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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

If aquatic exposure occurs, the substance will be mainly taken up by ingestion and digested through common metabolic pathways providing a valuable energy source for the organisms as dietary fats. The substance is not expected to bioaccumulate in aquatic or sediment organisms and secondary poisoning does not pose a risk.

Key value for chemical safety assessment

Additional information

Experimental bioaccumulation data are not available for fatty acids, vegetable-oil, esters with dipropylene glycol (CAS 95009-41-9). The high log Kow (> 6.85) as an intrinsic property indicates a potential for bioaccumulation. However, this does not reflect the behavior of the substance in the environment and the metabolism in living organisms.

Environmental behavior

Due to ready biodegradability and high potential of adsorption, the substance can be effectively removed in conventional STPs either by biodegradation or by sorption to biomass. The low water solubility and high estimated log Kow indicate that the substance is highly lipophilic. If released into the aquatic environment, the substance undergoes extensive biodegradation and sorption on organic matter, as well as sedimentation. The bioavailability of the substance in the water column is reduced rapidly. The relevant route of uptake of glycol ester in organisms is considered predominately by ingestion of particle bounded substance. 

Metabolism of aliphatic esters

Should the substance be taken up by fish during the process of digestion and absorption in the intestinal tissue, aliphatic esters like glycol esters are expected to be initially metabolized via enzymatic hydrolysis in the corresponding free fatty acids (C8 - C10) and the dipropylene glycol. The hydrolysis is catalyzed by classes of enzymes known as carboxylesterases or esterases (Heymann, 1980). The most important of which are the B-esterases in the hepatocytes of mammals (Heymann, 1980; Anders, 1989). Carboxylesterase activity has been noted in a wide variety of tissues in invertebrates as well as in fish (Leinweber, 1987; Soldano et al, 1992; Barron et al., 1999; Wheelock et al., 2008). The catalytic activity of this enzyme family leads to a rapid biotransformation/metabolism of xenobiotics which reduces the bioaccumulation or bioconcentration potential (Lech &, 1980). It is known for esters that they are readily susceptible to metabolism in fish (Barron et al., 1999) and literature data have clearly shown that esters do not readily bioaccumulate in fish (Rodger & Stalling, 1972; Murphy & Lutenske, 1990; Barron et al., 1990). In fish species, this might be caused by the wide CaE distribution, high tissue content, rapid substrate turnover and limited substrate specificity (Lech & Melancon, 1980; Heymann, 1980).

Metabolism of enzymatic hydrolysis products

In-vitro studies with propylene glycol distearate (PGDS) demonstrated hydrolysis of the ester (Long et al., 1958). The hydrolysis of fatty acid esters in-vivo was studied in rats dosed with fatty acid esters containing one, two (like propylene glycol esters) or three ester groups. The studies showed that fatty acid esters with two ester groups are rapidly hydrolysed by ubiquitously expressed esterases and almost completely absorbed (Mattson und Volpenheim, 1968). Furthermore, the in-vivo hydrolysis of propylene glycol distearate (PGDS), a structurally related glycol ester, was studied using isotopically labeled PGDS (Long et al., 1958). Oral administration of PGDS showed intestinal hydrolysis into propylene glycol monostearate, propylene glycol and stearic acid, confirming above discussed metabolism of Fatty acids, vegetable-oil, esters with dipropylene glycol, as well.

Following hydrolysis and absorption, the alcohol component dipropylene glycol is readily converted into propylene glycol, which itself is converted to lactic and pyruvic acids. These conclusions are drawn from a metabolism study of tripropylene glycol and propylene glycol (Dow, 1995, as cited in OECD SIDS, 2001).

Following absorption into the intestinal lumen, fatty acids are re-esterified with glycerol to triacylglycerides (TAG) and included into chylomicrons for transportation via the lymphatic system and the blood stream to the liver. Additionally, MCFA may be transported directly to the liver through the portal vein and do not necessarily form micelles in the gastrointestinal tract as discussed above. In the liver, fatty acids can be metabolised in Phase I and II metabolism. Using the OECD QSAR ToolBox 2.3.0, liver metabolism simulation resulted in 40 metabolites.

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 which are 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 ß-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).

Data from QSAR calculation

Additional information about this endpoint could be gathered through BCF/BAF calculation using BCFBAF v3.01 (Müller, 2014). The estimated BCF value indicates a low bioaccumulation in organisms (BCF: 430.7 - 657.9 L/kg, regression based). When including biotransformation rate constants a BCF of 2.065 - 19.26 L/kg and a BAF of 2.445 - 19.33 L/kg resulted (Arnot-Gobas estimate, including biotransformation, upper trophic). Even though the substance is outside the applicability domain of the model it 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 a substance with high log Kow values (> 10) has 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, 2014).

Conclusion

Aliphatic esters are bio-transformed 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 substance. Taking all these information into account, it can be concluded that the bioaccumulation potential of fatty acids, vegetable-oil, esters with dipropylene glycol (CAS 95009-41-9) is assumed to be low.

For a detailed reference list please refer to the CSR or IUCLID section 13.