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Reaction-Mechanisms Of C3NeF

Departments of Medicine and Immunology, Royal Postgraduate Medical School, London, England

Abstract

The cofactor requirements of C3NeF have been examined by using purified factors, C3, B, and D. Our experiments indicate that in the presence of NeF two C3-cleaving enzymes can be generated: when D is not added to the reaction mixture, B cleavage was not observed, whereas the addition of quantities of D sufficient to allow C3b feedback activation by purified C3b or CVF led to B cleavage. Both the B cleaving and the non-B cleaving reactions were inhibited (in a dose and time-dependent fashion) by preincubation of the C3 source with C3b inactivator (KAF) indicating that C3b rather than C3 is required. We envisage two enzymes
(NeF) and
(NeF), NeF functioning (as suggested by Fearon and Austen) to stabilize them. Thus, in the presence of NeF trace amounts of C3b—provided, for example, by spontaneous C3 breakdown or by other plasma proteolytic enzymes ("C3 tickover")—would lead to the generation of an efficient C3 convertase.