2-Mercaptobenzothiazole (MBT) is used primarily as a vulcanization accelerator in the production of rubber, although it is also used to inhibit the corrosion of copper in water and as an ingredient in cutting oils and petroleum products (Whittaker et al., 2004, De Wever et al., 1998). The manufacture of MBT produces wastewaters with high levels of MBT and its derivatives, and these compounds can also be released into water by factories using MBT and by leaching from MBT-containing products, particularly rubber (De Wever et al., 1998). Many other industries also use benzothiazoles, which in some cases degrade to MBT. For example, MBT is a degradation product of a fungicide widely used in the leather and lumber industries, 2-(thiocyanomethylthio)benzothiazole (Reemtsma, Fiehn, Kalnowski, and Jekel (1995) Environ Sci Technol 29:478-485).
MBT and some of its degradation products are known to be toxic to humans and other organisms (Whittaker et al., 2004, Nawrocki et al., 2005). Both laboratory and epidemiological studies suggest that MBT acts as a carcinogen in mammals (Whittaker et al., 2004). It also has toxic effects on many bacteria, and its presence inhibits biodegradation of other compounds, including some of its own degradation products (De Wever et al., 1994, De Wever et al., 2001).
MBT is very resistant to biodegradation, and many of its degradation products are also recalcitrant. De Wever et al. (2001) review many of the biotransformations of MBT and other 2-substituted benzothiazoles (shown on the right below; most of the enzymes are postulated). Many bacteria, including strains belonging to Corynebacterium, Pseudomonas, and Escherichia coli, are capable of methylating the thiol group to produce 2-methylthiobenzothiazole (MTBT) (Drotar et al., 1987). MTBT is usually not degraded further, although Reemtsma et al. (1995) report a minor reaction producing 2-(methylsulfinyl)benzothiazole from MTBT, and Liu et al. (1983) describe a culture that can degrade MTBT.
De Wever (1995) describes the conversion of MBT to 2,2'-dithio-bis-benzothiazole (DM), benzothiazole-2-sulphonate (BTSO3), a recalcitrant red dye stuff, and recalcitrant polar compounds, as well as the partial mineralization of MBT (De Wever (1995) PhD Thesis, Faculty of Agricultural and Applied Biological Sciences, Katholieke Universiteit Leuven). Reemtsma et al. (1995) also report a very minor reaction producing benzothiazole (BT) from MBT. (This reaction is poorly characterized and thus is only balanced provisionally here.) Of these products, DM is resistant to further degradation, although Dufresne et al. (1996) describe an acidophylic Sulfobacillus disulfidooxidans sp. that can degrade it. BT is readily degraded, and a Rhodococcus erythropolis strain capable of mineralizing BTSO3 has been isolated (De Wever et al., 1998). The degradation pathways for these two metabolites appear to be similar and may share enzymatic machinery. Rhodococcus erythropolis BTSO31 converts both compounds to 2-hydroxybenzothiazole (OBT), which is then further hydroxylated to form 2,6-dihydroxybenzothiazole (De Wever et al., 1998, Besse et al., 2001). While most of these conversions require the presence of oxygen, the conversion of BTSO3 to OBT can also be carried out under anaerobic conditions (De Wever et al., 1998). A similar pathway for BT degradation has been observed in Rhodococcus rhodochrous OBT18 (De Wever et al., 1998, Besse et al., 2001) and Rhodococcus pyridinovorans PA (Haroune et al., 2002). The latter species uses a catechol 1,2-dioxygenase to open the benzene ring.
Another pathway for MBT degradation has been described (shown on the left below; enzymes are postulated) (Haroune et al., 2004). Rhodococcus rhodochrous OBT18 converts MBT into two metabolites, a cis-dihydrodiol derivative and a hydroxylated form, both of which are then converted to a diacid MBT derivative and further mineralized. Under the laboratory conditions used in this study, this pathway completely mineralized 30 percent of MBT.
The following is a text-format 2-mercaptobenzothiazole pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions.
Graphical Map (10k) | Graphical Map (16k) 2-Mercaptobenzothiazole 2-Mercaptobenzothiazole 2-Mercapto- 2-Mercapto- 2-Mercapto- Rhodococcus rhodochrous OBT18 Escherichia coli HB101 benzothiazole benzothiazole benzothiazole | Corynebacterium sp. strain SO1A microbial microbial microbial | Pseudomonas sp. strains consortium consortium consortium | PF4, PF12, FB1024 | | | +--------+--------+ | | | | | | | thiol | 2-mercapto- | B | thiol | | | S-methyl- | benzothiazole v | oxidase 2-mercapto- | | 2-mercapto- | transferase | desulfurase | | benzothiazole | | benzothiazole | | | | monooxygenase | | dioxygenase v v v v | | 2-(Methylthio)- Benzothiazole Benzothiazole- 2,2'-Dithio- v v benzothiazole | 2-sulfonate bis-benzothiazole 6-Hydroxy- 2-Mercapto- | | | 2-mercapto- benzothiazole- | | benzothiazole | benzothiazole- benzothiazol cis-6,7- | 2-(methylthio)- | monooxygenase | 2-sulfonate | dihydrodiol | benzothiazole | | hydrolase | | | monooxygenase | | 6-hydroxy- | | 2-mercapto- | | v 2-mercapto- | | benzothiazole- v +---------------> 2-Hydroxy- benzothiazole | | cis-6,7- 2-(Methyl- benzothiazole monooxygenase | | dihydrodiol sulfinyl)- | | | dehydrogenase benzothiazole | | | | 2-hydroxy- | | | benzothiazole +--------+--------+ | monooxygenase | | v v 6,7-Dihydroxy- 2,6-Dihydroxy- 2-mercaptobenzothiazole benzothiazole | | | | | catechol | 2,6-dihydroxy- | 1,2-dioxygenase | benzothiazole | | monooxygenase v | 4-[2-Carboxylatovinyl]- v 2-mercaptothiazole- 2,6,7-Trihydroxy- 5-carboxylate benzothiazole | | | | catechol | A | 1,2-dioxygenase v | | | | v v 4-[2-Carboxylatovinyl]- Carbon dioxide 2-hydroxythiazole- 5-carboxylate | | | C v | | v Carbon dioxide
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