Antibody subpopulation in antihorse cytochrome c serum.

A specific antibody subpopulation(s) in antihorse cytochrome c serum was detected for peptide fragment 81-104 of cyanogen bromide (CNBr) cleaved horse cytochrome c (HCytc). This antiserum was made in the rabbit against polymeric horse cytochrome c. The presence of the peptide-specific antibody subpopulation(s) was demonstrated utilizing HCytc, CNBr-peptide 81-104 and isolated chymotrypsin-digested HCytc fragments 60-67, 83-97 and 98-104 to compete with radio-labeled peptide 81-104 and antiHCytc serum in a competitive radioimmunoassay (RIA). This antibody subpopulation(s) in antiHCytc serum was demonstrated to be specific for peptide 81-104. At the 50% inhibition level in competitive RIA, 100- and 1000-fold molar excesses of HCytc and its peptide 1-65, respectively, were required to affect an equivalent binding to that of the HCytc peptide 81-104. Competitive RIAs have been performed utilizing three different kinds of antigen to compete with HCytc peptide 81-104 and antihorse cytochrome c sera. These three kinds of antigens are: endopeptidase digests of HCytc, cytochrome c peptides 81-104 of several species and several isolated chymotryptic peptide fragments of HCytc. The results have indicated that this peptide-specific antibody subpopulations(s) in antiHCytc serum is similar to antibodies made against peptide 81-104-BSA. Regions of antigenicity have been identified at positions 92, 100, 103 and 104 with both antisera.