A Whiff of Sulfur: One Wind a Day Keeps the Doctor Away

Abstract

Reactive Sulfur Species (RSS), such as allicin from garlic or sulforaphane from broccoli, are fre-quently associated with biological activities and possible health benefits in animals and humans. Among these Organic Sulfur Compounds (OSCs) found in many plants and fungi, the Volatile Sulfur Compounds (VSCs) feature prominently, not only because of their often-pungent smell, but also because they are able to access places which solids and solutions cannot reach that easily. Indeed, inorganic RSS such as hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) can be used to lit-erally fumigate entire rooms and areas. Similarly, metabolites of garlic, such as allyl methyl sulfide (AMS), are formed metabolically in humans in lower concentrations and reach the airways from inside the body as part of one's breath. Curiously, H₂S is also formed in the gastrointestinal tract by gut bacteria, and the question of if and for which purpose this gas then crosses the barriers and enters the body is indeed a delicate matter for equally delicate studies. In any case, nature is surprisingly rich in such VSCs, as fruits (for instance, the infamous durian) demonstrate, and therefore these VSCs represent a promising group of compounds for further studies.

Keywords: Reactive Sulfur Species (RSS); allicin; allyl methyl sulfide (AMS); broccoli; cellular thiolstat; durian; garlic; sulforaphane.

Figure 1

In mammals, H₂S is produced via several pathways. Cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE) interact with cysteine to produce H₂S following the reverse transsulfuration pathway and α, β-elimination, respectively [39,40]. The production of H₂S also involves enzymes such as methionine-γ-lyase and cysteine desulfhydrase [24]. The scheme is superimposed on a photo of the Elisenbrunnen in the German city of Aachen, a popular source of sulfur-rich water drunk frequently by the local population (Photo credit to Muhammad Jawad Nasim) [41].

Figure 2

In humans, SO₂ is produced from L-cysteine and metabolized to SO₃²⁻ and SO₄²⁻, which are excreted via the urine. A brief list of biological activities is provided and more details can be found in the text [62,63,64].

Figure 3

Allyl and propyl mono- and polysulfides found in garlic and onion may serve as multifunctional agents able to interact with thiol residues of proteins and enzymes, interfere with metal homeostasis, bind to metals, and induce oxidative stress via oxidative interactions.

Figure 4

Generic pathway for the conversion of allyl thiosulfinates and polysulfanes to AMS. Allyl polysulfides are metabolized mainly to allyl methyl sulfide with allyl mercaptan as an intermediate, see text for details.

Figure 5

A brief overview of volatile sulfur species found in natural products such as mustard, cruciferous vegetables, coffee, and shiitake mushroom (a), and in durian fruit (b).