The C5b-6 complex was isolated from zymosan activated, C7-depleted human serum by affinity chromatography on lysine-Sepharose 4B and gel filtration. This C5b-6 consisted of the noncovalently linked subunits C5b and C6. Upon reduction, C5b gave rise to the C5bα chain, molecular weight 110,000, and to the C5β chain, molecular weight 76,000. Twenty-five percent of the C5bα chain was cleaved by an unidentified protease and upon reduction gave rise to a C5bα1 chain and a C5bα2 chain whose molecular weights were 76,000 and 27,000 respectively. To evaluate the influence of proteolysis on the subunit composition and hemolytic activity of C5b-6, we treated the complex with several proteases. Incubation of C5b-6 with β1H and C3bINA at physiologic concentrations neither cleaved the C5bα chain nor changed the hemolytic activity of the complex. Likewise, thrombin had no effect on the subunit composition of C5b-6. However, plasmin and trypsin progressively cleaved the C5bα chain of C5b-6 into the C5bα1 and C5bα2 fragments but did not change the hemolytic activity of the complex. Neither the C5β chain nor C6 was altered by these proteases. For comparison, purified C5 was treated with plasmin. Although the C5β chain was resistant to proteolytic attack, the C5α chain was cleaved at multiple sites into several low molecular weight fragments. Consequently, limited proteolysis is not an important regulator of the initial phase of membrane attack pathway of complement; in fact, the C5bα chain within the C5b-6 complex is partially protected from proteolytic attack possibly by C6.
|Original language||English (US)|
|Number of pages||5|
|Journal||Journal of Immunology|
|State||Published - Jan 1 1980|
ASJC Scopus subject areas
- Immunology and Allergy