No abstract

Serum complement has traditionally been defined as a four component system, Cq, C'2, C~3, and C~4. Although several investigators had suggested the existence of additional components (for review of earlier literature, see reference 1), only recently has unequivocal evidence been available supporting the further complexity of complement. The third component, C'3, was shown first to consist of at least two activities (1--4) and then three (5, 6), designated C~3a, C'3b, and C'3c. A highly purified protein,/31o-globulin (7), would appear to be identical with C'3a (5).Miiller-Eberhard and Kunkel (8, 9) and Taranta, Weiss, and Franklin (10) have described a new serum protein, distinct from the classical components of complement and designated the llS component on the basis of its sedimentation constant. In earlier work on the sequence of action of the components of complement (11, 12), the first component, C'I, was the first factor recognized to interact with sensitized erythrocytes. However, evidence has been presented that the I1S component participates in immune hemolysis at a stage even earlier than Ctl (8, 9), an observation which would appear to require revision of previous concepts of sequence.It is the purpose of this paper to report the resolution of Cq into three activities by chromatography on diethylaminoethyl (DEAE) cellulose of a euglobulin fraction of normal human serum. It will be shown that all three factors are required for reconstitution of Cq activity in several hemolytic systems and for generation of Cq esterase in the absence of antigen-antibody complexes (13).

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Normal human and other mammalian serums contain a protein, properdin (1, 2), which is an important constituent of a natural defense mechanism of blood. Properdin, in conjunction with complement and Mg ++, participates in the destruction of certain bacteria and abnormal red cells and in the neutralization and inactivation of certain viruses. Properdin differs from antibody in many respects, particularly in its apparent lack of serological specificity, its requirements for Mg ++ and complement for its interactions, and in its physical and chemical properties.A method for the assay of properdin has been briefly described elsewhere (1). This method depends upon the requirement of properdin for the inactivation of the third component of complement (C'3) by zymosan (3). A unit of properdin is defined as the smallest amount of test sample which will reduce the C'3 titer of a properdin-deficient serum (RP) from 120 to 0 units during incubation with zymosan under standard conditions. While there are theoretical and practical objections to the zymosan assay of properdin, it has been found to be more reproducible and reliable than other types of assays now under investigation. The actual test is not difficult to perform, but careful selection and standardization of reagents are necessary.

Several years ago, an hypothesis was proposed that the first of the four recognized components of complement was the precursor of an enzyme (1).This view was based in part upon observations of the effect of proteolytic enzymes upon human complement. Streptokinase is an activator of plasmin, a proteolytic enzyme of plasma most effective at neutrality. The complement activity of human serum was inhibited by the addition of streptokinase (2). Analysis demonstrated that this loss of complement activity was primarily the result of inactivation of the first, second, and fourth components of complement. However, the inactivation of the second and fourth components occurred only in serums containing the first component (1). These experiments suggested that the streptokinase had activated plasmin which in turn had converted the first component of complement into an agent which inhibited the activity of the second and fourth components.The possibility that the first component of human complement might be a proenzyme has found support in two separate lines of study. In the first group of experiments, it was observed that under certain conditions a partially purified preparation of the first component rapidly lost its hemolytic activity. In its stead, the preparation acquired the capacity to inactivate complement and to hydrolyze certain synthetic amino acid esters (3~ 4). In the second group of experiments, aggregates of antigen and antibody were mixed with fresh human serum. This procedure resulted in a demonstrable loss from the serum of activity attributable to the first component. It was then possible to elute from the aggregates of antigen and antibody one or more factors which inactivated complement and hydrolyzed the same amino acid esters as the preparations derived from the partially purified first component (3, 5). These observations suggested that the first component of complement may have been the precursor of an enzyme tentatively identified as an esterase (3-5). The inhibition of the hemolytic action of guinea pig complement by diisopropyl fluorophosphate, an *