L-Amino acid oxidase from filamentous fungi: screening and optimization

Abstract

Twenty-seven fungal isolates recovered on medium containing L-lysine were found to have the potentiality for producing extracellular L-amino acid oxidase (L-AAO). Aspergillus oryzae displayed the highest yield of enzyme (2.6 U/mg protein) and antioxidants (2.3 mg/ml) followed by Aspergillus flavipes and Trichoderma viride. Upon optimization of the fermentation medium, the maximum enzyme yield (4.6 U/mg protein) was obtained on a medium containing L-lysine (80 mM), glucose (0.6%), KH2PO4 (0.1%), KCl (0.05%) and MgSO4⋅7H2O (0.05%), pH 7.0, under submerged conditions. Supplementing the medium with K+, Ni2+ and Cu2+ ions increased enzyme biosynthesis by about twofold compared to the control. The activity of extracellular L-AAO from the submerged cultures of A. oryzae was twofold higher than that of the intracellular L-AAO. Eleven agro-industrial byproducts were tested as substrates under solid state fermentation conditions; of these, rice bran (3.3 U/mg) was the optimum solid substrate for the induction of L-AAO and antioxidants, followed by wheat bran (2.8 U/mg) and cotton seed cake (2.5 U/mg), possibly due to their higher crude protein content than ash and fiber. Glucose (1%) and L-phenylalanine were the best co-metabolic and co-inductive carbon and nitrogen source, respectively, for maximum L-AAO production by A. oryzae under solid state fermentation, at an initial pH of 7.0. The productivity of L-AAO by A. oryzae under solid state fermentation was higher than that of the submerged cultures by about 1.4-fold under optimum conditions, justifying the former fermentation processes for overproduction of this enzyme in terms of both economics and practicality.