Alternative Pathway of Complement: Nonenzymatic, Reversible Transition of Precursor to Active Properdin

Abstract

A method for the purification of properdin in precursor state (pre-P) has been elaborated. Precursor properdin, in contrast to activated properdin (P̄), does not initiate the formation of a C3 convertase when added to properdin-depleted serum, rather it requires the presence of an activating substance such as zymosan for expression of its activity. Comparing the activities of pre-P and P̄ it was found that some P̄ preparations contained significant amounts of pre-P because they were fully active only in the presence of zymosan. This observation indicated that P̄ can partly revert to pre-P. Comparison by immunoelectrophoresis and sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed no charge or size differences between pre-P and P̄. Radiolabeled pre-P, when analyzed after its participation in the reactions of the pathway, displayed an unchanged subunit m.w. of 50,000. Taken together, these results strongly suggest that the transition of pre-P to P̄ is due to a partly reversible change in the conformation of the protein rather than the result of proteolytic cleavage.