Mapping of four discontiguous antigenic determinants on horse cytochrome c.

The epitopes (antigenic determinants) recognized by four different monoclonal antibodies on horse cytochrome c have been partially characterized by differential acetylation of lysine residues of free and antibody-bound cytochrome c. The degree of acetylation in the bound and free antigen molecule was assessed by a double-labeling procedure with [3H]acetic anhydride and [14C]acetic anhydride. Out of the 19 lysine residues of cytochrome c only very few were less reactive in the antigen-antibody complex, i.e. presumably located at the epitope for the antibody under study. The protection varied from 1.5-fold to over 20-fold lower reactivity in antibody-bound cytochrome c. The present results are complemented by previous data obtained by cross-reactivity analysis with cytochromes c from different species, with chemically modified cytochrome c derivatives, and by inhibition of proteolysis of cytochrome c in the presence of the antibodies. From the combined data we conclude that each of the four epitopes depends on the precise spatial folding of the antigen and contains residues which are brought together by the folding of the polypeptide chain. This work exemplifies that mapping of conformation-dependent epitopes can be achieved by applying a combination of mapping procedures of which each by itself provides partial information.