Sulforaphane

Medicine Atlas · Food & Nutraceuticals · Isothiocyanates

Source: Broccoli, Brassica vegetables | Active form: Sulforaphane (SFN) | Precursor: Glucoraphanin | Category: Isothiocyanate / Nrf2 inducer

Isothiocyanate generated from glucoraphanin by myrosinase upon tissue disruption. SFN covalently alkylates KEAP1 cysteines (C151, C273, C288), stabilising Nrf2 to drive ARE-mediated transcription of phase II enzymes (NQO1, HO-1, GSTs) and glutathione biosynthesis genes. Also a class I/II HDAC inhibitor; suppresses NF-κB via HO-1-derived CO; directly inhibits H. pylori. The Qidong trial (n=291, Egner 2014) demonstrated urinary reduction of aflatoxin B1-DNA adducts and mercapturic acid carcinogen metabolites — the strongest clinical evidence for dietary Phase II enzyme induction.

SFN 1-isothiocyanato-4-(methylsulfinyl)butane 4-methylsulfinylbutyl isothiocyanate glucoraphanin (precursor) broccoli isothiocyanate C₆H₁₁NOS₂

Overview

Sulforaphane (SFN; MW 177.3 Da; formula C₆H₁₁NOS₂) is an isothiocyanate — an organosulfur compound bearing the −N=C=S functional group — generated enzymatically in cruciferous vegetables. Unlike most dietary bioactives that exist preformed in plant tissue, sulforaphane is a prodrug that does not exist in intact plant cells. It is produced only when plant tissue is disrupted, triggering a two-component enzymatic reaction: the stable vacuolar precursor glucoraphanin (4-methylsulfinylbutyl glucosinolate) mixes with myrosinase (β-thioglucosidase; EC 3.2.1.147), which hydrolyses the glucose–sulfur bond to yield an unstable aglycone that rearranges to sulforaphane. The epithiospecifier protein (ESP) can divert this reaction to the less bioactive sulforaphane nitrile, so plant genetics, cooking method, and preparation all govern final SFN yield.

The richest dietary source is 3-day-old broccoli sprouts, which contain 20–50× more glucoraphanin per gram than mature broccoli heads (~100–400 mg sulforaphane potential per 28 g). Mature broccoli provides 10–100 mg glucoraphanin per 100 g fresh weight. Myrosinase is inactivated above 60°C, so boiling or microwaving destroys the enzyme — glucoraphanin survives intact but is then converted only by gut bacteria at 3–10% of the efficiency of plant myrosinase. Brief steaming al dente, thorough chewing, or adding raw mustard powder to cooked broccoli restores SFN yield by providing exogenous myrosinase activity.

Pharmacokinetically, approximately 80% of intact SFN is absorbed in the small intestine; plasma Cmax reaches 1–3 µM after a broccoli sprout-based dose; t½ ~2–3 h. Metabolism proceeds through the mercapturic acid pathway: SFN conjugates with glutathione via glutathione-S-transferase → GS-SFN → Cys-SFN → N-acetyl-Cys-SFN, excreted in urine. This urinary metabolite serves as a validated biomarker of exposure in epidemiological studies. Notably, ~50% of Caucasians carry a null GSTM1 genotype, metabolising SFN more slowly and attaining higher, more prolonged plasma SFN concentrations — potentially yielding greater Nrf2 activation.

Mechanism of Action

KEAP1/Nrf2 Pathway — Central Mechanism

  Glucoraphanin (vacuole)
       |
       |  Myrosinase (tissue disruption: chewing / cutting)
       v
  Sulforaphane (SFN)  ---N=C=S electrophilic isothiocyanate
       |
       |  Covalent alkylation: KEAP1 Cys151, Cys273, Cys288
       v
  KEAP1 conformational change
       |
       |  Impaired CUL3-dependent Nrf2 ubiquitination
       v
  Nrf2 stabilises (t1/2: <20 min -> >60 min) -> nuclear translocation
       |
       |  Heterodimerises with small Maf; binds ARE (5'-TGACnnnGCA-3')
       v
  Nrf2 target gene induction:
       |-- NQO1      (quinone reductase; CD value 0.2 uM for SFN)
       |-- HMOX1/HO-1 (heme oxygenase -> CO + bilirubin; anti-inflammatory)
       |-- GSTM1/GSTP1 (phase II conjugation: electrophile + GSH -> excretable)
       |-- GCLC/GCLM (rate-limiting GSH synthesis)
       |-- SLC7A11/xCT (cystine import -> GSH precursor)
       |-- TXNRD1, PRDX1/6 (thioredoxin/peroxiredoxin H2O2 clearance)
       `-- FTH1/FTL  (ferritin -> iron sequestration -> less Fenton OH*)

