3-mercaptopropionic acid

Endpoint summary

Additional information

Interpreting test results of thiochemicals is complicated by the fact that these substances generally have to be regarded as difficult substances according to OECD 23. The main problem is identifying the test item over time in the bioassays. Firstly, in most cases only a fraction of the nominal concentration can be detected even at the start of the tests. The discrepancy between nominal and measured concentrations may often be due to immediate transformation of the test item on contact with aquatic medium. Sometimes, however, limited solubility may be the reason.

It cannot be distinguished between a toxic effect of the original test substance and the oxidation product because they are both present in the test solution representing a dynamic process, which differs depending on the thiochemical regarded as well as on the test applied. Moreover, in available test protocols the transformation products are usually neither described nor tested.

As the transformation products are less toxic than the parent substances (no free –SH groups) regulatory endpoints calculated on the basis of nominal concentrations represent a realistic worst case approach.  Moreover, for thiochemicals nominal concentrations represent the real environmental conditions, because the test substance is rapidly oxidized by the oxygen content in the aqueous phase.  The effect values therefore reflect the toxicity of the system – in the sense of the UVCB approach – rather than the toxicity of the original chemical substance. Under these conditions it is more meaningful to assess their toxicities on the basis of nominal concentrations instead of measured concentrations during exposure. This may better correspond to the situation under environmental conditions as recommended for multi-component-substances (OECD23).

 

Short-term toxicity to fish

3-Mercaptopropionic acid was tested for acute toxicity towards fish according to OECD-Test Guideline 203. In order to investigate the influence of the test substance towards fish the animals were exposed to different concentrations of the test substance semistatically. Daily the test solutions were prepared freshly and exarnined for dead animals. Under the conditions used for the test, toxic effects of the test substance towards fish were observed at concentrations beginning at 200 mg/L prepared in drinking water. The following LC-values were obtained after 96h:

o LCO (calculated value): 110 mg/L

o LC50 (calculated value): 270 mg/L

o LC100 (observed): 400 mg/L (directly taken from the nominal concentration being tested)

Within TOC-analyses performed with a test solution freshly prepared in tap water, and after 96h (this is the time schedule for a fish toxicity test), it could be shown, that the organic matter of the test substance remained constant in the aqueous phase. Upon chemical analyses of test solutions performed with freshly prepared test solutions and after 48h of incubation in a daphnia toxicity test (Study-No. IF-99/21946-00) it could be demonstrated, that the test substance was partially oxidized, and thus there are two components, the original test substance and the oxidation product, present in the test solution. It cannot be distinguished between a toxic effect of the original test substance and the oxidation product because they are both present in the test solution representing also a dynamic process. For this reason, the results of this test are based on the nominal concentrations tested. As an alternative calculation using the measured concentration of the original test substance 3-mercaptopropionic acid after 24h of incubation, respectively (semi-static test), the following LC-values were obtained after 96h:

LC0 (calculated value): 15 mg/L

o LC50 (calculated value): 98 mg/L

o LC100 (observed): 174 mg/L (directly taken from the concentration being measured)

These values don't represent the real environmental conditions because the test substance is rapidly oxidized by the oxygen content in the aqueous phase.

 

Long-term toxicity to fish

A full set of short-term toxicity data is available for 3-MPA, but no long-term ecotoxicity data. However, before proposing testing in vertebrate animals as a last resort, a Daphnia reproduction study with 3-MPA is proposed to be conducted first. Based on these data, an acute-to-chronic-ratio (ACR) will be calculated. The same ACR is assumed to apply to fish due to the mode of action of the thiochemicals. If the exposure and risk assessment results in PEC/PNEC ratios well below 1, a chronic fish study would not be acceptable for animal welfare reasons.

