Absence of non-DFP-Inhibitable Activities of Factor D̄

Abstract Factor D is an active serine esterase capable of specifically cleaving factor B in the presence of C3b and thereby generating the active C3- and C5-convertase C3bBb. In the absence of C3b when nephritic factor (NF) or activated properdin (P̄) is available, D̄ appears to have a different function. Native C3, B, and NF or P̄ form a complex which, dependent on traces of D̄ has C3 cleaving, C3b generating activity. B seems to remain in this complex in its uncleaved form [Schreiber et al., J. Exp. Med. 142, 760 (1975)]. This suggested to us that D̄ might have other than hydrolytic activities, the more so as we found it difficult to completely block its functional activity in the generation of B-dependent enzymes by the esterase inhibitor, DFP. The relative activities of active (D̄) and DFP-treated D̄ (D̄DFP) have therefore been compared in three systems: 1) Human C3b, B, and D̄ or D̄DFP were incubated in the presence of 5 mM Mg++, for 10 min at 25°C, then EDTA was added to stop further enzyme generation, and the C3 cleaving activity was assessed by incubation with C3. 2) Same system as in 1) but C3b was replaced by cobra venom factor. 3) Active properdin (P̄), C3, B, and D̄ or D̄DFP were incubated in the presence of 5 mM Mg++. While in sets 1) and 2) C3 cleaving activity depended only on the Bb fragment, i.e. was a measure of hydrolytic activity of D̄, in set 3) uncleaved B could have acted in addition, depending on nonenzymic activities of D̄. The ratios of activities D̄/D̄DFP were 1 to 5 in all three systems. This gives no support to the assumption that factor D has other than enzymic, hydrolytic activity in the generation of B-dependent enzymes.