Pathway for Biodegradation of p -Nitrophenol in a Moraxella sp

Abstract

A Moraxella strain grew on p -nitrophenol with stoichiometric release of nitrite. During induction of the enzymes for growth on p -nitrophenol, traces of hydroquinone accumulated in the medium. In the presence of 2,2′-dipyridyl, p -nitrophenol was converted stoichiometrically to hydroquinone. Particulate enzymes catalyzed the conversion of p -nitrophenol to hydroquinone in the presence of NADPH and oxygen. Soluble enzymes catalyzed the conversion of hydroquinone to γ-hydroxymuconic semialdehyde, which was identified by high-performance liquid chromatography (HPLC)-mass spectroscopy. Upon addition of catalytic amounts of NAD + , γ-hydroxymuconic semialdehyde was converted to β-ketoadipic acid. In the presence of pyruvate and lactic dehydrogenase, substrate amounts of NAD were required and γ-hydroxymuconic semialdehyde was converted to maleylacetic acid, which was identified by HPLC-mass spectroscopy. Similar results were obtained when the reaction was carried out in the presence of potassium ferricyanide. Extracts prepared from p -nitrophenol-growth cells also contained an enzyme that catalyzed the oxidation of 1,2,4-benzenetriol to maleylacetic acid. The enzyme responsible for the oxidation of 1,2,4-benzenetriol was separated from the enzyme responsible for hydroquinone oxidation by DEAE-cellulose chromatography. The results indicate that the pathway for biodegradation of p -nitrophenol involves the initial removal of the nitro group as nitrite and formation of hydroquinone. 1,4-Benzoquinone, a likely intermediate in the initial reaction, was not detected. Hydroquinone is converted to β-ketoadipic acid via γ-hydroxymuconic semialdehyde and maleylacetic acid.