In human, small amounts of hydrolyzed C3 (C3(H20) or C3i) are continuously formed in the fluid phase following spontaneous hydrolysis of the internal thioester bond of C3. The thioester bond in the alpha chain of C3 is formed between cysteine residue 1010 and glutamine residue 1013. The hydrolysis of this bond generates a sulfhydryl group on Cys-1010 and a carboxyl group on deamidated Gln-1013 [Pangburn MK and Muller-Eberhard HJ 1983]. Purified and characterized chicken C3 has 54% amino acid identity with both human and mouse counterparts [Mavrodis M et al 1995]. Like its mammalian ortholog, chicken pro-C3 is cleaved to generate a mature two-chain molecule. The alpha chain of chicken C3 contains an internal thioester and three potential N-glycosylation sites [Laursen I and Koch C 1989; Mavrodis M et al 1995]. The function of chicken C3 is dependent on the thioester because treatment of purified chicken C3 with methylamine inactivates it [Laursen I and Koch C 1989]. Multiple sequence alignment of human and chicken C3 orthologues reveals that chicken residues Cys-1002 and Gln-1005 correspond to human Cys-1010 and Gln-1013.