Transesterification in the Microbial Degradation of Phthalate Esters

Abstract

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 CH 3 OH 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 CD 3 OH instead of CH 3 OH 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 trans- * To whom correspondence should be addressed: Faculty of Pharmacy, Meijo University, 150 Yagotoyama, Tempaku-ku, Nagoya 468-8503, Japan. Tel.: +81-52-839-2676; Fax: +81-52-834-8090; E-mail: kojiman@meijo-u.ac.jp formed 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.

Potential Risks of Phthalate Esters: Acquisition of Endocrine-disrupting Activity during Environmental and Metabolic Processing

“…50) To determine the source of the methyl group in methyl propyl phthalate, we carried out a stable-isotope tracer experiment using CD 3 OH as a PE-solvent. 48) An HPLC peak with a retention time consistent with methyl propyl phthalate was detected in the experiment; however, the molecular weight of the compound as determined by LC/MS was 3 mass units larger than that of methyl propyl phthalate. Furthermore, no 1 H-NMR signal that could be assigned as the methyl ester group of methyl propyl phthalate was observed.…”

“…[46][47][48] However, it is unclear whether additional transformation products contribute to the toxicity of PEs. Since the biological effects of PEs fluctuate depending on the length and degree of branching of the alkyl chain, 32,33,37,44) it is important to determine the environmental fate of these alkyl chains in order to perform adequate human and environmental risk assessments.…”

Biotransformation of Phthalate Plasticizers and Bisphenol A by Marine-Derived, Freshwater, and Terrestrial Fungi

“…Potentially, the addition of methanol to fungal cultures (applied to improve DBP solubility) may have resulted in overestimations of methylated DBP derivatives presumably formed by transesterification. Fungal, as well as bacterial, cutinases, esterases, and lipases are well known to utilize short-chain alcohols such as methanol as substrates in transesterification reactions Okamoto et al, 2011;Lotti et al, 2015). However, such possible experimental artifacts would not have impaired the identification of most of the major reaction steps indicated by blue solid arrows in Figure 4.…”

“…Thus, hydrolytic enzymes (e.g. cutinases, esterases, lipases) and intracellular oxidative cytochrome P450s can be considered as primary candidates for initiating the biocatalytic breakdown of PEs in fungi Ghaly et al, 2010;Okamoto et al, 2011;Ahuactzin-Perez et al, 2016;Cantú Reinhard and De Visser, 2017;Ren et al, 2018), with major evidence for the latter remaining to be established. Aside from enzymatic removal, biosorption via physio-chemical processes such as adsorption, absorption, and ion interactions may also contribute and must be accounted for when investigating PE fate within the environment (Liang et al, 2008;Gadd, 2009;Net et al, 2015).…”

Surface molecularly imprinted polymers based ZnO quantum dots as fluorescence sensors for detection of diethylhexyl phthalate with high sensitivity and selectivity

“…Phthalates are commonly known as phthalate acid esters (PAEs) and have been used extensively in the production of plastics and other consumer products . PAEs added to these products may find their way into the food chain, water, ecosystems and human organisms after or during their life cycle. The presence of PAEs in the endocrine systems of organisms may lead to cancer, deformities and genetic mutations .…”