Enzyme immunoassay for alpha-human atrial natriuretic polypeptide--direct measurement of plasma level

Ultrasensitive enzyme immunoassay

Ultrasensitive enzyme immunoassay methods are reviewed not only for antigens but also for antibodies and haptens with emphasis on factors which limit the sensitivity. Ultrasensitive immunoassays can be developed by noncompetitive solid phase assay systems rather than competitive ones for antigens and antibodies. However, no noncompetitive immunoassays have been available for hapten molecules which cannot be bound simultaneously by two different antibody molecules. This has been overcome by developing methods to derivatize haptens with amino groups so that the derivatized haptens may be measured by two-site noncompetitive assays. For ultrasensitive noncompetitive solid phase immunoassays, the nonspecific binding of labeled reactants (background noise) should be minimized. This has been achieved by developing methods to transfer the complex of analytes and labeled reactants from solid phase to solid phase with minimal dissociation of the complex. Thus, the sensitivity for antigens, haptens and antibodies has been markedly improved and some applications have been made.

Synthesis of recombinant atrial natriuretic peptide (rANP) using hybrid fusion protein-phage fr coat/ANP (CP/ANP)

Recombinant atrial natriuretic peptide (rANP) was expressed in and isolated from E. coli. rANP was purified using HPLC. Amino acid analysis, partial sequencing, and molecular mass were determined. Fused protein was used to rise polyclonal antibodies and to develop of immunoenzymatic assays of rANP and CP/ANP. Experiments were designed to study rANP effects on isolated rabbit aortic strips and to examine hypotensive, diuretic, and natriuretic activity, as well as renal creatinine clearance, in an in vivo rat model. Identity of recombinant and commercial ANP has been confirmed. Physiological activity of CP/ANP has allowed the investigators to predict the conformation of CP/ANP, pro-ANP processing, and the method by which fusion protein interacts with ANP receptors.

Brain natriuretic peptide as a novel cardiac hormone in humans. Evidence for an exquisite dual natriuretic peptide system, atrial natriuretic peptide and brain natriuretic peptide

Using a specific radioimmunoassay for human brain natriuretic peptide (hBNP) with a monoclonal antibody, we have investigated its synthesis, secretion, and clearance in comparison with those of atrial natriuretic peptide (ANP) in normal subjects and patients with congestive heart failure (CHF). Mean BNP-like immunoreactivity (-LI) levels in normal atrium and ventricle were 250 and 18 pmol/g, respectively. The plasma BNP-LI level in normal subjects was 0.90 +/- 0.07 fmol/ml, which was 16% of the ANP-LI level. In contrast, the plasma BNP-LI level markedly increased in patients with CHF in proportion to its severity, and surpassed the ANP-LI level in severe cases. There was a significant step-up of the plasma BNP-LI level in the coronary sinus (CS) compared with that in the aortic root (Ao) and the difference between these BNP-LI levels, delta(CS-Ao)BNP, also increased with the severity of CHF. In addition, the step-up of the BNP-LI level in the anterior interventricular vein [delta(AIV-Ao)BNP] was comparable to delta(CS-Ao)BNP, indicating that BNP is secreted mainly from the ventricle. Predominant BNP synthesis in the ventricle was also confirmed by Northern blot analysis. Catheterization and pharmacokinetic studies revealed that hBNP is cleared from the circulation more slowly than alpha-hANP; this was in part attributed to lower (about 7%) binding affinity of hBNP to clearance receptors than that of alpha-hANP. A predominant molecular form of BNP-LI in the heart and plasma was a 3-kD form corresponding to hBNP. These results indicate that BNP is a novel cardiac hormone secreted predominantly from the ventricle, and that the synthesis, secretion and clearance of BNP differ from those of ANP, suggesting discrete physiological and pathophysiological roles of BNP in a dual natriuretic peptide system.

