Radioimmunoassay of vasoactive intestinal polypeptide

Abstract

A sensitive radioimmunoassay for plasma vasoactive intestinal polypeptide (VIP) has been developed based on preparations of fully immunoreactive 125I-labeled VIP and hightiter specific antiserum as well as elimination of plasma interference substance(s). Fully immunoreactive 125I-labeled VIP (specific activity = 520 μCi/nmol) was prepared by lactoperoxidase iodination and purified by gel filtration followed by chromatography on an O-(carboxymethyl) (CM)-Sephadex C-25 column. Specific anti-VIP serum produced from New Zealand white rabbits had a titer of 1:500,000 and the following binding parameters: effective affinity constant ( K eff), 2.9 × 10 11 m −1; heterogeneity index (α), 0.57; average affinity constant ( K 0), 2.4 × 10 10 m −1. Interfering substance(s) in plasma samples was proved to be present by direct radioimmunoassay and eliminated by an XAD-2 resin adsorption technique, leading to a minimal overall sensitivity of 0.48 p m for plasma samples. The average plasma VIP concentration of 78 normal fasting human subjects was 5.7 ± 3.4 (SD) p m, and that of 5 patients with watery diarrhea syndrome was 359 ± 93 p m, which reduced gradually to the normal basal value after clinical treatment.