# Isothiocyanates: An Overview of Their Antimicrobial Activity against Human Infections
## 2 Isothiocyanates in Nature  
Due to their markedly electrophilic chemical function, ITCs usually show a poor stability in most biological media: e.g., sulforaphane (4-methylsulfonylbutyl ITC) undergoes degradation in many aqueous solvent mixtures [18]. In Nature, ITCs mostly appear in a masked form known as glucosinolates (GLs). Acting generally as bio-precursors of ITCs, GLs are important thiosaccharide metabolites which occur in all 17 plant families of the order Brassicales, particularly in the predominant family, the Brassicaceae, but also in Capparaceae, Moringaceae, Resedaceae and Tropaeolecae species [19]. All known GLs--ca. 130 characterized molecules according to [PERSON] [20]--display a remarkable structural homogeneity invariably based on a \(\beta\)-d-glucopyranosyl unit and an O-sulfatedomeric (\(Z\))-thiohydroxinate function connected to a side chain which constitution, depending on plant species, is the sole structural variant. In Nature, GLs usually co-exist with the enzyme myrosinase (EC 3.2.3.147), the only known glucohydrolase able to break the anomeric C-S bond of GLs [21]. This coexistence, commonly known as the \"glucosinolate-myrosinase\" system, has been broadly investigated [22; 23]. The myrosinase-catalyzed hydrolysis of GLs releases D-glucose and produces aglycones, which can, by expulsion of a sulfate ion, undergo a fast Lossen-type rearrangement to finally deliver the corresponding ITCs (Scheme 1):  
GLs can be divided into several classes according to the chemical structure of their aglycon: aliphatic GLs, like gluccoapparin (methyl GL) or sinigrin (2-propenyl GL), thiofunctionalized GLs, like gluccoerule (4-methylsulfonylbutyl GL) or glucoraphanin (4-methylsulfonylbutyl GL), indole-type GLs like glucobrassicin (3-indolylmethyl GL, arylaliphatic GLs, including glucotropaeolin (benzyl GL), gluconasturitin (2-phenylethyll GL) and glucomoringin (4-(\(\alpha\)-L-rhanno-pyranosyloxy)benzyl GL) [19; 20]. A similar classification can be given for the corresponding ITCs: aliphatic, like methyl or allyl ITC (methyl or allyl mustard oils), thiofunctionalized, like erucin (4-methylsulfonylbutyl ITC) or sulforaphane (4-methylsulfonylbutyl ITC), arylaliphatic like benzyl and 2-phenylethyll ITC. Last but not least, the more structurally complex compound morgini  
Figure 1: Myrosinase-assisted hydrolytic cleavage of glucosinolates to produce isothi cyanates.  
(4-(\(\alpha\)-L-rhamnopyranosyloxy)benzylITC), obtained from glucomoringin should also be placed into the arylaliphatic class (Table 1).  
The beneficial impact of GL-containing vegetables on human health has received much attention and ITCs produced by enzymatic hydrolysis of GLs, which are present in edible parts, are regarded as most likely to maintain human health through their regular consumption in diets [24]. A recent study has provided evidence that enhancing the GRA content in soups made with novel broccoli varieties results in increased levels of SFN in human plasma [25]. Following absorption, SFN is metabolized through the mercapturic acid pathway and excreted in urine, predominantly as an \(N\)-acetylcysteine conjugate [25]. Recently, a great deal of attention has been paid to the Moringaceae family, due to their bioactive ingredients ubiquitously available in every edible part of the plants--leaves, seeds, bark, roots, exudates, flowers, and pods [26]. A number of reports on MGN, the ITC derived from glucomoringin isolated from _Moringa oleifera_, highlight various biological activities such as anticancer [27], anti-inflammatory [28], antioxidant [29] and also antibacterial, antifungal and antiviral activities [30; 31; 32]. Moreover, ITCs produced from an extract of _Cleome chrysantha_ showed good antimicrobial activity against Gram-negative bacteria, moderate activity against Gram-positive bacteria, and a good inhibitory activity against the growth of _Saccharomyces cerevisiae_[33]. _Capparis ovata_ and _Capparis spinosa_ extracts were similarly investigated for their antibacterial activity [34].