Laccase activity in soils: Considerations for the measurement of enzyme activity

► Optimal pH and kinetic parameters of isolated laccases and soil extracts are similar. ► The laccase assay with ABTS has the lowest detection limit of 13.5pg of protein. ► Assays with DOPA and 2,6-dimethoxyphenol suffer from interference with tyrosinase. ► Laccase is inhibited in the presence of low molecular mass soil compounds. Laccases (benzenediol: oxygen oxidoreductases, EC 1.10.3.2) are copper-containing enzymes that catalyze the oxidative conversion of a variety of chemicals, such as mono-, oligo-, and polyphenols and aromatic amines. Laccases have been proposed to participate in the transformation of organic matter and xenobiotics as well as microbial interactions. Several laccase assays have been proposed and used in soils. Here, we show that the optimal pH conditions for the laccase substrates 2,2′-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS, pH 3–5), 2,6-dimethoxyphenol (4–5.5), L-3,4-dihydroxyphenylalanine (DOPA; 4–6), guaiacol (3.5–5), 4-methylcatechol (3.5–5), and syringaldazine (5.5–7.0) are similar between purified laccases from Trametes versicolor and Pyricularia sp. and soil extracts; the substrate affinities of purified enzymes (KM) and soil extracts were also similar. The laccase assays showed specificity overlap with tyrosinase and ligninolytic peroxidases when hydrogen peroxide is present. The ABTS oxidation assay is able to reliably detect the presence of 13.5pgmL−1 or 0.199×10−12molmL−1 of T. versicolor laccase, which is three times more sensitive than the 2,6-dimethoxyphenol-based assay and more than 40 times more sensitive than any of the other assays. The low molecular mass soil-derived compounds and the isolated fulvic and humic acids influence the laccase assays and should be removed from the soil extracts before measurements of the enzyme activity are performed.