Transesterification in the Microbial Degradation of Phthalate Esters

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Our previous study revealed that phthalate esters (PEs), a group of suspected endocrine-disrupting chemicals, acquire estrogenic activities by ring 4-hydroxylation. In addition, the estrogenic activities are modified depending on alkyl chain structures (chain length and branching), which can be altered in the environmental conditions such as microbial degradation. Therefore, it is important to determine the environmental fate of these alkyl chains to evaluate the biological impact of PEs on humans and wildlife. PEs are known to undergo biodegradation via sequential hydrolysis, resulting in the formation of monoester and dicarboxylic acid forms. In this study, dipropyl phthalate chosen as one of PEs was cultivated with Acinetobacter lwoffii, a known PE-degrading bacterium, in the presence of a limited amount of CH3OH as a PE-solvent. As a result, several unknown biotransformation products were detected. The products were characterized as methyl propyl phthalate, dimethyl phthalate, and monomethyl phthalate, suggesting that environmental PEs are processed through novel biotransformation pathways. The products can be produced both by esterification of monoester forms and transesterification of diester forms. However, when monobutyl phthalate—a monoester of dibutyl phthalate—was used as a substrate, esterified products were not detected, indicating phthalate methyl esters were formed via transesterification. A stable-isotope tracer experiment using CD3OH instead of CH3OH revealed the production of phthalate methyl esters, the molecular ions of which shifted by 3 or 6 atomic mass units. These results revealed that PE was bacterially transformed via transesterification in the presence of alcohol. We demonstrated that PEs are transformed in the environment via more diverse ways than expected, although the environmental concentration of alcohols is very low. It would be worthwhile to perform a systematic assessment on the possibility that transesterification products may be associated with the potential adverse effects of PEs in the environment.

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