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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

The substance exhibits a low potential for bioaccumulation.

Key value for chemical safety assessment

Additional information

Experimental bioaccumulation data are not available for 1,2,3-Propanetriol, homopolymer, diisooctadecanoate (CAS 63705-03-3). The high log Kow (> 4.3) as an intrinsic chemical property indicates a potential for bioaccumulation. However, the information gathered on environmental behavior and metabolism, in combination with QSAR-estimated values, provide enough evidence (in accordance to the 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 1907/2006, Annex IX to state that the substance is likely to show negligible bioaccumulation potential.

Environmental behavior

Due to ready biodegradability and high potential of adsorption, 1,2,3-Propanetriol, homopolymer, diisooctadecanoate can be effectively removed in conventional Sewage Treatment plants (STPs) either by biodegradation or by sorption to biomass. The low water solubility (< 0.15 mg/L) and high estimated log Kow values (log Kow > 4.3) indicate that, 1,2,3-Propanetriol, homopolymer, diisooctadecanoate is highly lipophilic. If released into the aquatic environment, the substance undergoes extensive biodegradation and sorption on organic matter, as well as sedimentation. Thus, the bioavailability of this substance in the water column is reduced rapidly. The relevant route of uptake of, 1,2,3-Propanetriol, homopolymer, diisooctadecanoate in organisms is considered predominately by ingestion of particle bounded substance.

Metabolism of aliphatic esters

Should 1,2,3-Propanetriol, homopolymer, diisooctadecanoate be taken up by fish during the process of digestion and absorption in the intestinal tissue, aliphatic esters like 1,2,3-Propanetriol, homopolymer, diisooctadecanoate are expected to be initially metabolized via enzymatic hydrolysis to the corresponding free fatty acids and the free fatty alcohols. 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 distribution of carboxylesterase, high tissue content, rapid substrate turnover and limited substrate specificity (Lech & Melancon, 1980; Heymann, 1980).

Metabolism of enzymatic hydrolysis products

1,2,3-Propanetriol, homopolymer, diisooctadecanoate are hydrolysed to the corresponding alcohols (glycerol, diglycerol and triglycerol) and fatty acid by esterases (Fukami and Yokoi, 2012). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different sites in the organism: after oral ingestion, esters of polyglycerol and fatty acids will undergo chemical changes already in the gastro-intestinal fluids as a result of enzymatic hydrolysis. In contrast, substances which are absorbed through the pulmonary alveolar membrane or through the skin enter the systemic circulation directly before entering the liver where hydrolysis will basically take place.

In an in vitro enzymatic digestion study using fresh pancreatic juice plus bile described by King et al. 1971 the fatty acid labelled polyglycerol esters were investigated. Thin layer chromatography (TLC) and radioassay procedures were used to determine the distribution of 14C among the products of digestion. After enzymatic digestion of oleate-labelled polyglycerol ester, 89-92% of the recovered 14C was present as free oleic acid, whereas the remaining 8 and 11% was unhydrolysed or partially hydrolysed starting material. Hydrolysis of the eicosanoate-labelled polyglycerol ester was much slower than the oleate ester and only 21% of the 14C was recovered as free eicosanoic acid (Michael and Coots, 1971)

After hydrolysis, the cleavage products, fatty acids, are stepwise degraded by beta-oxidation based on enzymatic removal of C2 units in the matrix of the mitochondria in most vertebrate tissues. The C2 units are cleaved as acyl-CoA, the entry molecule for the citric acid cycle. For the complete catabolism of unsaturated fatty acids such as oleic acid, an additional isomerization reaction step is required. The omega and alpha-oxidation, alternative pathways for oxidation, can be found in the liver and the brain, respectively (CIR, 1987).

The other cleavage products polyol polyglycerol, are assumed to be rapidly excreted ad metabolism via cleavage of the ether bond to glycerol will not occur as for the related triglycerol (Michael and Coots, 1971).

Data from QSAR calculation

Additional information on bioaccumulation could be gathered from BCF/BAF calculations of representative fatty acid components (C18 FA monoester (glycerol, diglycerol and triglycerol); C18 FA diester (glycerol, diglycerol and triglycerol and C18 FA triester (glycerol)) using BCFBAF v3.01 (Hopp, 2011 and Marwitz, 2012). The estimated BCF/BAF values of 0.89 – 36.61 L/kg and 0.89 – 36.75 L/kg indicate low bioaccumulation potential in organisms, when including biotransformation (Arnot-Gobas estimate, including biotransformation, upper trophic). Even though that the components with diester and triester are outside the applicability domain of the model they can 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 (> 7) have a lower potential for bioconcentration as summarized in the ECHA Guidance R.11 and that they are not expected to meet the B/vB criterion (ECHA, 2012).

Conclusion

The biochemical process metabolizing aliphatic esters is ubiquitous in the animal kingdom. Based on the enzymatic hydrolysis of aliphatic esters and the subsequent metabolism of the corresponding carboxylic acid and alcohol, it can be concluded that the high log Kow, which indicates a potential for bioaccumulation, overestimates the true bioaccumulation potential of 1,2,3-Propanetriol, homopolymer, diisooctadecanoate since it does not reflect the metabolism of substances in living organisms. BCF/BAF values estimated with the BCFBAF v3.01 program also indicate that 1,2,3-Propanetriol, homopolymer, diisooctadecanoate will not be bioaccumulative (all well below 2000 L/kg). Taking all these information into account, it can be concluded that the bioaccumulation potential of 1,2,3-Propanetriol, homopolymer, diisooctadecanoate is low.