'Inactivated' third component of complement (C3b-like C3; C3i) acquires C5 binding capacity and supports C5 activation upon covalent fixation to a solid surface.

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.