 

Short-term toxicity to aquatic invertebrates

3-Mercaptopropionic acid was tested for acute toxicity towards Daphnia according to OECD 202, Part I. In order to investigate the influence of the test substance towards the daphnia, the swimming behaviour of the animals was recorded. Under the conditions used for the test, immobilisation of the daphnia was observed at concentrations >12.5 mg/L. The following EC-values were calculated on basis of the nominal concentrations being tested: 24 h EC50 = 31 mg/L, 48 h EC50 = 17 mg/L

Within TOC-analyses performed with a test solution freshly prepared in tap water, and after 96h (this is the time schedule for a fish toxicity test), it could be shown, that the organic matter of the test substance remained constant in the aqueous phase. Upon chemical analyses of test solutions performed with freshly prepared test solutions and after 48h of incubation it could be demonstrated, that the test substance was partially oxidized, and thus there are two components, the original test substance and the oxidation product, present in the test solution. It cannot be distinguished between a toxic effect of the original test substance and the oxidation product because they are both present in the test solution representing also a dynamic process. For this reason, the results of this test are based on the nominal concentrations tested. As an alternative calculation (not recommended by the OECD Guideline 202 Part D using the measured concentration of the original test substance 3-mercaptopropionic acid, the following EC-values were calculated: 24 hEC50 = 14 mg/L, 48 h EC50 = 4 mg/L

These values do not represent the real environmental conditions because the test substance is rapidly oxidized by the oxygen content in the aqueous phase.

 

Long-term toxicity to aquatic invertebrates

A testing proposal is included in the dossier. When the study data are available, a robust study summary will be prepared and submitted within an update of the dossier. Evaluation will be reconsidered based on the outcome of the Daphnia reproduction test.

 

Toxicity to algae

The influence of the test item 3-Mercaptopropionic acid on the growth of the freshwater green algal species Pseudokirchneriella subcapitata (formerly Selenastrum capricornutum) was investigated in a 72-hour static test according to the OECD Guideline 201 (2006) and the EU Commission Directive 92/69/EEC, C.3 (1992).
The nominal concentrations of the test item of 2.2, 4.6, 10, 22, 46 and 100 mg/L were tested in parallel with a control.
The measured concentrations of the test item in the freshly prep ared test media of the nominal concentrations of 2.2 to 100 mg/L were betw een 65 and 90% of the nominal values. After 72 hours test duration, the concentrations of the test item were determined to be below the LOQ (1.27 mg/L) at the nominal test concentrations of 4.6 to 46 mg/L. At the highest nominal test concentration of 100 mg/L, 45% of the nominal value was found at the end the test period.

The test item had a significant inhibitory effect on the growth of the algae (growth rate and yield) after the test period of 72 hours at the concentr ation of 8.0 mg/L (initially measured) and at all higher test concentrations. The microscopic examination of the algal cells at the end of the test showed no difference between the algae growing at the nominal test concentration of 100 mg/L and the algal cells in the control.

The results can be summarized as follows (on the basis of the initially measured concentrations of the test item): ErC10 = 17 (6.0-21) mg/L, EcR50 =26 (31-34) mg/L, NOErC = 4.1 mg/L.

Due to instability of the substance in aqueous media and since the transformation products are less toxic than the parent substances (no free –SH groups) regulatory endpoints calculated on the basis of nominal concentrations represent a realistic worst case approach.

The effect levels based on nominal concentrations are: ErC10 = 21.3 mg/L; ErC50 = 32.6 mg/L; NOEC = 4.6 mg/L

 

Toxicity to microorganisms

The influence of the test item 3-mercaptopropionic acid on the activity of activated sludge was evaluated by measuring the respiration rate under defined conditions in a study according to OECD Guideline for Testing of Chemicals, Section 2, No. 209: “Activated Sludge, Respiration Inhibition Test",
adopted April 04, 1984.. The respiration rate (oxygen consumption) of an aerobic activated sludge fed with a standard amount of synthetic sewage was measured in the presence of various concentrations of the test item after an incubation period of 3 hours.

In comparison to the inoculum controls the respiration rate of the activated sludge was not or only slightly inhibited by 0 and 5% at the two lowest nominal concentrations of 10 and 32 mg/L, respectively. At the next higher nominal concentrations of 100 and 320 mg/L, the respiration rate was moderately inhibited by 32.5 and 45%.

At the highest nominal test concentration of 1000 mg/L an inhibition rate of 67.5% was found.
The resulting inhibitions of the respiration rates based on nominal concentrations of the test item were EC10 = 32.1, EC20 = 76.1, and EC50 = 393.3 mg/L, respectively.