Atrial natriuretic peptide assayed after immunoextraction with magnetic iron particles

We investigated a new method for the immunoextraction of atrial natriuretic peptide (ANP) from plasma. Anti-ANP antibody was coupled to magnetizable iron particles by various coupling methods; both carboxyl terminal and amine terminal iron particles were examined. Introducing a chemical spacer between the iron particles and the antibody improved binding with ANP. The most efficient and reliable immunoextraction involves coupling the ANP antibody to amine terminal iron particles with bis-succinimidyl-suberate. Analytical recovery of ANP from this antibody-coupled suspension was 104% (SD 7.8%). We measured extracted samples by RIA. Standards were included in the extraction step. The detection limit of the assay was 3 ng/L. Patients' samples were assayed after both immunoextraction (y) and Sep-Pak C18 extraction (x); ANP values obtained with these methods correlated highly significantly y = 1.02x - 5.57 ng/L [r = 0.9302, n = 19, SE intercept = 5.7 (not significant), SE slope = 0.10 (P less than 0.0001)]. The method presented is potentially a reliable, efficacious immunoextraction method for ANP.