2-methylpropane-2-thiol

Endpoint summary

Additional information

There are no in vivo data on the toxicokinetics of 2-methylpropane-2-thiol.

The following summary has therefore been prepared based on the predicted and measured physicochemical properties of the registered substance. The data have been used in algorithms which are the basis of many physiologically based pharmacokinetic and toxicokinetic (PBTK) prediction models. Although these algorithms provide quantitative outputs, for the purposes of this summary only qualitative statements or predictions will be made because of the remaining uncertainties that are characteristic of prediction models.

The main input variable for the majority of the algorithms is the log Kow. By using this and, where appropriate, other known or predicted physicochemical properties of 2-methylpropane-2-thiol, reasonable predictions or statements may be made about its potential absorption, distribution, metabolism and excretion (ADME) properties.

Absorption

Oral

When oral exposure takes place, it can be assumed that, except for the most extreme of insoluble substances, uptake through intestinal walls into the blood takes place. Uptake from intestines can be assumed to be possible for all substances that have appreciable solubility in water or lipid. Other mechanisms by which substances can be absorbed in the gastrointestinal tract include the passage of small water-soluble molecules (molecular weight up to around 200) through aqueous pores or carriage of such molecules across membranes with the bulk passage of water (Renwick, 1993).

As 2-methylpropane-2-thiol is soluble (water solubility 1480 mg/l at 20°C) and has a molecular weight of 90.19 g/mol, it meets both of these criteria and therefore, should oral exposure occur, it is reasonable to assume systemic exposure will occur also. 

High intestinal absorption rates (93%) are also predicted using the pkCSM method (Pires et al, 2015).

No systemic effects were reported in the key acute oral toxicity study at dose as high as 4729 mg/kg bw (Fairchild, 1958) or in the sub-acute oral repeated dose toxicity study at 200 mg/kg bw/day (MHLW, 2006).

Dermal

The fat solubility and therefore potential dermal penetration of a substance can be estimated by using the water solubility and log K_ow values. Substances with log K_ow values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal) particularly if water solubility is high.2-Methylpropane-2-thiol is soluble and has a predicted log K_ow of 2.24. Absorption of 2-methylpropane-2-thiol across the skin is considered to be favourable, so some systemic exposure via this route is likely. However, 2-methylpropane-2-thiol is very volatile and losses from the skin via evaporation are also likely.

Dermal absorption rates of 2-methylpropane-2-thiol have been estimated with the IH SkinPerm v2.04 model (AIHA, 2018). In this model, 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. The fraction absorbed is calculated to be very low.

	Fraction absorbed (%) after instantaneous deposition (1000 mg)	Fraction absorbed (%) with deposition over 8 h (1 mg/cm^2/h)
2-methylpropane-2-thiol	0.4	0.8

 

No systemic effects were reported in the key acute dermal toxicity study at dose as high as 2000 mg/kg bw (Latven, 1976).

Inhalation

There is a QSPR to estimate the blood:air partition coefficient for human subjects as published by Meulenberg and Vijverberg (2000). The resulting algorithm uses the dimensionless Henry coefficient and the octanol:air partition coefficient (K_oct:air) as independent variables.

2-Methylpropane-2-thiol is soluble in water (water solubility 1470 mg/l at 20°C) and also volatile (vapour pressure 19000 Pa at 20°C). This results in a moderate blood:air partition coefficient (around 0.8:1).Therefore, uptake into the systemic circulation from the lungs is expected. The moderate partition coefficient and low molecular weight of 2-methylpropane-2-thiol are also favourable for absorption directly across the respiratory tract epithelium by passive diffusion.

No systemic effects were reported in the sub-chronic repeated dose toxicity study by the inhalation route at the highest dose tested (Ulrich, 1984).

Distribution

For blood:tissue partitioning a QSPR algorithm has been developed by DeJongh et al. (1997) in which the distribution of compounds between blood and human body tissues as a function of water and lipid content of tissues and the n-octanol:water partition coefficient (K_ow) is described. Using this algorithm for 2-methylpropane-2-thiol predicts that it will distribute approximately equal into liver, muscle, brain and kidney and to a higher degree to fat.

Table 1: Tissue:blood partition coefficients

	Log Kow	Kow	Liver	Muscle	Fat	Brain	Kidney
2-methylpropane-2-thiol	2.24	174	3.2	2.2	67	2.2	1.7

 

Distribution parameters were also predicted using the pkCSM method (Pires et al, 2015). The results are shown in the Table below.

