Bio-Fenton reaction involved in the cleavage of the ethoxylate chain of nonionic surfactants by dihydrolipoamide dehydrogenase from Pseudomonas nitroreducens TX1

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 (OPEO(n)) 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 (Mn(2+), Mg(2+), Zn(2+), and Cu(2+)). 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 OPEO(n). 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 (OPEO(6)) compared to OPEO(9) (m/z = 625.3), the main undegraded surfactant in the no enzyme control. It revealed that the concentration of OPEO(15) and OPEO(9) decreased by 90% and 40% after 4 hours, respectively. The disappearance rates for the OPEO(n) 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).