Isothiocyanates are a group of organic compounds containing sulfur, carbon, and nitrogen. Found in certain plants, they are responsible for their distinct pungent flavor and aroma. Isothiocyanates are not directly present in plants but form through an enzymatic process. They are studied for their diverse biological activities.
Where Isothiocyanates Come From
Isothiocyanates come from specific plant families, especially cruciferous vegetables. These vegetables, including broccoli, cabbage, cauliflower, kale, Brussels sprouts, and mustard, contain precursor compounds called glucosinolates. Glucosinolates are stable, inactive thioester compounds.
Isothiocyanates form when plant tissue is damaged, such as through chewing, chopping, or some cooking methods. This damage brings glucosinolates into contact with myrosinase, an enzyme typically stored separately within plant cells. Myrosinase hydrolyzes glucosinolates, leading to the formation of isothiocyanates. Different glucosinolates yield different isothiocyanates; for example, glucoraphanin yields sulforaphane, and sinigrin yields allyl isothiocyanate.
How Isothiocyanates Support Cellular Health
Isothiocyanates support cellular health through several biological activities. A primary mechanism involves their role in antioxidant defense, by activating the Nuclear factor erythroid 2-related factor 2 (Nrf2) pathway. Under normal conditions, Nrf2 is sequestered in the cytoplasm by a protein called Keap1. Isothiocyanates can react with sulfhydryl residues on Keap1, causing Nrf2 to be released.
Once released, Nrf2 translocates to the cell nucleus and binds to specific DNA sequences known as the antioxidant response element (ARE). This binding initiates the transcription of genes for cytoprotective proteins and enzymes, such as glutathione S-transferases (GSTs), NAD(P)H quinone oxidoreductase 1 (NQO1), and heme oxygenase-1 (HO-1). These enzymes enhance the cell’s capacity to neutralize reactive oxygen species and detoxify harmful electrophilic agents, reducing oxidative stress and protecting cellular components from damage.
Isothiocyanates also support detoxification pathways by modulating the activity of Phase I and Phase II enzymes. Phase I enzymes, such as cytochrome P450 (CYP450), can metabolize various chemicals, including procarcinogens, sometimes activating them into more reactive forms. Isothiocyanates can downregulate certain Phase I enzymes, inhibiting the activation of potential carcinogens.
Conversely, isothiocyanates are inducers of Phase II detoxification enzymes, including GSTs, UDP-glucuronosyl transferases (UGTs), and NQO1. These enzymes facilitate the conjugation of toxic compounds, making them more water-soluble and easier for the body to excrete. This dual action of inhibiting Phase I and inducing Phase II enzymes enhances the elimination of xenobiotics and their harmful metabolites from the body.
Beyond their roles in antioxidant defense and detoxification, isothiocyanates exhibit anti-inflammatory properties. For example, sulforaphane can downregulate the expression of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and interleukin-6 (IL-6). It also influences signaling pathways like nuclear factor kappa B (NF-κB), which plays a central role in inflammatory responses. By inhibiting NF-κB activation, isothiocyanates can reduce the production of pro-inflammatory mediators, including cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS), reducing inflammation within cells and tissues.
Adding Isothiocyanates to Your Diet
Incorporating more isothiocyanates into your diet involves prioritizing cruciferous vegetables and understanding how preparation methods can influence their availability. Vegetables like broccoli, cabbage, cauliflower, kale, Brussels sprouts, and watercress are rich sources of glucosinolates, the precursors to isothiocyanates. To maximize the formation of these beneficial compounds, it is best to consume these vegetables raw or lightly cooked.
The enzyme myrosinase, necessary for converting glucosinolates into isothiocyanates, is heat-sensitive. Excessive heat, such as prolonged boiling or stewing, can inactivate myrosinase, significantly reducing isothiocyanate yield. Light cooking methods like steaming, stir-frying, or microwaving for short durations are more effective at preserving myrosinase activity and increasing isothiocyanate levels. For instance, stir-frying can increase isothiocyanate yield in broccoli by nearly 11-fold and in cauliflower by eight-fold compared to raw.
Chopping or crushing cruciferous vegetables before cooking also helps release myrosinase and promote isothiocyanate formation. Allowing chopped vegetables to sit before cooking can further enhance this conversion. While cooking can inactivate plant myrosinase, gut microbiota can also hydrolyze glucosinolates into isothiocyanates, though at a slower rate.
Beyond common vegetables, other sources include mustard seeds and wasabi, which can be added to meals to boost isothiocyanate intake. For example, sprinkling mustard powder on cooked vegetables or adding fresh grated horseradish to dishes can enhance the nutritional profile. Diversifying your intake of cruciferous vegetables, such as adding kale to smoothies, roasting Brussels sprouts, or making a quick stir-fry with broccoli and cauliflower, provides varied types of isothiocyanates and ensures a consistent dietary supply.