Metabolism of Pentachlorophenol (PCP) by Soil Microorganisms

Abstract

Pentachlorophenol (PCP) and its sodium salt have been widely used as agricultural and industrial pesticides, especially as herbicides in paddy fields during the decade from 1960 in Japan. Degradation of PCP in soils and metabolism of PCP by isolated soil microorganisms have been intensively studied. The rate of PCP dissipation from soils under moist conditions varied among soils, and half-life of PCP was 7-14 days. The dissipation of PCP was influenced by the presence of certain fungicides as well as autoclaving. Another studies clarified the dissipation of PCP was also reduced in submarged condition. Repeated application of PCP to soils resulted in acceleration of the PCP dissipation. The evolution of ¹⁴CO₂ from soil treated with ¹⁴C-PCP was observed. From these results it appeared that the dissipation of PCP depended on microbial decomposition. Pseudomonas sp. isolated from a soil degraded ¹⁴CPCP rapidly with release of approximately 50% of the ¹⁴C as ¹⁴CO₂ and incorporated the ¹⁴C into the bacterial cell constituents such as amino acids. Tetrachlorocatechol (TCC) and tetrachlorohydroquinone (TCHQ) were identified as degradation products. Metabolic pathways of ortho or para hydroxylation of PCP as initial degradation step followed by rapid ring cleavage were proposed. PCP was methylated to pentachloroanisole (PCA) by isolated Mycobacterium sp. The isolate simultaneously metabolized PCP to TCHQ followed by successive methylation to form their dimethyl ethers, tetrachloro-1, 2-dimethoxybenzene or tetrachloro-1, 4-dimethoxybenzene, via monomethyl ethers, tetrachloro-2-methoxyphenol or tetrachloro-4-methoxyphenol. Chlorothiophenols as well as chlorophenols were also methylated by this bacterium. When either methionine, ATP, and Mg⁺⁺ or S-adenosylmethionine was incubated with the cell-free extracts of Mycobacterium sp. in phosphate buffer containing PCP, PCA was produced. Incubation of methyl-¹⁴C labeled S-adenosylmethionine with PCP by cell-free extracts resulted in transfer of the radioactivity to PCA. These results revealed that the methylation of PCP by isolated Mycobacterium sp. was attributable to the enzyme(s) concerned with the transferance of methyl group to PCP.