Registration Dossier

Registration Dossier

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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

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Administrative data

Link to relevant study record(s)

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Endpoint:
basic toxicokinetics in vitro / ex vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
test procedure in accordance with national standard methods
Objective of study:
metabolism
Qualifier:
no guideline available
Principles of method if other than guideline:
To identify the number and proportions of its metabolites, dimethyl disulphide was incubated with fresh rat hepatocytes. Samples were removed after 0, 1, 2 and 4 hours and quenched in liquid nitrogen. All samples were analysed by GC-MS.
GLP compliance:
no
Remarks:
Although a claim of GLP compliance has not been made for this study, the laboratory procedures were conducted in accordance with the current GLP requirements of the UK MHRA and OECD.
Radiolabelling:
no
Species:
rat
Strain:
Sprague-Dawley
Sex:
male/female
Route of administration:
application in vitro
Vehicle:
other: incubation medium
Duration and frequency of treatment / exposure:
up to 4 hours
Dose / conc.:
588 other: nmol/L
Dose / conc.:
177 other: nmol/L
Details on study design:
Dissolution of Test Substance
Stock solutions of Dimethyl disulphidewere prepared in incubation medium. The actual concentration of the formulation was determined after the incubation.
Reagents
General purpose reagents and solvents were of Analar grade (or a suitable alternative) and were obtained principally from VWR International Ltd, Rathburn Chemicals Ltd, Aldrich Chemical Company Ltd and Vickers Laboratories Ltd. Suitable liquid scintillants were obtained from Perkin Elmer Life Sciences Ltd.
Equipment
Radio-HPLC was performed on an Agilent 1100 chromatographic system with a Packard FSA 525TR radio-detector. HPLC data were captured on line with Laura software (version 3.4.7.52 SP8).
Test System
Source
Liver samples were obtained from one male and one female Sprague-Dawley rat. Hepatocytes were prepared by a two-step collagenase perfusion method.
The source of all tissue was recorded in the raw data and documented in the final report. The test system was identified by means of a label containing information such as species, lot number etc.
Cell Viability and [14C]7-Ethoxycoumamarin Metabolism
Cell viability of the hepatocyte suspensions was estimated by trypan blue exclusion. Metabolic capacity of the suspension cultures was determined by quantification of [14C]7-ethoxycoumarin metabolism.
Experimental Procedures
Cell Culture Medium
Leibovitz L-15 medium, pre-warmed toca.37°C, was used for all incubations.
Control matrix analysis
A sample of the control rat hepatocyte matrix (1 x 106cells/mL) was supplied to the Department of Environmental Sciences for analysis along with the incubation samples.
Dimethyl disulphide Incubations
All incubations and GC analysis was carried out in Headspace vials.
Dimethyl disulphide was incubated at a nominal concentration of 10 µmol/L with freshly prepared rat hepatocytes in suspension in Leibovitz L-15 medium (1 x 106cells/mL). Incubations were performed atca.37°C in a shaking water bath and terminated at 0, 1, 2 and 4 hours. The incubations were terminated by samples being frozen in liquid nitrogen. A blank incubation was carried out with either Leibovitz L-15 medium in the place of hepatocytes or in the place of DMDS.
Eight replicates at each time point were generated and supplied to the Department of Environmental Sciences and analysed by GC-MS.
[14C]7-Ethoxycoumarin Incubations
[14C]7-Ethoxycoumarin, at a nominal concentration of 50 µmol/L, was incubated in a suspension of freshly prepared rat hepatocytes at a nominal cell density of 1 x 106viable cells/mL in cell culture medium. Incubations were performed atca37°C in a shaking water bath and terminated after 0 and 4 hours. All incubations were carried out in duplicate.
The formation of 7-hydroxycoumarin and the corresponding glucuronide and sulphate conjugates was quantified by radio-HPLC.
Details on dosing and sampling:
sample analysis
Determination of 7-Ethoxycoumarin Metabolism
The formation of 7-hydroxycoumarin and the corresponding glucuronide and sulphate conjugates in the hepatocyte incubations was estimated by radio-HPLC.
Chemical Analysis
The concentration of Dimethyl disulphide and potential metabolites (dimethyl sulphide, dimethyl sulfone, dimethyl sulfoxide and methyl mercaptan) in the hepatocyte samples was measured using GC-MS.
Metabolites identified:
yes
Details on metabolites:
Dimethyl disulphide was metabolised extensively in fresh rat hepatocytes with the parent molecule concentrations of 66.0 and 92.3 nmol/L after 4 hours for replicate 3 and 4, respectively. As the amount of dimethyl disulphide decreased, the levels of metabolites were observed to increase at all time points
Two metabolites, dimethyl sulphide (DMS) and methyl mercaptan, were observed at measurable levels, greater than background levels in fresh rat hepatocytes after incubation for 4 hours. The major metabolite was attributed to methyl mercaptan with concentrations of 472 and 738 nmol/L at 4 hours in replicate 3 and 4, respectively.
Notably, the formation of the oxidation products of DMS, dimethylsulfoxide (DMSO) and dimethylsulfone (DMSO2)was not observed during the incubation with fresh rat hepatocytes.

