Attachment of [³⁵S]cysteine to the labile site of the third component of complement via the alternative pathway

Agarose beads to which [³⁵S]cysteine had been attached by a disulfide bond were incubated in serum under conditions which permitted complement fixation via the alternative pathway, then washed and treated with dithiothreitol to release the cysteine. The dithiothreitol eluate contained C3b fragments which had been labeled covalently by the radioactive cysteine. Labeling was postulated to involve the reaction of the amino group of the cysteine with the "labile site," an acylating group which is exposed when C3 is converted to C3b during the operation of the alternative complement-fixing pathway.