Human serum (pH 7.0) was incubated for 30 min at 0°C with ovalbumin-IgG anti-ovalbumin complexes in the presence of 50 µg/ml soybean trypsin inhibitor. After centrifugation and three washes at 0°C in a buffer containing 5 mM CaCl2 and 40 µg/ml STI (pH 7.0), C1̄r and C1̄s were resolubilized. From the C1̄s esterase activity and the SDS-polyacrylamide gel electrophoresis pattern, extracted C1̄r and C1̄s were found to be 75% activted. Complete activation was obtained by incubation for 30 min at 30°C before extraction.

Extraction of C1q, C1r and C1s in the presence of calcium. At pH 7.5, C1̄r and C1̄s were both readily extracted by increasing relative salt concentrations above 0.15. In contrast, C1q extraction started at RSC = 0.30 and increased more slowly.

C1̄r extraction paralleled C1̄s extraction as a function of pH, with a minimum at pH 7.0 and maxima at extreme pH values.

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