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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

If aquatic exposure occurs, Glycol Esters category members 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 category members are 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

Aquatic bioaccumulation

Experimental bioaccumulation data are not available for the members of the Glycol Esters category. The high log Kow as an intrinsic property of the category members indicates a potential for bioaccumulation. But it 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 category members 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 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 and the free glycol alcohols such as ethylene glycol and propylene 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; Suldano 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

Ethylene glycol and propylene glycol are the expected corresponding alcohol metabolites from the enzymatic reaction of the Glycol esters category members. Ethylene glycol and propylene glycol are rapidly absorbed from the gastrointestinal tract and then undergo rapid biotransformation in liver and kidney (WHO, 2002a, ATSDR, 1997). Propylene glycol will be further metabolized in liver by alcohol dehydrogenase to lactic acid and pyruvic acid which are endogenous substances naturally occurring in mammals (Miller & Bazzano, 1965, Ritchie, 1927). Ethylene glycol is first metabolised by alcohol dehydrogenase to glycoaldehyde, which is then further oxidized successively to glycolic acid, glyoxylic acid, oxalic acids by mitochondrial aldehyde dehydrogenase and cytosolic aldehyde oxidase (ATSDR, 2010; WHO, 2002a). The metabolite ethylene glycol is toxic to human and mammalian, but it has generally low toxicity to aquatic organisms and is not expected to bioaccumulate based on the low log Kow (WHO, 2002a; WHO, 2002b). Safety evaluation of propylene glycol indicated a low toxicity to human and mammalian and the aquatic compartment. (OECD, 2001).

 

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 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, 2011). The estimated BCF value indicates a low bioaccumulation in organisms (BCF: 3.16 L/kg, regression based). When including biotransformation rate constants a BCF of 0.893 - 89.4 L/kg and a BAF of 0.893 - 90.15 L/kg resulted (Arnot-Gobas estimate, including biotransformation, upper trophic). Even though the members of the Glycol 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, 2008).

Conclusion

Aliphatic esters 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 Glycol esters category members. Taking all these information into account, it can be concluded that the bioaccumulation potential of the Glycol Esters category members is assumed to be low.

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