The Utilization of Properdin in the Alternate Pathway of Complement Activation: Isolation of Properdin Convertase

Normal human serum contains an α-globulin, designated properdin convertase, which after activation by zymosan will interact with native properdin to generate a C3 reactive enzyme. The activation of properdin convertase as well as its interaction with properdin after activation requires no additional factors. The C3 reactive material which is generated reacts directly with C3 and contains properdin in an altered state; the exact composition of this C3 reactive enzyme, however, is not completely clear. Removal of properdin convertase from serum aborts the consumption of C3-C9 when zymosan is added. In addition, although C3 proactivator is not required for the action of properdin convertase on properdin or for the subsequent cleavage of C3, it, nevertheless, can be activated by products of the properdin convertase, properdin, C3 reaction. Hence, properdin convertase may be involved as an initiating step in the alternate pathway of complement activation.