The immune response to oxidized ferredoxin. I. Specificity of the response to the amino terminal determinant.

Several synthetic peptide analogues of the amino terminal antigenic determinant (ala-tyr-lysile-ala-asp-ser) of oxidized ferredoxin (O-Fd) were tested for their ability to inhibit the complement fixation reaction between O-Fd and homologous rabbit antiserum, and to inhibit the migration of spleen cells from guinea-pigs immunized to O-Fd or to a conjugate of the amino terminal heptapeptide (N7) and bovine serum albumin (N7-BSA). The results of the migration inhibition assay suggest that the tetrapeptide and longer peptides of the native sequence were all recognized and stimulated the production of migration inhibition factor. Peptides modified at the aspartic residue were partially active while the serine modified peptide was not. Modification at the amino end of the heptapeptide had no effect on migration inhibition. As specificity controls, it was shown that the N7-BSA conjugate inhibited migration in O-Fd immunized animals, while O-Fd inhibited migration in N7-BSA immunized animals. The hexa-, hepta-, aspartic-deleted and serine-modified peptides were able to inhibit the complement fixation reaction with O-Fd and specific rabbit antiserum. Inhibition found with the serine-modified peptide and the lack of inhibition with the amino-modified peptide or the di-, tri-, tetra- and pentapeptides indicates the determinant recognized by the rabbit antibodies is either larger or is located nearer the middle of the heptapeptide than the determinant which induced the production of MIF.