Abstract
Bacteria in the environment play a major role in the degradation of widely used man-made recalcitrant organic compounds. Pseudomonas nitroreducens TX1 is of special interest because of its high efficiency to remove nonionic ethoxylated surfactants. In this study, a novel approach was demonstrated by a bacterial enzyme involved in the formation of radicals to attack ethoxylated surfactants. The dihydrolipoamide dehydrogenase was purified from the crude extract of strain TX1 by using octylphenol polyethoxylate (OPEOn) as substrate. The extent of removal of OPEOs during the degradation process was conducted by purified recombinant enzyme from E. coli BL21 (DE3) in the presence of the excess of metal mixtures (Mn2+, Mg2+, Zn2+, and Cu2+). The metabolites and the degradation rates were analyzed and determined by liquid chromatography-mass spectrometry. The enzyme was demonstrated to form Fenton reagent in the presence of an excess of metals. Under this in vitro condition, it was shown to be able to shorten the ethoxylate chains of OPEOn. After 2 hours of reaction, the products obtained from the degradation experiment revealed a prominent ion peak at m/z = 493.3, namely the ethoxylate chain unit is 6 (OPEO6) compared to OPEO9 (m/z = 625.3), the main undegraded surfactant in the no enzyme control. It revealed that the concentration of OPEO15 and OPEO9 decreased by 90% and 40% after 4 hours, respectively. The disappearance rates for the OPEOn homologs correlated to the length of the exothylate chains, suggesting it is not a specific enzymatic reaction which cleaves one unit by unit from the end of the ethoxylate chain. The results indicate the diverse and novel strategy by bacteria to catabolize organic compounds by using existing housekeeping enzyme(s).
Highlights
Octylphenol polyethoxylates (OPEOn, commercial name Triton X-100) is a common nonionic surfactant that is used in industrial and household products[1,2]
We investigated the enzyme activities involved in OPEOn degradation in strain TX1 and a mechanism involved in the degradation of polyethoxylate chains was reported
Existence of metabolic enzymes related to OPEOn degradation in strain TX1
Summary
Octylphenol polyethoxylates (OPEOn, commercial name Triton X-100) is a common nonionic surfactant that is used in industrial and household products[1,2]. OPEOn can be degraded into shorter exothylate chain to form octylphenol or octylphenol mono- to tri-ethoxylates[4]. These OPEOn metabolites that are known more estrogenic-like than their parent compounds can mimic the natural hormones and act as endocrine disruptors in wildlife[5,6]. The biodegradation of OPEOn has been studied using both pure and mixed cultures that grow solely on OPEOn. Several bacterial strains have been reported to degrade the ethoxylate (EO) chain of OPEOn 1,7–11. We investigated the enzyme activities involved in OPEOn degradation in strain TX1 and a mechanism involved in the degradation of polyethoxylate chains was reported. The extent of removal for residual OPEOs during the degradation process by a pure recombinant enzyme was investigated
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