Some Properties of Acid Protease from the Thermophilic Fungus, Penicillium duponti K1014

A purified acid protease from a true thermophilic fungus, Penicillium duponti K1014, was most active at pH 2.5 for milk casein and at pH 3.0 for hemoglobin. The enzyme was stable at a pH range of 2.5 to 6.0 at 30 C for 20 h. The acid protease retained full activity after 1 h at 60 C at a pH range between 3.5 and 5.5. At the most stable pH of 4.5, more than 65% of its activity remained after heat treatment for 1 h at 70 C. These thermal properties show the enzyme as a thermophilic protein. The enzyme activity was strongly inhibited by sodium lauryl sulfate and oxidizing reagents such as potassium permanganate and N-bromosuccinimide. No inhibition was caused by chelating reagents, potato inhibitor, and those reagents which convert sulfhydryl groups to mercaptides. Reducing reagents showed an activating effect. The enzyme showed the trypsinogen-activating property at an acidic pH range; optimal trypsinogen activation was obtained at a pH of approximately 3.0. The isoelectric point of the enzyme was estimated to be pH 3.89 by disk electrofocusing. By using gel filtration, an approximate value of 41,000 was estimated for the molecular weight.