Glycerol tristearate

Bioaccumulation: aquatic / sediment

Description of key information

Glycerol tristearate shows low bioaccumulation potential

Additional information

5.3.1 Aquatic bioaccumulation

Experimental data evaluating the bioaccumulation potential of glycerol tristearate (CAS No. 555-43-1) are available, showing low bioaccumulation of the substance in fish (BCF < 10). Nevertheless, since this study was not performed according to a standard OECD or internationally accepted guideline, the results are combined with other available data in a Weight of Evidence approach (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. 1907/2006, Annex IX).

 

Experimental data

The bioaccumulation potential of glycerol tristearate (CAS No. 555-43-1) in fish, algae and activated sludge has been investigated (Freitag et al., 1985). Leuciscus idus melanotus, Chlorella fusca and activated sludge microorganisms were exposed for 3 days, 24 hours and 5 days respectively to the test substance, within a static water regime at a nominal concentration of 0.05 mg/L. After the exposure period, the resulting BCF values were < 10 L/kg (fish), 15840 (algae) and 3600 (activated sludge). The large differences between the results reported for fish with respect to those from algae and activated sludge suggest the role of a higher and more developed metabolic system which will allow for biotransformation as described below. In view of these results, the bioaccumulation potential of glycerol tristearate in fish species is expected to be low.

Intrinsic properties and fate

All substances included in the Glycerides category are readily biodegradable. According to the Guidance on information requirements and chemical safety assessment, Chapter R.7b, readily biodegradable substances can be expected to undergo rapid and ultimate degradation in most environments, including biological Sewage Treatment Plants (STPs)(ECHA, 2008). Therefore, after passing through conventional STPs, only low concentrations of these substances are likely to be (if at all) released into the environment.

 

The Glycerides category covers aliphatic (fatty) acid esters of glycerol. The category contains both monoconstituent and UVCB substances with aliphatic acid carbon chain lengths of C2 (acetate) and from C7-C22 (uneven/even-numbered, including saturated, unsaturated, branched, linear, hydroxylated and acetylated chains) building mono-, di- and tri-esters with glycerol in variable proportions. The majority of the substances in the category are insoluble in water (water solubility values < 1 mg/L). The Guidance on information requirements and chemical safety assessment, Chapter R7.b (ECHA, 2008) states that once insoluble chemicals enter a standard STP, they will be extensively removed in the primary settling tank and fat trap and thus, only limited amounts will get in contact with activated sludge organisms. Some category members (generally substances with a high mono- and diester content and short to medium fatty acid carbon chain lengths) are more soluble in water, with water solubility values ranging from 1.3 to 46 mg/L. Despite this difference, all the members of the Glycerides category have log Kow > 3. Therefore, besides being extensively biodegraded in STPs (due to their ready biodegradability), a significant degree of removal of these substances from the water column due to adsorption can be expected (Guidance on information requirements and chemical safety assessment, Chapter R.7a (ECHA, 2008)). Discharged concentrations into the aquatic compartment are therefore likely to be low.

 

Should the substances be released into the water phase, their behavior will largely depend on their water solubility values. The majority of the (insoluble) substances within the Glycerides category will tend to bind to sediment and other particulate organic matter due to their hydrophobicity and expected adsorption potential, being the actual dissolved fraction available to fish via water low (Mackay and Fraser, 2000). Thus, the most relevant exposure route for aquatic organisms such as fish will be via food ingestion or contact with suspended solids. On the other hand, those category members with higher water solubility but adsorption potential due to log Kow values > 3, will be bioavailable to aquatic organisms partially via water and partially via feed and contact with suspended solids.

 

Metabolism

After lipid content, the degree of biotransformation seems to be the most relevant factor regarding the bioaccumulation of organic chemicals in aquatic organisms (Katagi, 2010). Biotransformation consists in the conversion of a specific substance into another/other (metabolites) by means of enzyme-catalyzed processes (ed. van Leeuwen and Hermens, 1995). Carboxylesterases are a group of ubiquitous and low substrate specific enzymes, involved in the metabolism of ester compounds in both vertebrate and invertebrate species, including fish (Leinweber, 1987; Barron et al., 1999).