Hepatocyte Viability and Metabolic Capacity

The cell viabilities of the fresh hepatocytes as measured by trypan blue exclusion were 71% and 67% in male and female rat, respectively. The cells were considered acceptable for use and were pooled prior to use.

Phase I metabolic capacity was assessed by the quantitative appearance of 7-hydroxycoumarin following incubation of [14C]7-ethoxycoumarin in suspensions of hepatocytes from rats (male and female pooled). Phase II activity was assessed by the quantitative appearance of the corresponding glucuronide and sulphate conjugates. The metabolic capacity assessment is shown in Table1. All hepatocyte preparations were found to have acceptable Phase I and Phase II metabolic capacity when compared to historical data.

Concentration of Dimethyl Disulphide in Hepatocyte incubations

The concentration of dimethyl disulphide was 588 and 177 nmol/L in the incubations with rat hepatocytes from replicate 3 and 4, respectively. This was lower than the anticipated concentration due to the volatile nature of handling the test compound. However, the concentration achieved still allowed the evaluation of the formation of the metabolites of dimethyl disulphide.

Incubation of Rat Hepatocytes in the Absence of Dimethyl Disulphide

Control incubations containing buffer instead of dimethyl disulphide were performed (Table2).

Following incubation of rat hepatocytes in the absence of dimethyl disulphide three peaks were observed at retention times corresponding to dimethyl disulphide, dimethyl sulphide and methyl mercaptan. Methyl mercaptan was the most abundant substance, whereas the 2 other metabolites were present at low levels. Therefore the concentrations in the control incubations were subtracted from the corresponding concentrations of parent and metabolites in the incubations with dimethyl disulphide and hepatocytes.

Incubation of Dimethyl Disulphide in the Absence of Hepatocytes

Control incubations containing buffer instead of hepatocytes were performed (Table3).

Following incubation of dimethyl disulphide in the absence of hepatocytes for 4 hours only dimethyl disulphide and methyl mercaptan were detected in the incubate samples.

As dimethyl disulphide is hydrolytically stable it is likely that methyl mercaptan was formed during the analysis, however, this could not be confirmed from data.

Incubation of Dimethyl Disulphide with Rat Hepatocytes

Dimethyl disulphide was metabolised extensively in fresh rat hepatocytes with the parent molecule concentrations of 66.0 and 92.3 nmol/L after 4 hours for replicate 3 and 4, respectively (Table4). As the amount of dimethyl disulphide decreased, the levels of metabolites were observed to increase at all time points

Two metabolites, dimethyl sulphide (DMS) and methyl mercaptan, were observed at measurable levels, greater than background levels in fresh rat hepatocytes after incubation for 4 hours. The major metabolite was attributed to methyl mercaptan with concentrations of 472 and 738 nmol/L at 4 hours in replicate 3 and 4, respectively (Table4).

Notably, the formation of the oxidation products of DMS, dimethylsulfoxide (DMSO) and dimethylsulfone (DMSO2)was not observed during the incubation with fresh rat hepatocytes.

