Xylenes metabolism

Williams PA, MJ Worsey (1976) Ubiquity of plasmids in coding for toluene and xylene metabolism in soil bacteria evidence for the existence of new TOL plasmids. J Bacteriol 125 818-828. 

Verfiirth K, AJ Pierik, C Leutwein, S Zorn, 1 Heider (2004) Substrate specificities and electron paramagnetic resonance properties of benzylsuccinate synthesis in anaerobic toluene and m-xylene metabolism. Arch Microbiol 181 155-162. 

There are several cases where o-xylene degradation did not occur under nitrate-reducing conditions (see Table 4.5). Possible reasons for these observations were considered above. However, two reports not considered in the previous subsection failed to measure the biodegradation of o-xylene (Morgan et al., 1993 Flyvbjerg et al., 1993) In both cases there was probably very little microbial biomass as only groundwater was used as an inoculum and rates of o-xylene metabolism were likely too slow to measure. 

Positive results in field investigations of o-xylene metabolism were obtained at... 

Other organisms and routes of xylene metabolism are also known. Azoarcus sp. strain Td-15 was the only one of eight toluene-degrading, nitrate-reducing bacteria isolated by Fries et al. (1994) capable of metabolizing w-xylene, but the... 

Kim D, J-C Chae, GJ Zylstra, Y-S Kim, MH Nam, YM Kim, E Kim (2004) Identification of a novel dioxygenase involved in metabolism of o-xylene, toluene, and ethylbenzene by Rhodococcus sp. strain DK17. Appl Environ Microbiol 70 7086-7092. 

The metabolism of C-labeled BTX has been examined in soil cultures, and a mass balance constructed after 4 weeks of aerobic incubation (Tsao et al. 1998). Mineralization of all substrates was ca. 70% but ca. 20% of the label in toluene and ca. 30% in o-xylene were found in humus. It was suggested that alkylated catechol metabolites were responsible for this association. 

It has been shown that pure cultures of bacteria under anaerobic denitrifying conditions may produce benzylsuccinate as a metabolite of toluene (Evans et al. 1992 Migaud et al. 1996 Beller et al. 1996). Demonstration of this and the corresponding methylbenzyl succinates from xylenes has been used to demonstrate metabolism of toluene and xylene in an anaerobic aquifer (Beller et al. 1995, 2002). 

Davey JF, DT Gibson (1974) Bacterial metabolism of para- and mefa-xylene oxidation of a methyl substitnent. J Bacterial 119 923-929. 

It has become clear that benzoate occupies a central position in the anaerobic degradation of both phenols and alkylated arenes such as toluene and xylenes, and that carboxylation, hydroxylation, and reductive dehydroxylation are important reactions for phenols that are discussed in Part 4 of this chapter. The simplest examples include alkylated benzenes, products from the carboxylation of napthalene and phenanthrene (Zhang and Young 1997), the decarboxylation of o-, m-, and p-phthalate under denitrifying conditions (Nozawa and Maruyama 1988), and the metabolism of phenols and anilines by carboxylation. Further illustrative examples include the following ... 

Di Lecce C, M Accarino, F Bolognese, E Galli, P Barbieri (1997) Isolation and metabolic characterization of a Pseudomonas stutzeri mutant able to grow on all three isomers of xylene. Appl Environ Microbiol 63 3279-3281. 

Tsao C-W, H-G Song, R Bartha (1998) Metabolism of benzene, toluene, and xylene hydrocarbons in soil. Appl Environ Microbiol 64 4924-4929. 

Co-exposure to approximately equal concentrations of xylene or toluene (Nylen et al. 1989) has also prevented 77-hexane-induced testicular atrophy in Sprague-Dawley rats. The protective effects of xylene and toluene on peripheral neuropathy and testicular atrophy caused by 77-hexane may result from competition for metabolism, resulting in a slowing of 77-hexane conversion to 2,5-hexanedione. 

Kim. S.K. Kim, Y.C. (1996) Effect of a single administration of benzene, toluene or zw-xylene on carboxyhaemoglobin elevation and metabolism of dichloromethane in rats. J. appl. Toxicol., 16, 437-444... 

Tardif et al. (1992, 1993 a, 1997) have developed a physiologically based toxicokinetic model for toluene in rats (and humans—see Section 4.1.1). They determined the conditions under which interaction between toluene and xylene(s) occurred during inhalation exposure, leading to increased blood concentrations of these solvents, and decreased levels of the hippurates in urine. Similar metabolic interactions have been observed for toluene and benzene in rats (Purcell et al., 1990) toluene inhibited benzene metabolism more effectively than the reverse. Tardif et al. (1997) also studied the exposure of rats (and humans) to mixtures of toluene, we/a-xylene and ethylbenzene, using their physiologically based pharmacokinetic model the mutual inhibition constants for their metabolism were used for simulation of the human situation. 

Xylenes are metabolized primarily by oxidation to the methylbenzyl alcohols, followed by further oxidation to the corresponding methylbenzoic acids (toluic acids). These can be conjugated with glycine to form mcthylhippurates, or with UDP-glucuronate to form acyl glucuronides (see Figure 1). 

The pharmacokinetics and metabolism of the xylenes have been reviewed (Low... 

Xylenes are metabolized in humans primarily to the corresponding methylhippuric... 

In vitro, mixtures of xylenes did not induce sister chromatid exchanges or chromosomal aberrations either in Chinese hamster ovary CHO cells or in human lymphocytes, in the absence of an exogenous metabolic system. 

Elovaara, E., Engstrom, K., Hayri, L., Hase, T. Aitio, A. (1989) Metabolism of antipyrine and OT-xylene in rats after prolonged pretreatment with xylene alone or xylene with ethanol, phenobarbital or 3-methylcholanthrene. Xenobiotica, 19, 945-960... 

In the selection of a microbial system and bioremediation method, some examination of the degradation pathway is necessary. At a minimum, the final degradation products must be tested for toxicity and other regulatory demands for closure. Recent advances in the study of microbial metabolism of xenobiotics have identified potentially toxic intermediate products (Singleton, 1994). A regulatory agency sets treatment objectives for site remediation, and process analysis must determine whether bioremediation can meet these site objectives. Specific treatment objectives for some individual compounds have been established. In other cases total petroleum hydrocarbons total benzene, toluene, ethyl benzene, and xylene (BTEX) or total polynuclear aromatics objectives are set, while in yet others, a toxicology risk assessment must be performed. 

In a single field study (Cozzarelli etal., 1990) where Fe(III) served as the terminal electron acceptor, oxidized products of m- and /(-xylene were observed in the groundwater at Bemidji, Minnesota. While it is not inconceivable that similar products can be formed by either aerobic or anaerobic metabolic routes, the study provides presumptive evidence for xylene degradation under iron-reduci ng condi tions. 

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