Laccase-catalyzed oxidation of naphthol in the presence of soluble polymers

Abstract

The oxidation of 1-naphthol and 2-naphthol was studied in presence of recombinant laccases (benzenediol:oxygen oxidoreductase, EC 1.10.3.2) from Polyporus pinsitus (rPpL), Myceliophthora thermophila (rMtL), Coprinus cinereus (rCcL) and Rhizoctonia solani (rRsL). During 1-naphthol oxidation a violet colored insoluble product was formed. The product of 2-naphthol oxidation was a white insoluble precipitate. Almost two moles of 1-naphthol was oxidized by one mole of oxygen. Kinetic measurements of oxygen consumption showed that laccases have been inactivated during naphthols oxidation. Water soluble polymers and albumins changed oxidation profile. The polymers investigated were divided into three groups. The first group includes polymers which increase total oxygen consumption without changing an initial rate: polyvinyl alcohol, ficoll, a copolymer of acrylamide, N-vinylpyrrolidone and ethyl acrylate, hydroxyethyl cellulose, dextrans 500, 110 and 20, polyvinylpyrrolidone, polyethylene glycol and albumins. These polymers intermediately bound hydrophobic naphtholic radicals preventing inactivation of laccases. Cationic polymers such as diethylaminoethyl-dextran, poly- l-lysine and protasan that inhibited the initial oxidation rate, but did not change total oxygen consumption, were attributed to the second group. The inhibition was associated with complexation of positively charged polymers with negatively charged laccases since isoelectric points of rPpL and of rMtL were 3.5 and 4.2, respectively. The polymers such as alginic acid, polyacrylic acid and heparin were attributed to the third group. These polymers carried negative charges and formed random coil in solution due to electrostatic repulsion. They did not interact with laccases and did not combine intermediates. Therefore, they practically did not change total oxygen consumption and initial rate of naphthol oxidation.