Purification and characterization of laccases from the white-rot basidiomycete Dichomitus squalens

Two chromatographic forms of laccase c1 and c2 were purified approximately 225-fold from the extracellular culture fluid of ligninolytic cultures of Dichomitus squalens, using DEAE-Sepharose and Mono-Q fast protein liquid chromatography. Each homogeneous laccase (c1 and c2) has a molecular mass of approximately 66 kDa as determined by SDS-PAGE. Both forms are glycoproteins, and each contains four copper atoms per molecule of protein. The first 20 amino acids of the N-terminal sequences of these two laccases are identical and are similar to those of laccases from other lignin-degrading fungi. The electronic absorption spectra of these laccases exhibit bands at 610 and 330 nm, indicative of type I and type III copper. The EPR spectrum of laccase c1 exhibits bands indicative of type I and type II copper. Each laccase oxidizes a variety of phenolic substrates, has a pH optimum of 3.0 for the oxidation of 2,6-dimethoxyphenol, and is inhibited strongly by fluoride and azide.