COMPLEMENT AS A MEDIATOR OF INFLAMMATION

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Abstract
Purified preparations of human C'1 esterase, C'4, C'2, C'3, and C'5 were labeled with 125I. Reaction mixtures were prepared containing a single labeled component and other unlabled components. After incubation at 37°C for 10 min at pH 7.4 in the presence of 5 x 10–4 M Mg2+, they were adjusted to pH 3.5 and subjected to sucrose density gradient ultracentrifugation and gel filtration at pH 3.5. In all cases, an activity capable of contracting guinea pig ileum with tachyphylaxis was obtained in low molecular weight fractions. However, these fractions were labeled only when 125I-C'3 was employed, indicating that biological activity was associated with a cleavage product of C'3. This fragment has been designated F(a)C'3 in a nomenclature consistent with that of immunoglobulin degradation products. The much larger, residual portion of the C'3 molecule has been designated F(b)C'3.