초록
<P><B>Abstract</B></P> <P>Isolated strain <I>Aspergillus niger</I> WA 2017 was selected as potential protease producer and was identified on the basis of 18S rDNA gene homology. Optimization of protease production conditions was performed using statistical methodology. The most significant factors were identified by Plackett-Burman design (PB) and were optimized by Central Composite design (CCD). The enzyme production was increased by 3.6-fold with statistically optimized medium when compared to the basal medium. Based on the protease activity, 25–50% ethanol fraction exhibited the highest specific activity. The partially purified enzyme showed its highest activity (4.7-fold) after 10 min incubation at pH 10.0 and 60 °C. The enzyme was stable over a wide range of pH (7–11) and salt concentration (up to 20%). Kinetic parameters Michaelis constant (<I>K</I> <SUB> <I>m</I> </SUB>) and maximum velocity (<I>V</I> <SUB> <I>max</I> </SUB>) were calculated at varying casein concentrations. Additionally, thermal stability of the enzyme was substantially improved by NaCl. The enzyme showed excellent stability and compatibility in presence of organic solvents and detergents retaining 115.3 and 114.5% of its activity in presence of ethanol and Tide, respectively at 40 °C for 1 h. The results revealed that the produced enzyme was able to recover silver from used X-ray film under optimized condition using statistical methodology (CCD).</P> <P><B>Highlights</B></P> <P> <UL> <LI> Isolation of new fungus strains from the garden soil of NRC (Egypt), genetic identification of the potent isolates (its accession MG873558). </LI> <LI> Improve protease production using Response Surface methodology (PBD and CCD). </LI> <LI> Alkaline protease is highly tolerant in the presence of NaCl, organic solvents, detergents and denaturants. </LI> <LI> Catalytic and stability properties of enzyme under harsh conditions suggest that it can be used in industrial applications. </LI> <LI> This study is the first one optimized silver recovery by enzymes using CCD. </LI> </UL> </P>