Assay of Radiolabeled VIP Binding and Hydrolysis by Antibodies

High-specific-activity radiolabeled polypeptide probes permit sensitive detection of antigen-antibody interactions. In the case of small polypeptides labeled with ¹²⁵ I, various iodinated forms can be separated by reversed-phase HPLC and resolved peaks characterized by protein chemistry methods. We describe here methods for preparation and purification of vasoactive intestinal (VIP) labeled at Tyr¹⁰ with ¹²⁵ I, and the use of (Tyr^10_125 I)VIP to characterize antibody binding and enzymatic activities.