Conclusions:
Dimethyl disulphide, dimethyl sulphide and methyl mercaptan are present in the control hepatocyte solutions. Dimethyl disulphide was metabolised rapidly after incubation with fresh rat hepatocytes for up to 4 hours. Formation of both dimethyl sulphide and methyl mercaptan was observed over the 4 hour incubation. No formation of DMSO or DMSO2 was observed during the incubation with fresh rat hepatocytes.
Executive summary:

The objective of this study was to identify the number and proportions of the metabolites generated following incubation of dimethyl disulphide with fresh rat hepatocytes using a GC-MS method. Dimethyl disulphide was incubated with fresh rat hepatocytes. Samples were removed after 0, 1, 2 and 4 hours and quenched in liquid nitrogen. All samples were analysed by GC-MS. [14C]-7-ethoxycoumarin was also incubated with rat hepatocytes suspended in Leibovitz L-15 medium. Samples were terminated after 0 and 4 hours and all samples quantified by radio-HPLC for the formation of 7-hydroxycoumarin and glucuronide and sulphate conjugates. Dimethyl disulphide was metabolised rapidly after incubation with fresh rat hepatocytes for up to 4 hours. Methyl mercaptan was the main metabolite observed over the 4 hour incubation, dimethyl sulphide (DMS) was found at a low level. No formation of the oxidation products of DMS, dimethylsulfoxide (DMSO) or dimethylsulfone (DMSO2) was observed during the incubation with fresh rat hepatocytes.

Endpoint:
basic toxicokinetics in vivo
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Objective of study:
metabolism
Qualifier:
no guideline followed
Principles of method if other than guideline:
The intraperitoneal administration of 1 µl of dimethyl disulfide to mice to evaluate the pulmonary excretion.
GLP compliance:
no
Radiolabelling:
no
Species:
mouse
Strain:
CD-1
Sex:
female
Details on test animals or test system and environmental conditions:
TEST ORGANISMS:
- Source: Charles River Laboratories, Wilmington, MA, USA
- Age: no data
- Weight at study initiation: 25-30 g
- Adaptation period: no data

HOUSING
The animals were housed 5 per cages

FOOD and WATER
- Food: ad libitum
- Water: ad libitum

ENVIRONMENTAL CONDITIONS
- Temperature : no data
- Relative humidity : no data
- Light/dark cycle : no data
- Ventilation : no data
Route of administration:
intraperitoneal
Vehicle:
unchanged (no vehicle)
Duration and frequency of treatment / exposure:
Single administration
Dose / conc.:
1 other: µL/mouse
No. of animals per sex per dose / concentration:
5
Control animals:
yes, concurrent no treatment
Positive control reference chemical:
not appropriate
Details on excretion:
Intraperitoneal administration of 1 µl DMDS/mouse resulted in its appearance in the expired air, as well as much smaller amounts of methyl mercaptan and dimethyl sulfide. The excretion of DMDS reached a peak between 3 and 6 minutes after i.p. injection, and the total amount eventually excreted was about 6% of the dose. The amount of dimethyl sulfide and methanethiol excreted accounted for about 0.5% of the injected dose
Metabolites identified:
yes
Details on metabolites:
Methyl mercaptan and dimethyl sulfide

Mice injected with 1 microliter of dimethyl disulfide (specific gravity 1.06 gram/ml at 20° C) excreted three volatile sulfur compounds in their expired breath. These were positively identified by gas chromatography/mass spectrometry as dimethyl disulfide, dimethyl sulfide and methanethiol. The pattern of the pulmonary excretion of these campounds with time was then followed using the gas chromatograph, and the results are summarized in the Table. The excretion of dimethyl disulfide reached a peak between 3 and 6 minutes after intraperitoneal injection, and the total amount eventually excreted was about 6%, of the dose. The amounts of dimethyl sulfide and methanethiol excreted accounted for about 0.5% each of the injected dose, and the peak excretion for each occurred at 6 minutes after injection. These results strongly suggest that dimethyl disulfide is split in vivo to yield two moles of methanethiol. Part of the latter is presurcably methylated to yield dimethyl sulfide.


This postulated sequence was proved by injecting methanethiol into one mouse and showing that both it and dimethyl sulfide appeared on the expired breath. When dimethyl sulfide was injected into another mouse, it alone appeared an the expired breath.


