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Environmental fate & pathways

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A study dating back to 1981 is available in which the biodegradability of isoleucine was tested according to the Offhaus method. In this study 19% degradation was found after 5 days, and the COD/BOD5 ratio was determined to be 5.3. These results indicate that L-isoleucine will undergo rapid biological degradation.

The inhibition study showed negligible and decreasing disturbance of degradation of 1%.

Based on the observed respiration activity, the degradation should take place undisturbed, even at higher concentrations and especially after certain adaptation.

A second study on the biodegradability of L-isoleucine is available. In this study dating back to 1969, the ability of activated sludges from 3 municipal waste treatment plants to oxidise amino acids was investigared. The activated sludge (concentration 2500 mg/L) was exposed to L-isoleucine at a concentration of 500 mg/L. The biodegradation was assessed during 24h by measurement of the O2 consumption using Warburg flasks. The average biodegradation (as % of theoretical oxygen demand) over the 3 activated sludges was 14.8 %.

As the results available from the two biodegradation studies on L-isoleucine are not directly comparable to the CLP criteria for biodegradation, additionally a read-across to L-valine was considered to further support the conclusion on this endpoint.

Read-across to the test results available for structural analogue L-valine is deemed justified based on a comparison of the main factors driving environmental toxicity: pH, water solubility, log Kow and chemical reactivity (functional groups).

- both are essential amino acids that are in the zwitterion state at physiological pH

- the chemical structure differs only in that L-isoleucine has one extra methylene group in the aliphatic side chain

- the pKa values of the α-COOH group is very similar: 2.36 for ILE, 2.32 for VAL

- the pKa values of the α-NH2 group is very similar: 9.60 for ILE, 9.62 for VAL

- the substances both have a high water solubility: 34 - 41 g/L for ILE, 58 g/L for VAL

- the substances both have a low log Kow value: -1.72 for ILE (experimental value obtained from EpiSuite's WSKOW model database), -2.08 for VAL (calculated by EpiWin QSAR)

As the chemical structure and reactivity, and the water solubility, low Kow and pKa values of L-isoleucine and L-valine are almost identical, it can be concluded that read-across for aquatic toxicity information between those 2 substances is acceptable.

The tests available for L-valine are carried out according to OECD guidelines and reveal that the substance is readily biodegradable within 28 days. As a consequence, it can be concluded that L-isoleucine will also be readily biodegradable.