Studies in vivo of cobra factor and murine C3

The effect of the isolated C3-cleaving factor (CoF) of cobra venom on murine C3 in vivo and in vitro was studied. Optimal quantities of 100-200 units (0.5 minus 1.0 mg) of CoF per kg administered to mice by intraperitoneal injection in divided doses caused plasma C3 levels to fall to less than 5 per cent of normal from 1 to at least 4 days afterwards. Passive anti-CoF serum promptly abrogated the in vivo plasma C3 depletion, and under optimal conditions C3 levels reached 50 per cent of normal after approximately 15 hours. Injection of as little as 20 mug per mouse of CoF in saline induced a precipitating anti-CoF antibody response which prevented subsequent depletion of plasma C3 by CoF. The in vivo half-life of 125I-labelled CoF in normal mice estimated by whole body elimination and clearance from the blood was 24 hours. The presence in vivo of antibodies to CoF caused rapid clearance from the blood and elimination of 125I-labelled CoF, and also localization of some CoF in the spleen, liver and kidneys.