Bilirubin oxidase expression and activity enhancement from Myrothecium verrucaria in Aspergillus species

We constructed a new Aspergillus expression vector (pSENSU2512nid) under the control of the enolase promoter with 12 tandem repeats of cis-acting elements (region III) and the heat shock protein 12 (Hsp12) 5′ untranslated region (UTR). Bilirubin oxidase (EC: 1.3.3.5) from Myrothecium verrucaria, which catalyzes the oxidation of bilirubin to biliverdin, was overexpressed in Aspergillus oryzae and A. niger. The productivity was estimated to be approximately 1.2 g/L in the culture broth, which was approximately 6-fold higher than that of recombinant bilirubin oxidase (BOD) expressed in Pichia pastoris (Komagataella phaffii). BOD was purified using hydrophobic interaction chromatography, followed by ion exchange chromatography. The specific activity of the purified BOD against 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) substrate was 57.6 U/mg and 66.4 U/mg for A. oryzae and A. niger, respectively. l-Ascorbic acid (4 mM) addition and storage under deoxygenated conditions for 3–7 d increased the specific activity of these Aspergillus-expressed BODs approximately 2.3-fold (154.1 U/mg). The BOD specific activity was enhanced by incubation at higher temperature (30–50 °C). Further characterization of the enzyme catalytic efficiency revealed that the K_m value remained unchanged, whereas the k_cat value improved 3-fold. In conclusion, this high-level of BOD expression meets the requirements for industrial-level production. Additionally, we identified an effective method to enhance the low specific activity during expression, making it advantageous for industrial applications.