IMMUNE RECOGNITION OF CYTOCHROME-C .1. INVESTIGATION BY SYNTHESIS WHETHER ANTIGENIC SITES OF POLYMERIC CYTOCHROME COINCIDE WITH LOCATIONS OF SEQUENCE DIFFERENCES BETWEEN THE IMMUNIZING AND HOST CYTOCHROMES

Previous studies have shown that the locations of the antigenic sites of myoglobin, lysozyme, hemoglobin and serum albumin are independent of the host species and that their antigenicity is inherent in their structural locations. However, studies on cytochrome c (Cyt c) had been consistently analyzed on the basis of the assumption that protein antigenic sites are localized at locations of amino acid differences between the injected protein and its counterpart in the host. The intriguing possibility that immune recognition of Cyt c may proceed via a unique mechanism was tested using beef Cyt c as the antigen in rabbits. Regions around the 4 locations of amino acid differences between beef Cyt c and rabbit Cyt c (positions 44, 62, 89 and 92) were synthesized, and their ability to bind 125I-labeled antibodies against beef Cyt c was determined by immunoadsorbent titration studies. Of these peptides, only the peptide around the substitution at position 44 (i.e. peptide 42-50) had antibody binding activity which was almost equal to that of the homologous antigen and which also fully accounted for an autoreactivity of the antibodies with rabbit Cyt c. Since, of the 4 positions of amino acid differences, only 1 was localized in an antigenic site, this occurrence evidently was a chance happening. The antigenicity (and autoreactivity) of the region 42-50 was most likely inherent in its conformational location and not related to sequence differences between beef Cyt c and rabbit Cyt c.