Radio-immunoassay of atrial natriuretic peptide (ANP) and characterization of ANP immunoreactivity in human plasma and atrial tissue

Novel Chemiluminescent Enzyme Immunoassays for Individual Quantification of 3 Endogenous Molecular Forms of Atrial Natriuretic Peptide in Human Plasma

Background

Atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) are cardiac peptide hormones with pivotal roles in maintaining cardiovascular homeostasis. BNP and its precursor fragment are accepted as gold standard markers for heart failure (HF). Human ANP is present in the atria of the heart and plasma as 3 endogenous molecular forms designated α-ANP, β-ANP, and proANP. A previous study indicated that the ratios of these 3 ANP forms are altered in the plasma of HF patients. The purpose of our study was to establish immunoassays for quantifying the individual ANP forms to collect clinical information.

Methods

We developed 3 plate-based chemiluminescent enzyme immunoassays (CLEIAs) for measuring total ANP (i.e., sum of α-ANP, β-ANP, and proANP), β-ANP, and proANP levels. To minimize background signals, we added single-step PEGylation targeting the immobilized antibody in the conventional plate-based sandwich CLEIA procedure.

Results

CLEIAs with PEGylation showed sensitivity, specificity, reproducibility, and accuracy satisfying clinical requirements. Two of the CLEIAs enabled direct measurement in plasma samples. During treatments, acute decompensated HF patients exhibited marked decreases in plasma β-ANP levels but moderate decreases in plasma proANP level. The plasma ratios of α-ANP/total ANP and proANP/total ANP in acute decompensated HF patients were maintained, whereas the β-ANP/total ANP ratio was significantly decreased at discharge.

Conclusions

The combination of the 3 CLEIAs enabled accurate quantification of α-ANP, β-ANP, and proANP, even in plasma samples, and indicated the potential of β-ANP and proANP as circulating biomarkers for HF, with different characteristics from that of BNP.

Molecular forms of natriuretic peptides in heart failure and their implications

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are important biomarkers in the diagnosis and risk stratification for heart failure (HF). These peptides are synthesised as inactive precursors, pro-ANP and pro-BNP, which are converted to biologically active 28 amino acid ANP and 32 amino acid BNP, respectively. Most immunoassays currently used in the clinical setting, however, do not determine precise molecular forms of these natriuretic peptides, which may vary depending on the pathophysiological state of HF. Analysis from chromatography-based studies reveals that in HF inactive pro-ANP and pro-BNP forms often predominate. This indicates that the bioactive forms of natriuretic peptides may not be processed proportionally in patients with advanced HF. Distinguishing the bioactive natriuretic peptides from their inactive forms in plasma may help to define the role of these peptides in the pathogenesis of HF and provide new insights into the treatment of the disease.

Dual detection of peptides in a fluorescence binding assay by employing genetically fused GFP and BFP mutants

A competitive fluorescence microplate assay based on a red-shifted green fluorescent protein (rsGFP) and a blue fluorescent protein (BFP) was developed for the detection of two model peptides in the same sample. The assay employed gene fusion to prepare the fluorescently labeled peptide conjugates. Specifically, plasmids were constructed in which the genes encoding for the two small peptides (less than 12 amino acids in length) were fused to either the gene of the rsGFP or the BFP, as desired. The newly constructed plasmids were transformed into E. coli for expression of the fusion proteins. By employing the technique of gene fusion, one-to-one homogeneous populations of peptide-rsGFP or -BFP conjugates were produced. These peptide-GFP mutant conjugates exhibited the same excitation and emission spectral characteristics as the unmodified proteins. The naturally fluorescent proteins act as labels to provide sensitive dual detection of the two selected small peptides in a competitive assay format. To our knowledge, this is the first time that mutants of GFP, such as the rsGFP and BFP, have been used as quantitative labels for the development of a dual-analyte fluorescence immunoassay.

Degradation and inactivation of rat atrial natriuretic peptide 1-28 by neutral endopeptidase-24.11 in rat pulmonary membranes

Atrial natriuretic peptide (ANP), a 28-residue peptide with cardiovascular and renal effects, is rapidly cleared from the circulation. Beside renal clearance, an extra-renal metabolism by the enzyme neutral endopeptidase-24.11 (NEP-24.11) has been proposed, since specific NEP-24.11-inhibitors increase endogenous plasma-ANP. NEP-24.11 is present in rat lung but its significance for ANP hydrolysis within the lung is unclear. The aim of this study was to investigate a possible degradation of rat ANP in a membrane preparation from rat lung. Hydrolysis products of ANP were separated by HPLC and further characterized by a pulmonary artery bioassay, by radioimmunoassay with different antisera, by peptide sequencing and by masspectrometry. Rat pulmonary membranes degraded ANP to one main metabolite lacking biological activity and with poor cross-reactivity to an antiserum recognising the central ring-structure of the peptide. Formation of the hydrolysis product was prevented by the NEP-24.11-inhibitor phosphoramidon (1 microM). Peptide sequencing of the metabolite revealed a cleavage between Cys7 and Phe8, which was confirmed by mass-spectrometry. The metabolite had an HPLC elution time identical to that of the product formed by purified porcine NEP-24.11. These findings suggest that ANP is metabolized and inactivated by endopeptidase-24.11 in rat lungs, the first organ exposed to ANP released from the heart.