8. For general digestion of proteins, suggested conditions are a 0.4% solution of pepsin in 10 mM HCl, and digestion for 30-90 minutes at 37 °C. Pepsin has optimal activity with native proteins at approximately pH 1.0, but with some denatured proteins the optimal activity is at approximately pH 1.5-3.5.9,10.inetics, Solubility and Solution Stability. Pepsin is soluble in deionized water at 1% (10 mg/ml) and at 0.4% (4 mg/ml) in cold 10 mM hydrochloric acid. Solutions at pH 4.4 are stable at -20 °C for about 2-3 months. 12 The pH optimum for activity for porcine pepsin is ~2.2.