The metabolic behavior of C′3 labeled with radioactive iodine was investigated in 10 normal subjects and in 20 patients with diseases in which complement is thought to play a pathophysiological role. The mean fractional catabolic rate of C′3 in normal subjects was 2.3 ± 1.0% of the plasma pool per hr, whereas the fractional catabolic rate of C′3i, the inactive conversion product of C′3 produced by complement activation, was at least five times as great.