Pulmonary Excretion of Volatile sulfur Compounds in Mice Following Intraperitoneal Injection of Dimethyl Disulfidea


 





































































Minutes after Injection



Amount in Microgramsb



Dimethyl Disulfide



Dimethyl Sulfide



Methanethiol



0c



0.023 ± 0.013



0.001



0.001



3



2.1 ± 0.3



0.005 ± 0.0008



0.003 ± 0.0005



6



2.0 ± 0.14



0.14 ± 0.034



0.1 ± 0.016



9



0.4 ± 0.14



0.036 ± 0.0053



0.002 ± 0.0002



15



0.28 ± 0.068



0.016 ± 0.0037



0.001



22



0.04 ± 0.02



0.001



0.001



27



0.008



0.001



0.001



30



0.001



0.001



0.001



40



0.001



0.001



0.001



 


a Dose of 1 microliter/mouse or 35 to 40 mg/kg.


b Mean ± SEM for five mice. Amounts shown represent collections for 15 sec only. The total amounts were estimated by planimetry under the derived curves of above amounts vs. time.


c Values immediately after injection. No endogenous sulfur compounds were found prior to treatment.

Conclusions:
The intraperitoneal administration of 1 µl of dimethyl disulfide to mice resulted in its appearance in the expired breath as well as much smaller amounts of both methanethiol and dimethyl sulfide.
Executive summary:

Mice injected with 1 microliter of dimethyl disulfide (specific gravity 1.06 gram/ml at 20° C) excreted three volatile sulfur compounds in their expired breath. These were positively identified by gas chromatography/mass spectrometry as dimethyl disulfide, dimethyl sulfide and methanethiol. The pattern of the pulmonary excretion of these campounds with time was then followed using the gas chromatograph, and the results are summarized in the Table. The excretion of dimethyl disulfide reached a peak between 3 and 6 minutes after intraperitoneal injection, and the total amount eventually excreted was about 6%, of the dose. The amounts of dimethyl sulfide and methanethiol excreted accounted for about 0.5% each of the injected dose, and the peak excretion for each occurred at 6 minutes after injection. These results strongly suggest that dimethyl disulfide is split in vivo to yield two moles of methanethiol. Part of the latter is presurcably methylated to yield dimethyl sulfide.


This postulated sequence was proved by injecting methanethiol into one mouse and showing that both it and dimethyl sulfide appeared on the expired breath. When dimethyl sulfide was injected into another mouse, it alone appeared an the expired breath.

Endpoint:
basic toxicokinetics, other
Remarks:
G.I. human passive absorption
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
test procedure in accordance with national standard methods with acceptable restrictions
Objective of study:
absorption
Guideline:
other: REACH Guidance on QSARs R.6
Principles of method if other than guideline:
Model to predict either high or low fraction absorbed for an orally administered, passively transported substance on the basis of a new absorption parameter. The model includes only two inputs: the octanol-water partition coefficient (Kow) and the dimensionless oversaturation number (OLumen). The latter is the ratio of the concentration of drug delivered to the gastro-intestinal (GI) fluid to the solubility of the compound in that environment.
Specific details on test material used for the study:
SMILES (used for QSAR prediction): CSSC
Species:
other: Human
Route of administration:
oral: unspecified
Type:
absorption
Results:
Absorption from gastrointestinal tract for 1 mg dose: 90%
Type:
absorption
Results:
Absorption from gastrointestinal tract for 1000 mg dose: 90%
Conclusions:
Using a model to predict either high or low fraction absorbed for an orally administered, passively transported substance, the rates of absorption were 90% for a dose of 1 and 1000 mg (Danish (Q)SAR Database).
Endpoint:
dermal absorption, other
Remarks:
QSAR
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Guideline:
other: REACH Guidance on QSARs R.6
Principles of method if other than guideline:
IH SkinPerm (v2.04) is a mathematical tool for estimating dermal absorption. The rate of mass build-up (or loss) on the skin comes from the deposition rate onto the skin minus the absorption rate into the Stratum Corneum (SC) and the amount evaporating from the skin to the air.
Species:
other: human
Type of coverage:
open
Vehicle:
unchanged (no vehicle)
Details on study design:
DATA INPUT
Molecular weight: 94.19 g/mol
Temperature: 20 °C
Vapour Pressure: 3000 Pa
Water solubility: 2700 mg/L
Log Kow: 1.91
Density: 1062 mg/cm3
Melting point: --84.7°C

