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.
Human complement is inactivated by plasmin, the proteolytic enzyme of plasma or serum active at or near neutrality.
The addition of streptokinase to human serum, which converts plasminogen to plasmin, also causes the inactivation of complement components C'2 and C'4 and varying amounts of C'1. C'3 is the most resistant to inactivation by plasmin. Chloroform-activated human plasmin and bovine plasmin also destroy these components of complement, but are less effective than the streptokinase-activated enzyme. The inactivation of complement by the addition of streptokinase to human serum is inhibited by high hydrogen ion concentrations, low temperature, and elevated ionic strength. The inactivation of the components of complement in various fractions of serum is influenced by the available plasminogen and the content of plasmin inhibitors in these fractions.
Certain similarities are pointed out between the components of complement and the factors in the plasmin system and between the inactivation of the components of complement by antigen-antibody reactions, by specific agents, and by plasmin.
The possible significance of these relationships in immune hemolysis and complement fixation, and the possible role of the plasmin system in the instability of complement and the development of anticomplementary properties in serum are discussed.
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