Purification and characterization of laccase secreted by Phellinus linteus MTCC-1175 and its role in the selective oxidation of aromatic methyl group.

A laccase from the culture filtrate of Phellinus linteus MTCC-1175 has been purified to homogeneity. The method involved concentration of the culture filtrate by ammonium sulphate precipitation and an anion exchange chro- matography on DEAE-cellulose. The SDS-PAGE and native-PAGE gave single protein band indicating that the enzyme preparation was pure. The molecular mass of the enzyme determined from SDS-PAGE analysis was 70 kDa. Using 2,6-dimethoxyphenol, 2,2'[azino-bis-(3-ethylbonzthiazoline-6-sulphonic acid) diammonium salt] (ABTS) and 4-hydroxy-3,5-dimethoxybenzaldehyde azine as the substrates, the Kin, kcat and kt/Km values of the laccase were found to be 160 microM, 6.85 s(-1), 4.28 x 10(4) M(-1) s(-1), 42 microM, 6.85 s(-1), 16.3 x 10(4) M(-1) s(-1) and 92 microM, 6.85 s(-1), 7.44 x 10(4) M(-1) s(-1), respectively. The pH and the temperature optima of the P. linteus MTCC-1175 laccase were 5.0 and 45 degrees C, respectively. The activation energy for thermal denaturation of the enzyme was 38.20 kJ/mole/K. The enzyme was the most stable at pH 5.0 after 1 h reaction. In the presence ofABTS as the mediator, the enzyme transformed toluene, 3-nitrotoluene and 4-chlorotoluene to benzaldehyde, 3-nitroben-zaldehyde and 4-chlorobenzaldehyde, respectively.