SCENARIO PARAMETERS
- Instantaneous deposition
Deposition dose*: 1000 mg
Affected skin area**: 1000 cm²
Maximum skin adherence***: 2 mg/cm²
Thickness of stagnant air****: 1 cm
Weight fraction: 1
Timing parameters
. Start deposition: 0 hr
. End time observation: 8 hr
Report parameters
. Calculation (intervals/hr): 10000
. Report (intervals/hr): 100

- Deposition over time
Affected skin area**: 1000 cm²
Maximum skin adherence***: 1 mg/cm²
Dermal deposition rate: 2 mg/cm²/hr
Thickness of stagnant air****: 1 cm
Weight fraction: 1
Timing parameters
. Start deposition: 0 hr
. Duration of deposition: 8hr
. End time observation*: 8 hr
Report parameters
. Calculation (intervals/hr): 10000
. Report (intervals/hr): 100

*Default value defined according to the internal validation study
**Estimated skin surface of two hands of an adult.
***The skin adherence field is greyed out and a default of -1 is indicated if the substance is a liquid at 25°C. Smart logic is built into IH SkinPerm; the program recognizes whether a substance is a solid or liquid at standard temperature (25°C) based on the physicochemical properties. For substances
that are solids at 25°C a maximum adherence value up to 2 mg/cm² is allowed based on studies of soil-on-skin adherence. If the deposition rate results in an increase above the input figure (0.2-2 mg/cm²), it is assumed that the surplus disappears just by removal from the skin.
*** 3 cm if clothing involved, 1 cm if bare skin involved

Time point:
8 h
Dose:
1000 mg
Parameter:
percentage
Absorption:
2.86 %
Remarks on result:
other: Instantaneous deposition
Time point:
8 h
Dose:
1 mg/cm²/h
Parameter:
percentage
Absorption:
5.7 %
Remarks on result:
other: Deposition over time for 8 hr
Conclusions:
The dermal absorption of DMDS is estimated to be low (<= 10%).
Executive summary:

The dermal absorption of DMDS leads to the following results, obtained using the SkinPerm v2.04 model according to the input data:

 

Instantaneous deposition

 

Deposition over time

End time observation 8 hr

Total deposition (mg) or deposition rate (mg/cm²/hr)

1000

1

Fraction absorbed (%)

2.86

5.7

Amount absorbed (mg)

 28.6

442

Lag time stratum corneum (min)

6.5

Max. derm. abs. (mg/cm²/h)

0.0396

Description of key information

DMDS can be absorbed through the skin, however, no dermal penetration data are available.
Dermal exposure to DMDS is not anticipated under normal conditions of use based on the use of enclosed systems that are required to maintain the highly volatile DMDS (vapour pressure 38.6 hPa @ 25 °C) as a liquid. The potential for skin irritation and skin sensitisation caused by DMDS will require workers to wear the recommended personal protective equipment (PPE). General population have no direct access to DMDS-containing products.
In consideration of the industrial use of DMDS, inhalation will be the most relevant route of exposure for operators/ workers and bystanders due to the volatility of DMDS (vapour pressure: 38.6 hPa @ 25°C, Henry’s law constant 105 Pa m3/mol at 20°C). Toxicology studies have identified a low level of systemic toxicity following inhalation exposure and the critical effect for the human health risk assessment is related to a concentration effect at the site of contact at concentrations of DMDS devoid of systemic effects.


 

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - oral (%):
90
Absorption rate - dermal (%):
10
Absorption rate - inhalation (%):
90

Additional information

No Good Laboratory Practice (GLP) and test guideline-compliant oral administration in vivo metabolism studies are available for dimethyl disulphide (DMDS). Nevertheless there is a wealth of data in the public domain on the natural occurrence of DMDS in the diet, the use of DMDS as a flavouring agent and the absorption, excretion, distribution and metabolism of simple disulphides following ingestion. Some supporting information is also available on the in vitro metabolism of DMDS in rat hepatocytes (Kilford, 2012). A study on pulmonary excretion of DMDS in mice when administered by intraperitoneal injection is also available (Susman et al., 1978).


