Fearon D T, Austen K F, Ruddy S
J Exp Med. 1974 Feb 1;139(2):355-66. doi: 10.1084/jem.139.2.355.
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.
通过产生具有溶血活性的细胞中间体EAC43B(D)来测定备解素因子D的活性，该中间体带有依赖C3b的替代途径C3转化酶。用二异丙基氟磷酸（DFP）处理因子D可阻止EAC43B(D)的形成；因此，丝氨酸酯酶对于替代途径C3转化酶的产生至关重要，这种情况类似于C1在经典C3转化酶C42形成中的作用。将因子D定义为丝氨酸酯酶促使人们寻找其酶原形式，并从血浆中通过色谱法分离出一个单一的胰蛋白酶诱导因子D活性峰，它与活化的因子D不同。分析性凝胶过滤表明其表观分子量为25,000。通过EAC4B(D)的形成、依赖C3转化酶前体因子（CoVF）的C3转化酶的产生以及在C3b存在下因子B的裂解来评估，这种经胰蛋白酶作用产生因子D活性的蛋白质被命名为“前体因子D”。前体因子D对DFP具有抗性，而其经胰蛋白酶激活的形式对DFP灭活敏感，这类似于其他血浆丝氨酸酯酶（包括C1）的行为。
