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EC number: 812-656-2 | CAS number: -
Bioaccumulation: aquatic / sediment
Administrative data
Link to relevant study record(s)
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
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