 

Glycerides, especially triglycerides, are the predominant lipid class in the diet of both marine and freshwater fish. Once ingested, they will be hydrolized into fatty acids and glycerol by a specific group of carboxylesterase (CaE) enzymes (lipases) as reported in different fish species (Tocher, 2003). Part of the free fatty acids will be re-sterified once more with glycerol and partial acyl glycerols to form triglycerides, that will be stored as long-term energy reserves. Glycerol is naturally present in animal and vegetable fats, rarely found in free state (mostly combined with fatty acids forming triglycerides) (ed. Knothe, van Gerpen and Krahl, 2005). If freely available in aquatic organisms, it will not bioaccumulate in view of its log Kow value of -1.76 (OECD SIDS, 2002). Especially in periods in which the energy demand is high (reproduction, migration, etc.), glycerides are mobilized from the storage sites as source of fatty acids. Fatty acid catabolism is the most important energy source in many species of fish, resulting in the release of acetyl CoA and NADH (throughβ-oxidation) and eventually, via the tricarboxylic cycle, the production of metabolic energy in the form of ATP. This fatty acid-catabolism pathway is the predominant source of energy related to growth, reproduction and development from egg to adult fish. A similar metabolic pathway is observed in mammals (see section 7.1.1 Basic toxicokinetics).

 

According to the Guidance on information requirements and chemical safety assessment, Chapter R.7c (ECHA, 2008), even though ready biodegradability does not per se preclude bioaccumulation potential, generally (depending on exposure and uptake rates) ready biodegradable substances are likely to be rapidly metabolised, and therefore, concentrations stored in aquatic organisms will tend to be low.

 

 

QSAR data

Additional information on the bioaccumulation of Glycerides in fish species is available. Estimated bioconcentration (BCF) and bioaccumulation (BAF) values were calculated for all substances using the BCFBAF v3.01 program (Estimation Programs Interface Suite™ for Microsoft® Windows v 4.10., US EPA), assuming biotransformation (Arnot-Gobas method). Even though not all substances or main fatty acid components (in the case of UVCBs) are within the applicability domain of the model (covering substances with log Kow values in the range 0.31-8.70), the estimated values can be considered as supporting data indicating low bioaccumulation of the substances. BCF and BAF values ranged from 0.89 to 57.1 L/kg across the category.

 

Conclusion

The bioaccumulation potential of glycerol tristearate (category member, CAS No. 555-43-1) was experimentally determined, resulting in a BCF < 10 L/kg in fish (Leuciscus idus melanotus). High BCF values were reported for algae (15840) and activated sludge (3600), reflecting the lack of a developed metabolic system as present in fish. This findings support the argument that in fish species, rapid metabolism takes place and therefore the bioaccumulation potential is low, even when log Kow values are above the trigger value of 3. The supporting BCF/BAF values estimated with the BCFBAF v3.01 program also indicate that these substance will not be bioaccumulative (0.89 L/kg). Furthermore, due to its readily biodegradable nature, extensive degradation of this substance in conventional STPs will take place and only low concentrations are expected to be released (if at all) into the environment. Once present in the aquatic compartment, further biodegradation will occur and, due to its high log Kow, low water solubility and adsorption potential, glycerol tristearate will be bioavailable to aquatic organisms such as fish mainly via feed and contact with suspended organic particles. After uptake by fish species, extensive and fast biotransformation of glycerol tristearate by carboxylesterases into fatty acids and glycerol is expected. Fatty acids will be further used by these organisms as their main source of energy throughout all the different life stages (early development, growth, reproduction,etc.).

The information above provides strong evidence supporting the statement that rapid metabolism and low bioaccumulation potential can be expected for glycerol tristearate. 

 

A detailed reference list is provided in the technical dossier (see IUCLID, section 13) and within the CSR.