Measurement of plasma levels of brain natriuretic peptide (BNP) has been used to assess left ventricular dysfunction and prognosis. Levels of the N-terminus of the precursor of BNP (NT-proBNP) have been reported to be elevated to a greater extent than BNP in left ventricular dysfunction. We have devised a non-radioactive sensitive and specific assay for NT-proBNP based on a competitive ligand binding principle. The chemiluminescent label 4-(2-succinimidyloxycarbonylethyl)phenyl-10-methylacridinium 9-carboxylate fluorosulphonate was used to label peptides representing domains in the middle and C-terminal sections of NT-proBNP. Assay of the C-terminal section of NT-proBNP (amino acids 65–76) in patients with proven left ventricular dysfunction [left ventricular wall motion index median 0.9 (range 0.3–1.4)] revealed elevated values [median 639 (386–911) fmol/ml] compared with normal controls [left ventricular wall motion index of 2 in all, NT-proBNP median 159 (120–245) fmol/ml, P< 0.001]. Measurement of the middle section of NT-proBNP (amino acids 37–49) was not a discriminating test. It is thus possible to derivatize small peptides with a methyl acridinium label and preserve immunodetection with specific antibodies. Such methodology may allow non-radioactive immunoluminometric assays to be devised.

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