Structure–Activity Relationships of Neuromedin U. V. Study on the Stability of Porcine Neuromedin U-8 at the C-Terminal Asparagine Amide under Mild Alkaline and Acidic Conditions

Abstract

Porcine neuromedin U-8 (X-Asn-NH₂, X=H-Tyr-Phe-Leu-Phe-Arg-Pro-Arg) is occasionally unstable in the biological fluids used for bioassay as well as in the acidic solutions used for purification of synthetic peptides. In this study, HPLC examination of an incubate solution of X-Asn-NH₂ revealed that the main decomposition products in Tyrode's solution (pH 7.4) were either α- or β-monocarboxylic acid analogs (X-Asn-OH or X-Asp-NH₂), and that no dicarboxylic acid analog (X-Asp-OH) was produced. Further investigation, employing a model peptide (Y-Asn-NH₂, Y=Benzoyl-Pro-Arg) incubated in a 0.1 M sodium bicarbonate solution at 60 °C, revealed that the decomposition of C-terminal Asn-NH₂ occurred through the formation of an aminosuccinimide intermediate (Y-Asu), at a rate faster than that of Y-Asn-Ser peptide but slower than that of Y-Asn-Gly peptide. Mild acid hydrolysis of X-Asn-NH₂ examined in a 1 M HCl solution at 60 °C yielded X-Asn-OH and X-Asp-NH₂, which further decomposed to yield X-Asp-OH. The C-terminal degradation of X-Asn-NH₂ resulted in reduced biological and immunochemical binding activities.