Covalent binding of [³⁵S]cysteine to the labile binding site of the third component of complement: A physiological approach

The alternative pathway of complement was activated physiologically by agarose beads to which [³⁵S]cysteine had been bound by a disulfide link. The activated form of the third component of complement, C3b, which had bound to the radioactive cysteine was then released from tharose bead with dithiothreitol. The [³⁵S]cysteine was shown to be covalently bound to the α′ chain of C3b, and to its known major breakdown product, the 66,000-Da polypeptide. This is highly suggestive that complement activation has led to formation of a peptide bond between the radioactive cysteine and the labile binding site on the α′ chain of C3b. This radioactive marker will enable the amino acid sequence of the labile binding site to be determined, with the knowledge that the labeling of the amino acid(s) has occurred during physiological activation.