Rabbit C3 has been isolated from normal rabbit plasma by a method based on that developed for human C3 by Tack and Prahl (Biochem. 15:4513, 1976). Fractionation of the rabbit plasma with polyethylene glycol was followed by DEAE-sepharose chromatography, gel filtration and final separation on hydroxyl apatite. The C3 obtained by this technique is hemolytically active and represents 7–15% of the plasma C3.

Rabbit C3 migrates as a single band on SDS gels with an estimated molecular weight of 186,000, and can be separated on SDS gels into α and β chains of 132,000 and 70,000, respectively, under reducing conditions.

Preliminary studies of the biologic activities of the rabbit C3 preparations indicate that it binds to rabbit peripheral blood neutrophils. The C3 remaining in the supernatant after incubation with the neutrophils is no longer hemolytically active, and SDS gels of this material show that it has been cleaved into two major fragments. Studies are in progress to determine the nature of the interaction between rabbit C3 and the neutrophils.

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