Beschreibung:
<jats:p>The antigenic determinants for two monoclonal antibodies directed against horse apo‐cytochrome <jats:italic>c</jats:italic>, a protein of disordered structure, as judged by spectroscopic and hydrodynamic criteria, have been studied by a combination of methods: antigen competition in solution by radio immunoassay and enzyme‐linked immunoassay, and differential acetylation of free and antibody‐bound antigen. In the latter method the accessibility of lysine residues of the antigen in the antigen‐antibody complex is compared to the accessibility in the free antigen.</jats:p><jats:p>The two antibodies against the heme‐free protein do not recognize intact native cytochrome <jats:italic>c</jats:italic>, but they crossreact with the heme‐containing peptides 1–38 and 1–65 of cytochrome <jats:italic>c</jats:italic>. The antigenic determinant recognized by monoclonal antibody SJL 2–4 is conformational and discontiguous, it is composed of residues close to the N‐terminus and around position 25. The other monoclonal antibody, Cyt‐1–59, seems to recognize a contiguous epitope close to the N‐terminus.</jats:p><jats:p>The present results show that even a seemingly disordered protein which is conventionally classified as a random coil may feature subtle spatial regularities. The presence of ordered conformational elements in apocytochrome <jats:italic>c</jats:italic> may be important for the enzyme‐catalyzed covalent attachment of the heme and the import of cytochrome <jats:italic>c</jats:italic> into mitochondria. A discontiguous determinant for SJL 2–4 is particularly interesting because this antibody inhibits the proliferation of a T‐cell clone specific for apo‐cytochrome <jats:italic>c</jats:italic> [Corradin & Engers (1984) <jats:italic>Nature (Lond.) 308</jats:italic>, 547–548].</jats:p>