Antibody response to the C-terminal peptide sequence in beef myoglobin

The nature of the surface determinants on the beef myoglobin molecule which direct the distinctive antibody response of sheep, have been further defined. Antisera raised to beef myoglobin in sheep, rabbits and mice have been compared for their ability to recognize the synthetic C-terminal beef myoglobin peptide (140-153), which contains four of the six amino acid substitutions between sheep and beef myoglobins. The antibodies raised in rabbits, directed to the entire surface of beef myoglobin, contain only a minor population directed to the C-terminal sequence: in mice, even fewer antibodies are specific for this sequence. In sheep, however, antibodies to beef myoglobin appear to be directed almost exclusively to topographic domains which include the (140-153) sequence. This specificity is most apparent in "early" antisera in which all antibodies display equal avidity for "native" beef myoglobin and the peptide; further immunization produces antibodies which recognize a larger overlapping set of domains and only 20% of these antibodies have effective avidity for the (140-153) peptide. The antibodies to beef myoglobin raised in sheep comprise two discrete populations. One ("common") population is directed to regions of similarity between the beef and sheep myoglobin molecules, in which the region represented by the C-terminal peptide of beef myoglobin is less important in defining the antibody-binding site and/or affinity, while still being directly involved in the topographic determinant. The other ("non-common") appears to be directed almost exclusively to the C-terminal sequence (140-153) of beef myoglobin. The findings are discussed in relation to our previous findings on the effect of the host species on the nature of the antibody response and in relation to views on the possibility of direct vs indirect effects of evolutionary amino acid substitution on immuno-cross-reactivity among homologous proteins.