Spontaneous hydrolysis of vasoactive intestinal peptide in neutral aqueous solution

Hydrolysis of radioiodinated vasoactive intestinal peptide (VIP) was observed in buffered aqueous solution at neutral pH and 38 degrees C. The reaction displayed apparent first-order kinetics at initial peptide concentrations below 3 nM (kobs = 1.5 x 10(-5) s-1), but the rate deviated below predicted values at higher peptide concentrations. The rate constant derived from the reaction progress curve over three half lives, starting at a concentration of 82 pM peptide, was also consistent with a first-order process. The reaction results in several products that were isolated and characterized as peptide fragments. Based on the identity of these fragments, we deduced hydrolysis at five different peptide bonds clustered between residues 17-25 of VIP. Control experiments were devised to eliminate trivial explanations for the peptide hydrolysis. Peptides representing the C-terminal segment 15-28 and the internal segment 14-22 assayed by analogous methods and under identical conditions were not degraded at a measurable rate. Sodium dodecyl sulfate and acetonitrile, agents known to influence the secondary structure of VIP, inhibited its spontaneous hydrolysis, as did chloride salts of sodium and calcium, albumin and a peptide unrelated to VIP. The rate and product distribution are inconsistent with known pathways of peptide degradation involving cyclic imide or anhydride formation at asparagine or aspartate residues. We suggest that the breakdown of VIP in dilute solutions represents an autolytic process.