A two-site immunoradiometric assay of proatrial natriuretic factor. Application to tissue extracts.

A "two-site" immunoradiometric assay (IRMA) was developed to specifically measure ANF (1-126), the precursor of ANF. This assay is based on the simultaneous use of antibodies against two different antigenic determinants: murine monoclonal antibody (2H2), which recognizes positions 101 through 103 of ANF, is linked to Immunobeads and employed to extract any ANF C-terminal; a second antibody, which is directed against positions 11 through 37, is radioiodinated and allows binding to any C-terminal-2H2-Immunobead material which bears the N-terminal antigenic site. A curvilinear relationship was obtained between radioactivity and the amount of proANF (1.5 to 400 fmol) added. Optimisation of IRMA was determined by the amount of 2H2-Immunobeads and labeled antibody used, incubation time as well as possible interference by both ANF (99-126) and ANF (1-98). Tissue extracts were used to validate the assay. proANF was detected in decreasing amounts in heart atria, heart ventricles, lungs, kidneys and adrenal glands. Its presence was further confirmed by reverse-phase HPLC followed by radioimmunoassay. IRMA is a simple and rapid method for the direct measurement of proANF in tissue extracts and chromatographic fractions. The presence of proANF in tissues strongly suggests local synthesis.