Partial purification and characterization of a yeast extracellular acid protease

During screening of 143 yeasts for proteolytic milk coagulating activity, a strain belonging to the species Cryptococcus albidus var. aerius was found which produced extracellular protease in shake culture. An enzyme preparation was obtained from the cell-free broth by ammonium sulphate precipitation. It possessed an optimum pH for milk-clotting at 5.5 to 5.7 at 35 degrees C. Maximum stability occurred between pH 3.5 and 5.5. The optimum temperature for the enzyme was 45 degrees C. Activity of the enzyme was inhibited by copper+2, iron+2, and mercury+2 ions.