2-methylpropane-2-thiol	Result	Comment
Volume of distribution steady state (VDss) (Human)	0.066 (log L/kg)	The steady state volume of distribution (VDss) is the theoretical volume that the total dose would need to be uniformly distributed to give the same concentration as in blood plasma. Low = log VDss <-0.15 High = log VDss >0.45
Fraction unbound (human)	0.603	Fraction bound to serum proteins.
Blood-brain barrier (BBB) permeability	0.601 (log BB)	Measures the ability of the substance to cross the blood-brain barrier. LogBB >0.3 indicates a substance that readily crosses the blood-brain barrier; logBB <-1 indicates a substance that is poorly distributed to the brain.
Central nervous system (CNS) permeability	-1.946 (log PS)	The blood-brain permeability-surface area product (logPS) is a more direct measurement of blood brain permeability. logPS > -2 indicates substances that penetrate the Central Nervous System (CNS); log PS<-3 indicates substances unable to penetrate the CNS.

 

Metabolism

There is little information in the open literature on the mammalian metabolism of 2-methyl-2-propanethiol (tertiary butyl mercaptan, TBM) and what information can be inferred has been deduced from information gleaned from studies using other low molecular weight thiols, such as allyl and 1-propane thiols. Although there is much data on the behaviour on how thiols react and are metabolised in biological systems, when reading across this information to infer the likely degradative metabolic route of TBM, it must be realised that this compound is a tertiary thiol, and has the potential to undergo elimination reactions, which would be unlikely for primary thiols. Additionally, the steric effect of the bulky tertiary butyl group may hinder reactivity in some metabolic enzyme-catalysed reactions, however, much of the metabolic chemistry of TBM, particularly thiol-disulfide exchange reactions can be shown to be common to other low molecular weight thiols. Experimental evidence that TBM can participate in this type of reaction is shown by a study on the decomposition of TBM (Karthikeyan et al. 2012) in water that contained microorganisms in which the di-tertiary-butyl-disulfide, its sulfoxide and sulfone were detected:

2(^tBuSH)⇌^tBu-S-S-^tBu+ O₂→^tBu-S(O)-S(O)-^tBu + O₂→^tBu-S(O₂)-S(O₂)-^tBu            

 

Oxidative and Thiol Exchange Metabolism in Mammals

 

Thiol-disulfide exchange reactions are common in vivo and result from nucleophilic substitution by sulfur compounds. Thiol-disulfide exchange reactions with endogenous cellular thiols (reduced glutathione, GuSH) or disulfides (oxidised glutathione, GuSSGu) will produce mixed disulfides that may also undergo reduction. Also, thiol groups on proteins (surface cysteine residues) or other nucleophilic groups may be involved and in many cases such thiolate substitution will affect the biological function of the protein. In blood plasma, the main protein-containing thiol is Cys34 of serum albumin, which constitutes ~80% of the free thiols in blood.

 

Further oxidative metabolism of any free thiols produced will generate sulfinates, sulfones and eventually polar sulfates, which are generally excreted. Oxidation of thiols is catalysed by cytochrome P450 and flavin mono-oxygenases, in the liver. Thiols may also be methylated via S-adenosylmethionine (SAM)-dependent thiol methylation to yield a methylthio-ether, which is usually oxidised to the sulfoxide (major) and sulfone (minor) polar metabolites, which are then excreted.

 

The above general metabolites are those typically seen in the metabolism of diallyldisulfide (DAD) by rat and human hepatocytes or perfused liver (Germain et al., 2003) anddi-1-propyl disulphide in rat and man (Germain et al., 2008) although in the more reactive diallydisulfide, oxidised, methylated and glutathione exchange metabolites were seen:

 

• DAD→H₂C=CH-CH₂-S(=O)-S-CH₂-CH=CH₂

• Diallylthiosulfinate + GuSH→GuS-S-CH₂-CH=CH₂(non-enzymatic)

• DAD + SAM→CH₃-S-CH₂-CH=CH₂→CH₃-S(=O)-CH₂-CH=CH₂→ CH₃-S(=O)₂-CH₂-CH=CH₂

• DAD + GuSH→Gu-S-CH₂-CH=CH₂

• DAD→H₂C=CH-CH₂-SH

 

The rate of metabolism of DAD by liver was very rapid, with a t½of approximately 6 min. It can thus be deduced that disulfides are rapidly metabolised in mammalian systems by a combination of thiol-disulfide exchange reactions and oxidation steps.

 

In the case of the less reactive di-1-propyldisulfide the following sequence of liver metabolites were detected in rats(Germain et al., 2008):

ⁿPr-S-S-ⁿPr →ⁿPr-SH + SAM →ⁿPr-S-CH₃+ O₂→ⁿPr-S(O)-CH₃+ O₂→ⁿPr-S(O₂)-CH₃

 

The rate of metabolism of di-1-propyldisulfide in rat liver was much slower (t½ =8.25 hr) than that of the allyl homologue, DAD, and it would be expected that metabolism of the more sterically hindered TBM would be slower still.  

