S protein, an inhibitor to the membrane attack complex of complement, was purified from human plasma. The procedure involved barium citrate adsorption and fractionation by poly(ethylene glycol) 4000 precipitation, followed by chromatography on DEAE-Sephacel, Blue Sepharose, Sephacryl S-200, and finally anti-albumin-Sepharose. Reduced glutathione was added throughout to inhibit spontaneous formation of disulfide-linked S-protein dimers. The recovery was 7%, resulting in approximately 10 mg of pure S protein from 1 L of starting plasma. S protein is a single-chain molecule; sedimentation equilibrium ultracentrifugation yielded a molecular weight of 83 000; the s20,w0 value was estimated to be 4.0 S. The purified protein contained a free, reactive thiol group causing spontaneous formation of disulfide-linked S-protein dimers. Alkylated and nonalkylated S proteins were equally active in inhibiting C9 polymerization, catalyzed by the C5b-8 complex. In parallel with the inhibition of C9 polymerization, nonalkylated S protein catalyzed the formation of disulfide-linked C9 dimers, presumably through disulfide interchanges.
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