J. Biochem, 1988, Vol. 103, No. 5 792-796
© 1988 Japanese Biochemical Society
research-article |
Heat-Induced Thiol-Disulfide Interchange Reaction on the Third Component of Human Complement, C3
* Department of Immunology, The Center for Adult Diseases Higashinari-ku, Osaka, Osaka 537
** Hokkaido University Kita-ku, Sapporo, Hokkaido 060
1To whom correspondence should be addressed.
The third component of human complement, C3 is composed of two disulfide-bridged polypeptide chains of Mr 120,000 (a chain) and Mr 70,000 (ß chain). C3 has a thioester bond that serves as a binding site for targets when C3 is activated. Heat treatment of C3 induces autolytic peptide bond cleavage at the thioester site in the a chain as well as rupture of the thioester bond. The a chain fragments are linked to each other and ß chain via disulfide bonds. This study, however, documented that prolonged heating gave rise to liberation of several fragments including, ß and the larger fragment of a chain. Using a fluorescent thiol reagent and [14C] iodoacetamide, we analyzed thiol residues present on each fragment, and elucidated that the thiol residue exposed by rupture of the thioester bond shifts in turn to another fragment resulting in the liberation of the fragments. The results were compatible with those on C4, and suggested that the generated thiol residue induces thiol-disulfide interchange reaction. On heating of plasma, fragments of C3 were not released, while the cleavage of the 
chain occurred more effectively. The heated C3 (56°C, 15 min) became insusceptible to C3b inactivator (I) and factor H, suggesting that additional conformational change is accompanied with cleavage of the thioester bond.