Production of Fenton's reagent by cellobiose oxidase from cellulolytic cultures of Phanerochaete chrysosporium.

Abstract

The reduction of dioxygen by cellobiose oxidase leads to accumulation of H2O2, with either cellobiose or microcrystalline cellulose as electron donor. Cellobiose oxidase will also reduce many Fe(III) complexes, including Fe(III) acetate. Many Fe(II) complexes react with H2O2 to produce hydroxyl radicals or a similarly reactive species in the Fenton reaction as shown: H2O2 + Fe2+----HO. + HO- + Fe3+. The hydroxylation of salicylic acid to 2,3-dihydroxybenzoic acid and 2,5-dihydroxybenzoic acid is a standard test for hydroxyl radicals. Hydroxylation was observed in acetate buffer (pH 4.0), both with Fe(II) plus H2O2 and with cellobiose oxidase plus cellobiose, O2 and Fe(III). The hydroxylation was suppressed by addition of catalase or the absence of iron [Fe(II) or Fe(III) as appropriate]. Another test for hydroxyl radicals is the conversion of deoxyribose to malondialdehyde; this gave positive results under similar conditions. Further experiments used an O2 electrode. Addition of H2O2 to Fe(II) acetate (pH 4.0) or Fe(II) phosphate (pH 2.8) in the absence of enzyme led to a pulse of O2 uptake, as expected from production of hydroxyl radicals as shown: RH+HO.----R. + H2O; R. + O2----RO2.----products. With phosphate (pH 2.8) or 10 mM acetate (pH 4.0), the O2 uptake pulse was increased by Avicel, suggesting that the Avicel was being damaged. Oxygen uptake was monitored for mixtures of Avicel (5 g.1-1), cellobiose oxidase, O2 and Fe(III) (30 microM). An addition of catalase after 20-30 min indicated very little accumulation of H2O2, but caused a 70% inhibition of the O2 uptake rate. This was observed with either phosphate (pH 2.8) or 10 mM acetate (pH 4.0) as buffer, and is further evidence that oxidative damage had been taking place, until the Fenton reaction was suppressed by catalase. A separate binding study established that with 10 mM acetate as buffer, almost all (98%) of the Fe(III) would have been bound to the Avicel. In the presence of Fe(III), cellobiose oxidase could provide a biological method for disrupting the crystalline structure of cellulose.