 

In general, such highly oxidised polar metabolites as the oxidised disulfides and thioethers are excreted in the urine, although no direct confirmatory data could be found.

 

Further Oxidative Metabolism and Conjugation

 

Thiols can be oxidized to form ultimately sulfonic (R-SO₃H) acids via the following sequence: R-SH + O₂→ R-SOH (unstable) + O₂→ R-SO₂H + O₂→ R-SO₃H. The sulfonic acid group is highly polar and makes molecules very soluble in water. Sulfonic acids are stable to metabolism and are generally excreted in the urine.

 

The S-glucuronidation of aromatic thiols has been reported, and this may be a pathway for the degradative metabolism and ultimate excretion of thiophenols (Jancova et al., 2010) and consequently of simple aromatic thiols, as glucuronyl transferases behave similarly towards hydroxyl and sulfydryl groups, and the two activities have the same subcellular location and optimal pH (Illing and Dutton, 1973). Since the primary metabolite of tertiary butanol (^tBu-OH) is its glucuronide conjugate, which is excreted in the urine in rabbits (Kamil et al., 1953) it could be inferred that a likely metabolite of TBM is also its (S-) glucuronide. 

 

Summary

 

From information on the metabolism of other short chain aliphatic thiols, it is proposed that TBM is oxidatively metabolised in mammals via the following pathways to polar metabolites that are excreted (Fig. 1):

 

1.     Formation of a glutathione conjugate (metabolite 1), either directly from TBM or via its oxidised disulphide

2.     Formation of a S-glucuronyl conjugate (metabolite 2)

3.     Methylation of TBM by S-adenosyl methionine followed by oxidation of the resultant thioether to the sulfone (metabolite 3)

4.     Direct oxidation of TBM to the sulfonic acid (metabolite 4)

 

Excretion

A determinant of the extent of urinary excretion is the soluble fraction in blood. QPSRs as developed by DeJongh et al. (1997) using log K_ow as an input parameter, calculate the solubility in blood based on lipid fractions in the blood assuming that human blood contains 0.7% lipids.

Using this algorithm, the soluble fraction of 2-methylpropane-2-thiol in blood is approximately 45% suggesting it is likely to be eliminated via the kidneys in urine. In addition, metabolism to more polar metabolites that are more effectively eliminated in urine is expected.

 

References

AIHA (2018), IH SkinPerm (v2.04), accessed at https://www.aiha.org/get-involved/VolunteerGroups/Pages/Exposure-Assessment-Strategies-Committee.aspx.

DeJongh, J., H.J. Verhaar, and J.L. Hermens, A quantitative property-property relationship (QPPR) approach to estimate in vitro tissue-blood partition coefficients of organic chemicals in rats and humans. Arch Toxicol, 1997. 72(1): p. 17-25.

Germain, E., Chevalie, J., Siess, M. H., Teyssier, C. (2003) Hepatic metabolism of diallyl disulphide in rat and man Xenobiotica., 33:1185-1199.

Germain, E., Semon, E., Siess, M.H. and Teyssier, C., (2008). Disposition and metabolism of dipropyl disulphide in vivo in rat. Xenobiotica, 38: 87-97.

Illing, H.P.A., Dutton, G.J., (1973_. Some properties of the uridine diphosphate glucuronyltransferase activity synthesizing thio-β-d-glucuronides. Biochem. J 3: 139-147.

Jancova, P., Anzenbacher, P. and Anzenbacherova, E., (2010). Phase II drug metabolizing enzymes. Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub, 154: 103-116.

Kamil, I.A., Smith, J.N., Williams, R.T., (1953). Studies in detoxication. 46. The metabolism of aliphatic alcohols. The glucuronic acid conjugation of acyclic aliphatic alcohols. Biochem. J., 53: 129-136.

Karthikeyan, R., Hutchinson, S.L.L. and Erickson, L.E., (2012) Biodegradation of tertiary butyl mercaptan in water. J. Bioremed. Biodegrad. 3(6)

Meulenberg, C.J. and H.P. Vijverberg, Empirical relations predicting human and rat tissue:air partition coefficients of volatile organic compounds. Toxicol Appl Pharmacol, 2000. 165(3): p. 206-16.

Pires D.E.V, Blundell T.L. and Ascher D.B (2015). pkCSM: predicting small-molecule pharmacokinetic properties using graph-based signatures, Journal of Medicinal Chemistry, 58 (0), p. 4066-4072.

Renwick A. G. (1993) Data-derived safety factors for the evaluation of food additives and environmental contaminants. Fd. Addit. Contam. 10: 275-305.