IgG and rabbit IgG in binding of complement subcomponent Clq

Treatment of covalently cross-linked or heat-aggregated oligomers of human IgG with 4 mMtetranitromethane abrogated their Clq-binding activity. In contrast, tetranitromethane modification of rabbit IgG oligomers, under identical conditions, had no effect upon their Clq-binding activity. The tetranitromethane treatment led to nitration of about ten tyrosine residues per IgG molecule in both species, and the modification was specific for tyrosine residues. Reduction of the nitrated protein with Na2S204 did not lead to recovery of Clq-binding activity in human IgG oligomers or to loss of activity in rabbit IgG oligomers. Tryptic peptides from the nitrated proteins were isolated and a peptide containing nitrotyrosine319 was recovered from human IgG, as well as peptides from both species corresponding to the region around nitrotyrosine-278. These data are consistent with the inactivation of Clq-binding activity in human IgG being the result of nitration of tyrosine-319; the rabbit IgG is unaffected by nitration because position 319 is phenylalanine. The evidence supports the Clq-receptor site proposed by Burton, Boyd, Brampton, Easterbrook-Smith, Emanuel, Novotny, Rademacher, van Schravendijk, Sternberg & Dwek [(1980) Nature (London) 288, 338-344]: residues 316-338.