Cleavage of human C5 by trypsin: characterization of the digestion products by gel electrophoresis.

Human C5 is composed of two nonidentical polypeptide chains, alpha and beta (m.w. 130,000 and 80,000, respectively) linked together by disulfide bonds and noncovalent forces. Cleavage of C5 by trypsin fragments with increased anodic mobilities. Limited digestion of C5 by trypsin (substrate to enzyme ratio 10:1 w/w at 37 degrees C for 1 min) resulted in the release of a small terminal alpha-chain peptide (alpha1, m.w. 15,000) probably analogous to C5a, from a large fragment, C5b (m.w. 195,000) composed of an intact beta-chain disulfide linked to an alpha-chain that has a lower m.w. (alpha' 115,000). Further digestion (37 degrees C, 5 min) resulted in cleavage of the alpha-chain at multiple sites with the production of three peptides from the alpha'-chain (alpha2I, 23,500; alpha2II 15,700 and alpha2III 10,200) and a residual fragment, C5c (m.w. 144,000). The alpha1 and alpha2 peptides are not covalently linked to the beta-chain nor to one another. The C5c fragment on the other hand is composed of small peptides of the alpha'c chain (alpha3 14,000; alpha4I 9,000; ALPHA 4II 11,000; alpha 5 23,000 to 30,000) which are linked to the beta-chain and also probably to one another by covalent bonds. Secondary cleavage occurred upon prolonged digestion with trypsin (37 degrees C, 20 min), and this resulted in the progressive erosion of the alpha'c peptides and the conversion of C5c to smaller C5c-like species.