Activated factor B (Bb) of the alternative pathway of complement activation cleaves and activates plasminogen
Sundsmo, J.S.; Wood, L.M.
Journal of Immunology 127(3): 877-880
1981
ISSN/ISBN: 0022-1767 PMID: 6911148 Document Number: 179200
Activated Factor B (Bb), the central serine esterase of the alternative pathway of complement (C) activation, exhibits restricted substrate specificity for C3 and C5. Bb can cleave and activate plasminogen in an experimental system containing purified plasminogen and Bb, C cellular intermediates bearing the Bb-enzyme, or cobra venom factor-stabilized Bb-enzyme (CVF,Bb). Cleavage of plasminogen by Factor Bb generated 2 disulfide-linked polypeptides with apparent MW of 64,000 and 25,000-32,000. Cellular intermediates containing the C3b,Bb-enzyme cleave 40-80% of 4.5 .mu.g 125I-labeled plasminogen during 30 min of incubation at 37.degree. C; native Factor B was inactive; and anti-Factor BIg inhibited by 100% the plasminogen cleavage mediated by C cellular intermediates bearing the Bb-enzyme. Fibrinolytic activity was detected in plasminogen activator (PA) assays when purified plasminogen and 125I-labeled fibrin tubes were incubated with Bb, CVF,Bb or C cellular intermediates bearing the C3b,Bb-enzyme: 10 .mu.g Bb released 40-65% of the 125I-fibrin released by 5 .mu.g urokinase in 4 h at 37.degree. C. Plasminogen activator activity of the C3b,Bb-enzyme was regulated in serum. At 1:50 dilutions of normal human serum, the PA-activity of 1.6 .mu.g Bb was 100% inhibited and at a 1:250 dilution, 50% inhibition was observed. A novel activity for the Bb-enzyme is described which constitutes the C3/C5-convertase of the alternative pathway of C activation.