Purification of laccase II from Armillaria mellea and comparison of its properties with those of laccase I

Armillaria mellea secretes two laccases during growth on malt extract medium, but only one (laccase II) during growth on soya flour glucose medium. It appears that laccase I is directly or indirectly induced by some component of malt extract, although in all media tested, laccase activity only accumulated during synthesis of rhizomorphs. Using preparative isoelectric focusing, gel filtration and lectin affinity chromatography, laccase I was purified from malt extract culture fluid and laccase II was purified from soya flour-glucose culture fluid. Laccase I and laccase II had similar pH optima (around pH 4, depending on the substrate used) and similar, but not quantitatively identical substrate and inhibitor specificities. Laccase II was less thermostable than laccase I, had a more acid pI and was glycosylated differently as the two enzymes behaved differently during lectin affinity chromatography. The N-terminal sequence of laccase I was similar to those of other white-rot basidiomycetes, but the N-terminus of laccase II was blocked. Thus A. mellea makes two laccases with very similar properties that are only secreted during rhizomorph synthesis, but which in the case of laccase I are not required for rhizomorph formation as during growth on soya flour glucose medium abundant rhizomorph synthesis occurred and large amounts of laccase were secreted, but this was entirely laccase II.