C3 modified at the thiolester site: acquisition of reactivity with cellular C3b receptors.

Isolated human C3 protein has been reported to have variable affinity for cellular C3b receptors. The question was investigated therefore as to whether native C3 or a derivative form of the protein exhibits receptor affinity. It was found that native C3 lacks the ability to react with C3b receptors. However, C3 modified at the thiolester site, either by treatment with chaotropic agents or methylamine or by freezing and thawing, expressed inhibitory activity in the C3b-mediated rosetting assay or immune adherence. The extent of inhibition was comparable to that caused by C3b. The ability of modified C3 to react with cellular C3b receptors constitutes an additional functional property of C3b-like C3.