Binding of vasoactive intestinal polypeptide to dispersed enterocytes results in rapid removal of the NH₂-terminal histidyl residue

Specific binding sites for ¹²⁵I-labelled vasoactive intestinal polypeptide (VIP) (half-maximal inhibition at 1.5 ± 0.2 nM VIP) were identified on dispersed porcine enterocytes. Radioactivity bound to the cell surface was internalized. At 37 ° C, a steady state was achieved after 45 min with a ratio of internalized to cell surface-bound radioactivity of approximately 1:2 but at 10 ° C, no radioactivity appeared intracellularly. Incubation of VIP with cells in the absence of inhibitors of proteolysis for as short a time as 30 s at 37 °C led to the formation of [des-His¹]VIP by the action of amastatin- and bestatin-sensitive aminopeptidase(s). This metabolite was formed in the presence of sodium azide and when incubations were performed at 10°C suggesting that internalization was not a prerequisite for degradation. As [des-His¹]-VIP has only 1% of the bioactivity of VIP, formation of this metabolite will effectivily terminate the action of VIP in the epithelial layer of the intestine.