Production and characterization of thermostable phenol oxidases of the ascomycete Thermoascus aurantiacus

Abstract

The effect of some culture conditions on extracellular phenol oxidase (PO) production by the thermophilic ascomycete Thermoascus aurantiacus was studied. The effect of different lignocellulosic and an aromatic compound (p‐anisidine) as inducers of PO activity was studied in liquid culture. The most effective substrate, wheat bran, induced between 1 and 2 units/ml PO activity when utilized at a concentration of 1.5% (w/v). The type and size of fungal inoculum influenced the PO production. However, the agitation of cultures did not increase PO production. An initial pH between 6 and 8 resulted in a higher PO production. The crude extracts of T. aurantiacus had activities sufficient to oxidize various typical substrates of POs in the absence of H2O2. Higher activities were attained with 2, 2'‐azinobis‐(3‐ethylthiazoline 6‐sulphonate), 2, 6‐dimethoxyphenol and o‐dianisidine. Typical inhibitors of metal‐containing oxidases were the most potent inhibitors of T. aurantiacus PO. The optimal pH for o‐dianisidine oxidation was 2.8 in 50 mM citrate/phosphate buffer; the optimal oxidation temperature was between 70 and 80°C. All characteristics of the crude extract indicated that the PO is a typical fungal laccase, except for the high optimal temperature.