'Inactivated' third component of complement (C3b-like C3; C3i) acquires C5 binding capacity and supports C5...

The haemolytic activity of human C3 was destroyed by freezing and thawing or by treatment with methylamine. This partially denatured C3 (C3i) was coupled to activated Thiol-Sepharose via its single SH group. The fixed C3i was capable of reversibly binding C5 and of supporting C5 cleavage in the presence of factors B and D. Soluble C3i lacked these properties. C3i was also fixed to sheep red cells which had been supplied with activated thiol groups by treatment with N-succinimidyl-3-(2-pyridyldithio)-propionate. The E-C3i thus obtained were lysed by factors B, D and late complement components C5-C9. The experiments demonstrate that upon fixation, C3i becomes C3b-like in functional activity directed to C5 utilization.