Properdin and nephritic factor dependent C3 convertases: requirement of native C3 for enzyme formation and the function of bound C3b as properdin receptor

The purpose of this paper is to demonstrate the functional similarity of activated properdin (P)¹ and nephritic factor (NF) in the generation of C3 convertase activity and to distinguish between these two related enzymes and the subsequently formed C3b dependent C3 convertase of the alternative pathway. Specifically, it will be shown that the P and NF dependent enzymes require native C3 and not C3b as a cofactor in addition to proactivator (factor B) and proactivator convertase (factor D); that the enzymes can efficiently turnover C3 in absence of formation of the C3b dependent convertase; and that bound C3b can function as a receptor for P and thus possibly as a focus for the assembly of P C3 convertase. The differential requirements of the formation of these three enzymes is delineated.