A study was carried out on the dioxygen reactivity of the laccases from Trametes villosa, Rhizoctonia solani, Myceliophthora thermophila, Scytalidium thermophilum, and Coprinus cinereus. At pH 5.5, these laccases showed an appaxently constant K-m (about 20-50 muM) for O-2 with either 2,2 ' -azinobis (3-ethylbenzothiazoline-6-sulfonic acid) or methyl syringate as the reducing substrate, in contrast to the k(cat), which varied up to 100-fold. O-2 reactivity of the Trametes and Myceliophthora laccases was also studied at various pH and NaF concentrations. The apparent K-m of Trametes and Myceliophthora laccases varied only slightly when pH changed from 3.0 to 8.0 or when the laccases were inhibited by F- at pH 5.5, although the apparent kc, were more significantly affected by both factors. The dependence of the apparent K-m in on the source of laccase, pH, and F- inhibition suggested that the fungal laccases might have a conserved O-2 pocket and that the F- or OH- inhibition might affect the O-2 reduction step (kcat) more than the O-2 binding step (K-m) under steady-state conditions.