Methyl ricinoleate

Endpoint summary

Description of key information

Low bioaccumulation potential can be expected for methyl ricinoleate.

Additional information

No experimental data evaluating the bioaccumulation potential of methyl ricinoleate (CAS 141-24-2) are available. The substance has a log Kow value above 3 (6.84, KOWWIN v1.68), suggesting a potential to bioaccumulate in biota. However, the information gathered on environmental behaviour and metabolism in combination with the QSAR-estimated BCF values provide enough evidence (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) to state that methyl ricinoleate is likely to show a low bioaccumulation potential.

 

Intrinsic properties and fate

Based on experimental data for suitable read across substances methyl ricinoleate is expected to be 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). The Guidance on information requirements and chemical safety assessment, Chapter R7.B (ECHA 2016) 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. Nevertheless, once this contact takes place, these substances are expected to be removed from the water column to a significant degree by adsorption to sewage sludge (Guidance on information requirements and chemical safety assessment, Chapter R.7a, (ECHA 2016)) and the rest will be extensively biodegraded (due to ready biodegradability). Thus, discharged concentration of the substance into the aqueous compartment is likely to be very low. Should the substance be released into the water phase, due to their hydrophobicity and expected high adsorption potential, it will tend to bind to sediment and other particulate organic matter, and therefore, the actual dissolved fraction available to fish via water will be reduced (Mackay and Fraser 2000). Thus, the main route of exposure for aquatic organisms such as fish will be via food ingestion or contact with suspended solids.

 

QSAR data

Estimated bioconcentration (BCF) and bioaccumulation (BAF) values were calculated for the substance using the BCFBAF v3.01 program (Estimation Programs Interface Suite™ for Microsoft® Windows v 4.10., US EPA), including biotransformation rates (Arnot-Gobas method). The substance is within the applicability domain of the QSAR model (log Kow 0.31-8.70) and therefore provides valid supporting information to be considered in the overall bioaccumulation assessment of the substance. BCF values were calculated to be 368.5 (regression based estimate) and 123.3 (Arnot-Gobas method), respectively. A BAF value was estimated to be 128. BCF calculations reflect the bioaccumulation potential after uptake via water, whereas the BAF gives an indication of the bioaccumulation when all exposure routes (water, food, etc.) are taken into account.

The obtained result supports the assumption that the bioaccumulation potential of methyl ricinoleate is low. According to Regulation (EC) No. 1907/2006, Annex XIII, 1.1.2, a substance only fulfils the bioaccumulation criterion (B) when BCF values are > 2000. Even though this condition is preferred to be confirmed with experimental data, in this case the estimated QSAR-based BCFs provide sufficient reliable evidence which suggests that methyl ricinoleate will not be bioaccumulative.

 

Biotransformation and 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/others (metabolites) by means of enzyme-catalyzed processes (e.g., 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). Fatty acid methyl esters are hydrolysed to the corresponding alcohol (methanol) and fatty acid by esterases (Fukami and Yokoi 2012). Particularly in fish rapid metabolism rates of two methyl esters (haloxyfop methyl ester and fluroxypyr methylheptyl ester) have been observed in vitro (fish liver homogenates) with half-lives of 5 and 1 minute respectively (Murphy and Lutenske 1990, Cowan-Elsberry et al. 2008). Furthermore, in vivo studies conducted with esters, including a methyl ester (according to OECD 305) resulted in experimental fish BCFs ranging from 1 to 70, even when the log Kow values of these substances are above 3, indicating once again rapid metabolism (Rodger and Stalling 1972, Barron et al. 1989, Barron et al. 1990).

 

According to the Guidance on information requirements and chemical safety assessment, Chapter R.7c (ECHA 2014), 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.

 

Regarding the biotransformation products of methyl ricinoleate, methanol will partially tend to evaporate from water surfaces (Henry's Law Constant of 4.55x10-6 atm m3/mole (Gaffney et al. 1987)) or stay in the water phase (low adsorption potential to sediment and organic particles according to a Koc of 2.75 (Schuurmann et al 2006), in which rapid biodegradation is expected to occur (92% biodegradation in 14 days; NITE, Japan 2012). Therefore, its bioavailability to aquatic organisms will be generally low. Methanol is a naturally occurring compound in living organisms. It is known to be metabolised and further excreted in the form of CO2 and H2O in several species such as mammals. The log Kow value of this substance (-0.77, Hansch et al. 1995) indicates that bioaccumulation in biota is not expected. In fish (Leuciscus idus), this was confirmed by a test in which a measured BCF < 10 was obtained for methanol (Freitag et al. 1985).

 

Ricinoleic acid is a naturally occurring component in living organisms.For instants, it is a major component of seed oil obtained from mature Castor plant (Ricinus communis L., Euphorbiaceae) seeds or in sclerotium of ergot (Claviceps purpurea Tul., Clavicipitaceae).Fatty acids are known to be metabolised quickly and participate in ubiquitous standard physiological processes (e. g. citric acid cycle, sugar synthesis and lipid synthesis) (Hochachka et al. 1977, Jump 2002). In fish species, fatty acids are the most important energy source 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 (Tocher 2003).

 

Conclusion

Methyl ricinoleate is not expected to be bioaccumulative. Due to its readily biodegradable nature, extensive degradation 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 log Kow, water solubility and adsorption potential, methy ricinoleate may be bioavailable to aquatic organisms via feed and contact with suspended organic particles. After uptake by fish species, extensive and fast biotransformation of the substance by carboxylesterases into ricinoleic acid and methanol is expected. Ricinoleic 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.). Rapid metabolism of analogue ester compounds (involving hydrolysis into fatty acids and methanol) in fish has been observed in vitro, with half-lives in fish liver homogenates below 6 minutes. In vivo fish tests reported BCF values ranging from 1 to 70 for similar ester substances, supporting the argument that rapid metabolism takes place 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 the substance will not be bioaccumulative (well below 2000).

 

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

 

Reference

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