Biodegradation of 2,4,6-TCA by the white-rot fungus Phlebia radiata is initiated by a phase I (O-demethylation)aphase II (O-conjugation) reactions system: implications for the chlorine cycle

SummaryThirteen species of white-rot fungi tested have been shown to efficiently biodegrade 1mM 2,4,6-trichloroanisole (2,4,6-TCA) in liquid cultures. The maximum biodegradation rate (94.5% in 10-day incubations) was exhibited by a Phlebia radiatastrain. The enzymes of the ligninolytic complex, laccase, lignin peroxidase (LiP), manganese peroxidase (MnP) and versatile peroxidase (VP) were not able to transform 2,4,6-TCA in in vitroreactions, indicating that the ligninolytic complex was not involved in the initial attack to 2,4,6-TCA. Instead, the first biodegradative steps were carried out by a phase I and phase II reactions system. Phase I reaction consisted on a O-demethylation catalysed by a microsomal cytochrome P-450 monooxygenase to produce 2,4,6-trichlorophenol (2,4,6-TCP). Later, in a phase II reaction catalysed by a microsomal UDP-glucosyltransferase, 2,4,6-TCP was detoxified by O-conjugation with d-glucose to produce 2,4,6-TCP-1-O-d-glucoside (TCPG). This compound accumulated in culture supernatants, reaching its maximum concentration between 48 and 72h of growth. TCPG levels decreased constantly by the end of fermentation, indicating that it was subsequently metabolized. A catalase activity was able to break in vitro the glycosidic link to produce 2,4,6-TCP, whereas ligninolytic enzymes did not have a significant effect on the biotransformation of that compound. Once formed, 2,4,6-TCP was further degraded as detected by a concomitant release of 2.6mol of chloride ions by 1mol of initial 2,4,6-TCA, indicating that this compound underwent almost a complete dehalogenation and biodegradation. It was concluded that P.radiata combines two different degradative mechanisms in order to biodegrade 2,4,6-TCA. The significance of the capability of white-rot fungi to O-demethylate chloroanisoles for the global chlorine cycle is discussed.