Conversion of the fourth complement component studied by crossed immunoelectrophoresis.

C4 in EDTA plasma and partially purified C4 give a β peak on crossed immunoelectrophoresis. During electrophoresis C4 in serum is converted to a product of fast β mobility, usually accompanied by a slow β peak. Conversion in serum is inhibited by EDTA. Storage of serum at room temperature results in a gradual increase of the slow β peak. Storage of EDTA plasma changes the configuration of the native β peak. C[unk]s, trypsin, chymotrypsin, plasmin or thrombin added to partially purified C4 is capable of producing a fast β C4 protein peak. C[unk]s, trypsin and chymotrypsin give this conversion product also when added to EDTA serum. C[unk]s, trypsin and chymotrypsin also give rise to a show β and an inter α C4 conversion product in serum, probably consisting of complex formations between C4 and other serum proteins. Enzyme inhibitors known to interfere with C[unk] inhibit the conversion of C4 in serum on agarose electrophoresis. The results suggest that such conversion is caused by an activation of C1 during electrophoresis.