Purification of [I]-vasoactive intestinal peptide by reverse-phase HPLC

VIP was labeled with sodium ¹²⁵iodide, and ¹²⁵I-VIP was purified by reverse-phase high performance liquid chromatography. Optimal separations of ¹²⁵I-VIP and unlabeled VIP were obtained using two C₁₈-Novapak columns in series and a gradient of acetonitrile in triethylamine phosphate for elution. The specific activity of the ¹²⁵I-VIP was 1.99±0.21 Ci/μmole, approaching the maximum specific activity of monoiodinated VIP (2.26 Ci/μmole). Radioimmunoassay and radioreceptorassay for VIP were more sensitive (2.6-fold, and 2.5-fold, respectively) using ¹²⁵I-VIP purified by HPLC compared to ¹²⁵I-VIP obtained from an open-end cellulose column. These results demonstrate the advantage of preparing purified ¹²⁵I-VIP by HPLC for the accurate assay of VIP and VIP-receptors in tissues and biological fluids.