Two complex enzymes were assembled that both converted C3 to C3b, one consisting of activated properdin (P), native C3, proactivator (PA) and proactivator convertase (PAase), and the other of nephritic factor (NF) and the same three cofactors. By maintaining a critical concentration of PAase, the P-C3 convertase and the NF-C3 convertase were shown to function efficiently without formation of th...

The complement fragments C3b and C4b are the main ligands for the membrane receptor CR1. We showed elsewhere that CR1 functions as an essential cofactor for the factor I-mediated enzymatic breakdown of membrane-bound C3b (*C3b) into C3c and * C3dg . One of the main findings of the present paper is that CR1 also promotes the degradation of bound C4b (*C4b) into C4c and *C4d. On a weight basis, t...

Cleavage of C3 by the physiologic C3 convertases or by trypsin results in the generation of a metastable form of C3b bearing a reactive internal thioester (1). This thioester may undergo hydrolysis by solvent water molecules or may form ester or amide linkages with suitable acceptors. Immunoglobulin G (IgG) is reported to display a measurable affinity for uncleaved C3 (2) and has been shown to ...

The C5 convertase of the classical complement pathway is a complex enzyme consisting of three complement fragments, C4b, C2a, and C3b. Previous studies have elucidated functional roles of each subunit (4, 6, 7), but little is known about how the subunits associate with each other. In this investigation, we studied the nature of the classical C5 convertase that was assembled on sheep erythrocyte...

Purified beta1H globulin (beta1H) was shown to bind to C3b coated cells by both immunofluorescent and radioactive tracer techniques. With EAC43, the amount of beta1H bound was directly proportional to the amount of C3 used to prepare the cells; EA, EAC14 and EAC14oxy2 bound very small amounts of beta1H. The C3b binding site on beta1H was labile in that not all of the purified 125I-beta1H was ca...

We have recently reported the isolation from rabbit alveolar macrophages of a receptor which retained its ligand-binding activity for the third component of complement (C3) and for its major proteolytic derived activation fragment (C3b). The isolated receptor demonstrated a greater ability to bind C3b than an equimolar amount of C3. C3b differs from C3 in at least two ways: it is a proteolytic ...

Several complement proteins interact with hemostatic factors. We discovered that von Willebrand factor (VWF) acts as a cofactor for factor I-mediated cleavage of complement C3b, thereby shutting down complement activation. The complement regulatory function of VWF multimers depends on their size. Smaller VWF multimers enhance cleavage of C3b but large and ultra-large VWF (ULVWF) multimers have ...

Mononuclear cells from normal human subjects and patients with chronic lymphocytic leukemia (CLL), chronic lymphosarcoma cell leukemia (LCL), and hairy cell leukemia (HCL) were labeled with fluoresceinated, purified human C3b (FI-C3b) and analyzed using the fluorescence-activated cell sorter (FACS). FI-C3b labeled 17.6% +/- 6.0% of peripheral blood mononuclear cells (PBM) from 20 normal subject...

Addition of isolated B3b to murine lymph node cell cultures induced increased DNA synthesis. The stimulation is dependent on the dose of C3b added and is potentiated by fetal calf serum present in the medium. Isolated C3 is less stimulatory than C3b; C3a and C3c had no effect on DNA synthesis in these cultures. 10-20% large immunoglobulin containing blastlike cells developed in lymph node cell ...

C3 and C3b, the components central to the complement activation, also play a damaging role in several inflammatory disorders. Vaccinia virus complement control protein (VCP) and curcumin (Cur) are natural compounds with different biological origins reported to regulate complement activation. However, both VCP and Cur have not been investigated for their interaction with the third component (C3)...