Effect of buffer pH, buffer concentration and skin with or without enzyme inhibitors on the stability of [Arg 8]-vasopressin

The stability of [Arg 8]-vasopressin (AVP) as a function of buffer pH, buffer concentration, salt concentration, temperature, and skin with and without enzyme inhibitors was investigated. AVP was analyzed by reverse-phase high-performance liquid chromatography. The results indicated that the buffer’s pH affected the degradation rate of AVP. Buffer ions (H 2PO 4 − and HPO 4 2−) and salt concentrations had no effect on the degradation of AVP. Maximum stability was achieved at pH 3.35 among pH values tested. The activation energy for the overall reaction was 21.5 kcal mol −1 at pH 3.35. From the Arrhenius equation, the shelf-life of AVP at 25°C and pH 3.35 was calculated to be 1.38 years. The degradation rate of AVP in the skin (area: 9 cm 2, thickness: 0.5 mm) was 0.22 h −1. Bestatin (an aminopeptidase inhibitor) had the best stabilizing effect on the degradation of AVP by skin among the three enzyme inhibitors (i.e. aprotinin, bestatin, and leupeptin) studied. The degradation rate of AVP in the skin was reduced to 0.059 h −1 in the presence of bestatin in comparison with no inhibitor (0.22 h −1).