3. Results
The production of enzymes from microorganisms is strongly influenced by the composition of the medium temperature, pH, percentage of culture inoculated and length of incubation. B. subtilis showed higher proteolytic activity on 0.03 (w/v) gelatin than on casein or skimmed milk in nutrient agar medium (Fig. 1). Carbon and nitrogen sources are also important factors for the production of protease in culture medium. Of the five carbon and nitrogen sources tested, galactose and peptone resulted in the highest protease activity (236.31 and 175.083 U/mL per min, respectively), while the least enzyme activity was observed with starch and ammonium chloride (Fig. 2). In tests to optimize the time, protease production increased gradually from 0 to 36 h, at which it was maximal, at 243.28 U/mL per min; it then decreased with time (Fig. 3). To determine the optimal H+ concentration for protease production, B. subtilis was cultivated Fig. 4. Effect of pH on activity of protease from B. subtilis. Fig. 5. Effect of temperature on activity of protease from B. subtilis. over the pH range 5–9 at room temperature. The highest protease activity was found at pH 7.4 (143.73 U/mL per min; Fig. 4). Residual protease activity was determined at different temperatures and was maximal at 40–50 ◦C, although considerable residual activity (50%) was observed even after incubation at 80 ◦C for 30 min (Fig. 5). Hence, protease obtained from B. subtilis can be considered a thermostable enzyme. To obtain proteins from the cell-free culture filtrate, they were precipitated with ammonium sulphate at 75% (w/v) saturation by adding powdered ammonium sulphate slowly with continuous shaking and a magnetic stirrer to avoid bubble formation. The protein precipitate was then dissolved and dialysed for 24 h against phosphate buffer. The yield of protein was 2.31 mg/mL, with a total activity of 475.56 U/mL per min.