Complement-independent activation of the fifth component (C5) of human complement: limited trypsin digestion resulting in the expression of biologic activity

Highly purified human C5 was subjected to limited trypsin digestion (0.42 mol affinity-purified trypsin/mol C5) at pH 7.4 and 37 degree C. Samples were removed after various time intervals, treated with phenylmethylsulfonyl fluoride to inhibit trypsin activity, and were analyzed by SDS-polyacrylamide slab gel electrophoresis (SDS-PAGE) in the presence or absence of beta -mercaptoethanol ( beta ME). Collectively, the results indicate that limited trypsin digestion of human C5 did not result in the production of C5a and C5b fragments. Nevertheless, limited trypsin digestion did result in the activation of human C5 to induce human neutrophil chemotactic and lysosomal enzyme-releasing activities. Detailed correlation experiments demonstrated that the first trypsin-mediated C5 alpha to C5 alpha sub(1) and C5 alpha sub(5) polypeptide fragments, resulted in the expression of C5a-like biologic activities by trypsin-modified C5.