Development of ultrasensitive enzyme immunoassay reviewed with emphasis on factors which limit the sensitivity

Development of ultrasensitive enzyme immunoassays for antigens, haptens and antibodies is reviewed with emphasis on factors which limit the sensitivity. One of the most important conditions for ultrasensitive immunoassays is the use of non-competitive solid phase assay systems rather than competitive ones. Although non-competitive immunoassays are available for antigens and antibodies, there are only competitive immunoassays for hapten molecules which can not be bound simultaneously by two different antibodies. In order to overcome this difficulty, methods have been developed to derivatize haptens with amino groups so that the derivatized haptens may be measured by two-site assays. The other condition for ultrasensitive immunoassays is to minimize the non-specific binding of labelled reactants. This has been achieved by developing methods to transfer the complex of analytes and labelled reactants from solid phase to solid phase without dissociation. Thus, the sensitivity for antigens, haptens and antibodies has been markedly improved. However, the sensitivity for the detection of label enzymes remains to be improved.

Novel and sensitive noncompetitive (two-site) immunoassay for haptens with emphasis on peptides

A novel and sensitive noncompetitive (two-site) enzyme immunoassay is described to measure attomole amounts of haptens, especially small peptides, which cannot be bound simultaneously by two different antibodies directed to the corresponding two different epitopes on the hapten molecules; consequently, they cannot be measured by the conventional two-site immunoassays. The principle of the assay is to label haptens to be measured with an appropriate substance, so that the hapten molecule can be bound simultaneously by both a binding substance molecule for the label, and anti-hapten antibody molecule, which allows two-site assay. Feasibility of the principle was demonstrated by using biotin as a label and angiotensin I, a 10 amino acid single chain peptide, as a model hapten. Angiotensin I was biotinylated and, after removal of unreacted biotin and other biotinylated substances, was measured using anti-angiotensin I Fab'-horseradish peroxidase conjugate and streptavidin-coated polystyrene balls. The detection limit of angiotensin I was 10 amol (13 fg); this was 100-fold lower than that by competitive radioimmunoassay using the same antibody and 125I-angiotensin. I. This principle was successfully applied to the measurement of arginine vasopressin, a nine amino acid single chain peptide with an intramolecular disulfide bridge. Possible modifications, advantages, and disadvantages of the assay method are discussed.

Development and applications of ultrasensitive enzyme immunoassays for antigens and antibodies

Some enzymes can be measured at lower molar concentrations than the radioisotopes used as labels in radioimmunoassay. Therefore, enzyme immunoassay should be capable of higher sensitivity than radioimmunoassay, if appropriate techniques are used for enzyme-labelling of antibodies and formulation of assay protocols. Techniques required for enzyme immunoassay with attomole sensitivity have been developed, and successfully applied to the measurement of clinically important ligands at levels below those possible using radioimmunoassay. In addition, a novel enzyme immunoassay technique (immune complex transfer enzyme immunoassay technique) is being developed to provide higher sensitivity.

A sensitive enzyme immunoassay for atrial natriuretic polypeptide

A sensitive sandwich enzyme immunoassay for mammalian alpha-atrial natriuretic polypeptide (alpha-ANP) has been developed using Fab' fragments of affinity purified rabbit anti-alpha-hANP[6-28] polyclonal antibodies (R3) conjugated to horseradish peroxidase. Immunoplates were coated with (F(ab')2) fragments of anti-alpha-hANP monoclonal antibodies (HA53). Without using a pre-concentration step the detection limit of alpha-hANP in plasma was 5 pg/ml. The assay was sufficient accuracy and reproducibility for general use of, i.e., the coefficients of within-assay and between-assay variation were 6.4-10.7% and 9.7-15.3%, respectively. Using the assay, the basal plasma alpha-hANP level of healthy persons was 15.3 +/- 8.3 pg/ml (mean +/- SD, n = 40).

Human atrial natriuretic factor (ANF). Characterisation of a monoclonal antibody panel and its use in radioimmunoassay

The production of nine monoclonal antibodies to human atrial natriuretic factor (ANF 1-28) is described. All possible combinations of two antibodies failed to reveal any which could simultaneously bind ANF. Studies with ANF analogues and the antibodies having the three highest affinity values (KD = 5, 25 and 21 pM) indicated that the antibodies are directed to the central portion of the antigen molecule. The highest affinity antibody was able to replace polyclonal antisera in the radioimmunoassay of ANF in extracts of plasma.