Thermostable alkaline protease produced by an Aspergillus niger strain

A thermostable alkaline protease was isolated from the Aspergillus niger Z1 strain in a liquid Czapek Dox medium, containing casein (1% w/v) as the sole nitrogen source. Enzyme extract was subjected to electrophoresis in SDS-polyacrylamide gel. Two protein bands were seen on polyacrylamide gel. Active enzyme was visualized in a zymogram and protease activity exhibited a molecular mass of 68 kDa on SDS-polyacrylamide gel. The optimum pH for activity was found to be 9.0. The temperature optima of the enzyme was found to be 40°C at pH 9.0 and it remained stable up to 90°C, with 48.4% of the original activity retained after heat treatment at 90°C for 15 min. Proteolytic activity was inhibited by PMSF, but slightly inhibited by SDS