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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

The potential for bioaccumulation of the TMP polyol esters category members is low based on all available data.

Key value for chemical safety assessment

Additional information

Experimental data on bioaccumulation of TMP esters is not available. The evaluation of the bioaccumulation potential of the substance is therefore based on a Weight of Evidence (WoE), combining all available related data. This is in accordance to the REACh Regulation (EC) No 1907/2006, Annex XI General rules for adaptation of the standard testing regime set out in Annexes VII to X, 1.2, to cover the data requirements of Regulation (EC) No. 1907/2007 Annex IX and X (ECHA guidance section R.7.11.5.3, page 121).
The bioaccumulation potential of a substance is driven by the physico-chemical properties of the substance triggering the bioavailability as well as by metabolism and excretion. The bioavailability of polyol esters is expected to be low. Though the polyols esters have a high estimated partition coefficient indicating the potential to bioaccumulate a significant accumulation is not expected based on the environmental fate and expected rapid metabolisation of the substances.

Environmental fate
Due to the ready biodegradability and the high adsorption potential an effective removal of the polyol esters in sewage treatment plants is expected. However, when released to the aquatic environment a rapid degradation is anticipated. The concentration in the water phase will be further reduced by adsorption to organic matter and by sedimentation. Thus a significant uptake of polyol esters through the water phase is not expected. Pelagic or benthic organisms may take up the substance by ingestion of food particles.

 

Metabolism of enzymatic hydrolysis products
Neopentylglycol (NPG), trimethylolpropane (TMP), pentaerythritol (PE) and dipentaerythritol (DiPE) are the expected possible corresponding alcohol metabolites from the enzymatic reaction of the polyol ester category members. In general, the hydrolysis rate of fatty acid esters and polyol ester in particular varies depending on the fatty acid chain length, and grade of esterification (Mattson and Volpenhein, 1969; Mattson and Volpenhein, 1972a,b).
In the gastrointestinal GI tract (GIT), metabolism prior to absorption via gut microflora or enzymes in the GI mucosa may occur. In fact, after oral ingestion, fatty acid esters with glycerol (glycerides) are rapidly hydrolized by ubiquitously expressed esterases and almost completely absorbed (Mattson and Volpenhein, 1972a).The result of the pancreatic digestion of one NPG ester shows a degradation of the ester of almost 90% within 4 hours (Oßberger, 2012). In contrast with regard to the Polyol esters it was shown that lower rate of enzymatic hydrolysis in the GIT were showed for compounds with more than 3 ester groups (Mattson and Volpenhein, 1972a,b). In vitro hydrolysis rate of pentaerythritol ester was about 2000 times slower in comparison to glycerol esters (Mattson and Volpenhein, 1972a,b).
When hydrolysis occurs the potential hydrolysis products are absorbed and subsequently enter the bloodstream. Potential cleavage products are stepwise degraded via beta–oxidation in the mitochondria. Even numbered fatty acids are degraded via beta-oxidation to carbon dioxide and acetyl-CoA, with release of biochemical energy. In contrast, the metabolism of the uneven fatty acids results in carbon dioxide and an activated C3-unit, which undergoes a conversion into succinyl-CoA before entering the citric acid cycle (Stryer, 1994). Alternative oxidation pathways (alpha- and omega-oxidation) are available and are relevant for degradation of branched fatty acids.
The other cleavage products Polyols (NPG, TMP and PE) are easily absorbed and can either remain unchanged (PE) or may further be metabolized or conjugated (e.g. glucuronides, sulfates, etc.) to polar products that are excreted in the urine (Gessner et al., 1960, Di Carlo et al., 1964).
Lipids and their key constituent fatty acids are, along with protein, the major organic constitute of fish and they play a major role as sources of metabolic energy in fish for growth, reproduction and movement, including migration (Tocher, 2003). In fishes, the fatty acids metabolism in cell covers the two processes anabolism and catabolism. The anabolism of fatty acids occurs in the cytosol, where fatty acids esterified into cellular lipids that is the most important storage form of fatty acids. The catabolism of fatty acids occurs in the cellular organelles, mitochondria and peroxisomes via a completely different set of enzymes. The process is termed beta-oxidation and involves the sequential cleavage of two-carbon units, released as acetyl-CoA through a cyclic series of reaction catalyzed by several distinct enzyme activities rather than a multienzyme complex (Tocher, 2003).
As fatty acids are naturally stored in fat tissue and re-mobilized for energy production is can be concluded that even if they bioaccumulate, bioaccumulation will not pose a risk to living organisms. Fatty acids (typically C14 to C24 chain lengths) are also a major component of biological membranes as part of the phospholipid bilayer and therefore part of an essential biological component for the integrity of cells in every living organism (Stryer, 1994).
Furthermore calculated BCF/BAF values indicate a low bioaccumulation potential of polyol esters (BCFBAF v3.01; Blum, 2011; Müller, 2013). Calculations including the normalized whole-body metabolic biotransformation rate constant gave a BCF of 0.89 – 39.11 and a BAF of 0.89 – 153.3 L/kg (Arnot Gobas, upper trophic). Even though the members of the polyol ester category are outside the applicability domain of the model they might be used as supporting indication that the potential of bioaccumulation is low. The model training set is only consisting of substances with log Kow values of 0.31 - 8.70. But it supports the tendency that substances with high log Kow values (> 10) have a lower potential for bioconcentration as summarized in the ECHA Guidance R.11 and they are not expected to meet the B/vB criterion (ECHA, 2012).

Conclusion
The bioaccumulation potential of polyol esters is expected to be low. The substances are characterized by a rapid degradation and low water solubility causing a low bioavailability. If taken up the substances are biotransformed to fatty acids and the corresponding alcohol component by the ubiquitous carboxylesterase enzymes in aquatic species. Based on the rapid metabolism it can be concluded that the high log Kow, which indicates a potential for bioaccumulation, overestimates the bioaccumulation potential of the polyol ester category members.

A detailed reference list is provided in the technical dossier (see IUCLID, section 13) and within CSR