Fatty acids, C16-18, esters with diethylene glycol

Bioaccumulation: aquatic / sediment

Description of key information

Bioaccumulation is assumed to be low.

Additional 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.

 

Experimental bioaccumulation data are not available for Fatty acids, C16-18, esters with diethylene glycol (CAS 85116-97-8). The high log Kow (> 6) 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 and the free glycol alcohols (e.g. diethylene 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 & Bend, 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 fatty acids

Since diethylene glycol has a log Kow of -1.47 (KOWWIN v1.68) it will not bioaccumulate. Thus, this section only focuses on the metabolism of the fatty acid as second enzymatic hydrolysis product. The metabolism of diethylene glycol is discussed in detail in the toxicokinetic section in IUCLID section 7.1.

 

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). Saturated fatty acids (SFA; C12 - C24) as well as mono-unsaturated (MUFA; C14 - C24) and poly-unsaturated fatty acids (PUFA; C18 - C22) were naturally found in muscle tissue of the rainbow trout (Danabas, 2011) and in the liver (SFA: C14 - C20; MUFA: C16 - C20; PUFA: C18 - C22) of the rainbow trout (Dernekbasi, 2012).

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). When including biotransformation rate constants a BCF of 0.893 - 69.79L/kg and a BAF of 0.903 - 69.93 L/kg resulted (Arnot-Gobas estimate, including biotransformation, upper trophic). Even though the substance 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, 2014).

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 Fatty acids, C16-18, esters with diethylene glycol (CAS 85116-97-8). Taking all these information into account, it can be concluded that the bioaccumulation potential of the substance is assumed to be low.