Spontaneous activation of the first component of human complement (C1) by an intramolecular autocatalytic mechanism

For biochemical characterization, the first component of human complement (C1) was reconstituted from physiologic concentrations of purified C1q, 125I C1r, and 131I C1s. Upon incubation at 37 degrees C, C1 spontaneously activated, as evidenced by the characteristic proteolysis of the C1r and C1s polypeptide chains as detected by SDS-PAGE analysis. This spontaneous C1 activation followed first-order kinetics (t 1/2 = 4 min and k = 0.173 min-1) with an activation energy of 19.1 kcal/mol. Spontaneous C1 activation was unaffected by the general protease inhibitor phenylmethylsulfonylfluoride (PMSF) but reversibly blocked by a known inhibitor of C1 activation, nitrophenylguanidinobenzoate (NPGB). Spontaneous C1 activation was measured at C1 concentrations ranging from 9 to 160 nM (i.e., 0.05 to 1.0 times physiologic concentrations). The data indicate that C1 spontaneously activates by an intramolecular autocatalytic mechanism, for first-order kinetics were observed over the entire concentration range with t 1/2 = 4 min at each concentration. However, the percentage of activable C1 decreased with dilution due to C1 dissociation (i.e., C1qr2s2 leads to C1q + C1r2s2). The observed concentration of C1 that spontaneously activated at each dilution equalled the concentration of C1 present as macromolecular C1. When reconstituted C1 was mixed with normal human serum (NHS) and then incubated at 37 degrees C, spontaneous C1 activation was completely inhibited. Pretreating NHS at 56 degrees C for 30 min destroyed its inhibitory activity. In conclusion, C1 spontaneously autoactivates at 37 degrees C by an intramolecular mechanism. This activation is suppressed in NHS.