lysine