 ABSORPTION


Oral route


DMDS has a molecular weight of 94 g/mol, and molecular weights below 500 are favourable for oral absorption. DMDS has a low water solubility (2.7 mg/L) but a moderate logP (1.91) which is also favourable for absorption by passive diffusion. Mortalities and signs of systemic toxicity observed in the acute oral toxicity studies are also indicative of a significant oral absorption.


Using a model to predict either high or low fraction absorbed for an orally administered, passively transported substance, the rates of absorption were 90% for a dose of 1 and 1000 mg (Danish (Q)SAR Database).


 


Inhalation route


In consideration of the industrial use of DMDS, inhalation will be the most relevant route of exposure for operators/ workers and bystanders due to the volatility of DMDS (vapour pressure: 38.6 hPa @ 25oC, Henry’s law constant 105 Pa m3/mol at 20°C). Toxicology studies have identified a low level of systemic toxicity following inhalation exposure and the critical effect for the human health risk assessment is related to a concentration effect at the site of contact at concentrations of DMDS devoid of systemic effects.


A gross estimate of the absorption by inhalation exposure can be obtained by comparison of the endogeneous dose levels in the acute oral and inhalation toxicity studies. The 4-h inhalation LC50 of DMDS is 5050 mg/m3 (Kirkpatrick, 2005). For a calculation of an endogenous dose using the alveolar ventilation rate it has to be considered that only 70% of the total ventilation air is available for exchange via the alveoli per unit time (REACH guidance Chapter R.7c, 2017). With a ventilation rate of 0.19 m3/kg for 4 hours of exposure, the equivalent endogenous dose level of the 4h LC50 is 671 mg/kg (5050 mg/m3 x 0.19 m3/kg x 0.7). This value is in the range of the acute oral LD50 (415-750 mg/kg, Yasso, 2016), indicating a similar absorption by inhalation and oral exposure. Therefore, for the purpose of risk assessment the inhalation absorption rate will be considered to be 90%.


 


Dermal absorption


Dermal exposure to DMDS is not anticipated under normal conditions of use based on the use of enclosed systems that are required to maintain the highly volatile DMDS (vapour pressure 38.6 hPa @ 25oC) as a liquid. The potential for skin irritation and skin sensitisation caused by DMDS will require workers to wear the recommended personal protective equipment (PPE). General population have no direct access to DMDS-containing products.


To be absorbed, the substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis. With a water solubility of 2.7 mg/l and a logP of 1.91, dermal uptake of DMDS is anticipated to be low to moderate.


The rate of absorption was estimated with the IH SkinPerm model (v2.04). For an instant or an over time skin deposition, the dermal absorption of DMDS is low.


Therefore, according to the REACH Guidance, a default value of 10% will be used for the skin absorption rate of DMDS. This default value is further supported by the lack of mortality in the dermal toxicity studies up to the dose level of 5000 mg/kg, but a potential of skin sensitisation.


DISTRIBUTION


Due to the low molecular weight, a wide distribution of DMDS is expected. As the molecule is lipophilic (log P >0), it is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration particularly in fatty tissues.


