Beschreibung:
<jats:p>The antigenic determinants for three monoclonal antibodies against cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub> from <jats:italic>Rhodospirillum rubrum</jats:italic> were partially characterized by differential chemical modification of free and antibody‐bound cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub> and by cross‐reactivity analysis with different antigens. Circular dichroism spectroscopy was used to probe the effect of antibody binding on the conformation of cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub>. The binding of two antibodies was strongly dependent on the native folding of the antigen. The first antibody bound to a determinant around the exposed heme edge on the ‘front side’ of the molecule which is not antigenic in mitochondrial cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub>. Binding of this antibody to cytochrome <jats:italic>c</jats:italic> increased the induced CD of the ferric heme in a manner similar to that observed previously when mitochondrial cytochrome‐<jats:italic>c</jats:italic> oxidase bound to the front side of cytochrome <jats:italic>c</jats:italic>. This observation points to a subtle conformational adaptation of the antigen induced by the antibody. The determinant for the second antibody, which also affected the heme CD spectrum of the antigen, was on a polypeptide loop where cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub> differs from mitochondrial cytochrome <jats:italic>c</jats:italic> by an eight‐residue insertion. The third antibody, which did not induce a change in CD, bound to a sequential determinant near the amino end of cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub>. Only this antibody cross‐reacted with isolated cytochrome‐<jats:italic>c</jats:italic>‐derived peptides and with apo‐cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub>.</jats:p><jats:p>A preliminary analysis of the polyclonal immune response of five rats against cytochrome <jats:italic>c</jats:italic><jats:sub>2</jats:sub> indicates that, unlike in eukaryotic cytochrome <jats:italic>c</jats:italic>, antigenic determinants are distributed over the whole polypeptide chain of the prokaryotic immunogen.</jats:p>