Mechanism for Biotransformation of Nonylphenol Polyethoxylates to Xenoestrogens in Pseudomonas putida

Abstract

ABSTRACT A strain of Pseudomonas putida isolated from activated sewage grew aerobically on the xenoestrogen precursor, nonylphenol polyethoxylate (NPEO x , where x is the number of ethoxylate units) as sole carbon source. Comparative growth yields on NPEO av6 , NPEO av9 , and NPEO av20 (mixtures with average ethoxylate numbers as indicated) were consistent with utilization of all but two ethoxylate units, and the final accumulating metabolite was identified by gas chromatography-mass spectroscopy as nonylphenol diethoxylate (NPEO 2 ). There was no growth on nonylphenol or polyethylene glycols, and there was no evidence for production of carboxylic acid analogs of NPEO x . Biodegradation kinetics measured by high-pressure liquid chromatography (HPLC) for each component in NPEO x mixtures showed that biodegradation proceeded via successive exoscission of the ethoxylate chain and not by direct scission between the second and third ethoxylate residues. The NPEO x -degrading activity was inducible by substrate, and cell extracts of NPEO av9 -induced cells were also active on the pure alcohol ethoxylate, dodecyl octaethoxylate (AEO 8 ), producing sequentially, under either aerobic or anaerobic conditions, AEO 7 , AEO 6 , AEO 5 , etc., thus demonstrating that the pathway involved removal of single ethoxylate units. HPLC analysis of 2,4-dinitrophenylhydrazone derivatives revealed acetaldehyde (ethanal) as the sole aldehydic product from either NPEO av9 or AEO 8 under either aerobic or anaerobic conditions. We propose a mechanism for biotransformation which involves an oxygen-independent hydroxyl shift from the terminal to the penultimate carbon of the terminal ethoxylate unit of NPEO x and dissociation of the resulting hemiacetal to release acetaldehyde and the next-lower homolog, NPEO x −1 , which then undergoes further cycles of the same reaction until x = 2.