METABOLISM


DMDS has been investigated by the World Health Organization (WHO) and European Food Safety Authority (EFSA) in respect to its use as a food flavouring agent. An evaluation of absorption, metabolism and elimination of DMDS was described in WHO Food Additive Series No. 59 (Williams GM & Bend J, 2000). DMDS was assigned to the group of substances ‘simple disulphides’, which are of low relative molecular mass and are sufficiently lipophilic to be absorbed from the intestine. As metabolism would usually result in increased polarity and a greater likelihood of excretion, these substances would not be expected to accumulate in the body. Disulphides would be able to form disulphide bonds with endogenous thiols. Disulphides formed with cysteine could be excreted in the urine as cysteine disulphide, whereas formation of disulphides with endogenous macromolecules would be eliminated more slowly.
The reduction of simple disulphides is believed to be extensive, and the reaction may be catalysed enzymatically by thioltransferases and chemically by exchange with glutathione, thioredoxin, cysteine and other endogenous thiols. Reduction of non-cyclic disulphides, such as dimethyl disulphide, would result in the formation of thiols of low relative molecular mass, which would then be metabolised by the various pathways described below for simple thiols.
Simple thiols can be metabolized via several pathways. Simple aliphatic thiols undergo S-methylation in mammals to produce the corresponding methyl thioether or sulphide. S-Methylation is catalysed by thiopurine-S-methyltransferase in the cytosol and thiol-S-methyltransferase in microsomes; both reactions require S-adenosyl-L-methionine as a methyl group donor. Thiopurine-S-methyltransferase is present in human liver, kidney and erythrocytes, and its preferred substrates include aromatic and heterocyclic thiols. S-Methylation of aliphatic thiols is catalysed by microsomal thiol-S-methyltransferase, and the resulting methyl thioether (sulphide) metabolite undergoes S-oxidation to give the corresponding methyl sulphoxide and methyl sulphone analogues, which are excreted in the urine.
Thiols may react with glutathione and other endogenous thiol substances to form mixed disulphides. Both microsomal and cytosolic thioltransferases have been reported to catalyse the formation of mixed disulphides. The resulting mixed disulphides can undergo reduction back to thiols, oxidative desulphuration or oxidation to the corresponding sulphonic acid via the intermediate thiosulphinate and sulphinic acid. The principal form in the circulation would probably be a mixed disulphide formed with albumin.
Thiols may be oxidised to form sulphenic acids (RSOH), which are unstable and readily undergo further oxidation to sulphinic (RSO2H) and sulphonic (RSO3H) acids or combine with nucleophiles. The sulphonic acid group is highly polar and renders molecules very soluble in water. In general, sulphonic acids are not extensively metabolised.
Alkyl thiols of low relative molecular mass undergo oxidative desulphuration in vivo to yield carbon dioxide and sulphate. This reaction has been shown to occur, for example, with methyl mercaptan. Whereas the carbon atoms from thiols may be used in the biosynthesis of amino acids, the sulphur atoms are not used significantly in the synthesis of sulphur-containing amino acids.
An in vitro metabolism study in rat hepatocytes has been performed to identify the metabolites that were generated following incubation of DMDS with fresh rat hepatocytes (Kilford, 2012). The study demonstrated rapid metabolism with the formation of methane thiol (also known as methyl mercaptan) and dimethyl sulphide at lower levels. This correlates with the reduced sulphur-sulphur bond cleavage and theS-methylation steps presented in the metabolism pathway shown above and confirms the role of the liver in the metabolism of DMDS.
A possible metabolism pathway for dimethyl disulphide is shown in the following Figure: see attachement


In mice that were injected intraperitoneally with 35 to 40 mg DMDS/kg (Susman et al, 1978), three volatile sulfur compounds were detected in breath samples: DMDS itself (parent compound), dimethyl sulphide (metabolite) and methyl mercaptan (metabolite). The excretion of the parent compound, DMDS, reached a peak between 3 and 6 minutes following injection, and the total amount excreted was approximately 6% of the administered dose. The amounts of the two metabolites accounted for approximately 0.5% each of the administered dose. The study concluded that pulmonary excretion appeared to be quantitatively insignificant as a means for the elimination of sulphide by mice.
The importance of the role of the liver in the metabolism of DMDS is further supported by increased concentrations (120 ppb) of DMDS in mouth air in hepatopathic subjects such as those with liver cirrhosis (Hiroshi et al., 1978; Kaji et al., 1978) in comparison to normal subjects (up to 6 ppb) (Tonzetich, 1971).
DMDS occurs naturally in a large number of food types and may also be ingested through its use as a food flavouring agent. In WHO Food Additive Series 59, consideration was made of combined intakes from use of flavouring agents and natural occurrence. It was concluded that under the current conditions of use as flavouring agents, the combined intakes of these substances would not saturate the metabolic pathways and combined intakes would not raise safety concerns. The estimated current intake in Europe from use as food flavouring agent is 11 µg/person/day (WHO TRS 896) whilst the threshold level of concern for structural class I is 1800 µg/person/day. Safe use of DMDS as a flavouring agent has been accepted by EFSA (EFSA Journal 2011; 9(12):2459) in the absence of DMDS-specific animal metabolism data.


EXCRETION


Due to the high vapour pressure, an excretion in exhaled air is expected. Also an excretion in the urines because DMDS has a low molecular weight.