Fahey and Talalay (1999) quantified sulforaphane's potency by the CD value — the concentration doubling NQO1 activity in Hepa1c1c7 murine hepatoma cells — at just 0.2 µM, approximately 10× more potent than any other component of broccoli extracts at Nrf2-target induction.

KEAP1 cysteine alkylation: SFN's −N=C=S group forms covalent carbamate/dithiocarbamate adducts with sensor cysteines C151, C273, C288 of KEAP1 (analogous to Michael addition chemistry).
Nrf2 escapes proteasomal degradation: Alkylated KEAP1 cannot ubiquitinate Nrf2 via CUL3/RBX1 complex; Nrf2 protein half-life rises from <20 min to >60 min.
Nuclear translocation and ARE binding: Stabilised Nrf2 heterodimerises with small Maf proteins and binds ARE sequences (5’-TGACnnnGCA-3’) in promoters of target genes.
Phase II enzyme transcription: NQO1, HO-1, GSTM1/GSTP1, GCL subunits, xCT cystine transporter, peroxiredoxins, ferritin chains — a coordinated antioxidant and detoxification programme.
Glutathione surge: Combined upregulation of GCL (rate-limiting GSH synthesis) and xCT (cystine import) substantially increases intracellular GSH — SFN is among the most potent physiological inducers of intracellular glutathione.

Pleiotropic Mechanisms

HDAC Inhibition

SFN inhibits class I/II HDACs (HDAC1, 2, 3, 6, 8; IC₅₀ in the µM range) → maintained histone acetylation → ↑tumour suppressor expression (p21/CDKN1A, p27) → G2/M arrest. Most relevant at gut lumen concentrations and high supplemental doses.

NF-κB Suppression

HO-1–derived CO inhibits IKK → ↓IκBα phosphorylation → ↓NF-κB nuclear translocation. GSH elevation reduces ROS-dependent IKK activation. Net: ↓TNF-α, ↓IL-6, ↓IL-1β, ↓COX-2 in macrophages and hepatocytes.

H. pylori Inhibition

Direct antimicrobial activity (MIC ~4–12 µg/mL) via isothiocyanate reactivity with bacterial proteins. Fahey 2002 demonstrated H. pylori eradication in mouse gastric mucosa; clinical pilot data: ↓H. pylori density, ↓IL-8 in gastric biopsies after broccoli sprout consumption.

Apoptosis / Anti-cancer

↑Bax/↓Bcl-2 → cytochrome c release → caspase-9/3; ↑p21 (HDAC inhibition) → G2/M arrest; mTOR suppression → autophagy. Anti-androgen receptor effects in prostate cancer cells via HDAC inhibition-dependent mechanism.

Dietary Sources & RDA

Sulforaphane has no established RDA; figures below reflect glucoraphanin content and estimated SFN yield under optimal myrosinase conditions.

Source	Serving	Glucoraphanin / SFN Potential	Notes
Broccoli sprouts (3-day)	28 g (1 oz)	100–400 mg SFN potential	Richest source; 20–50× mature broccoli; myrosinase intact
Mature broccoli (raw)	100 g	10–100 mg glucoraphanin	Full conversion if raw; thorough chewing improves yield
Mature broccoli (boiled)	100 g	~2–10 mg SFN equivalent	Myrosinase inactivated; only gut bacterial conversion
Brussels sprouts (raw)	100 g	High glucoraphanin	Also contains sinigrin; myrosinase intact if raw
Kale, cabbage, cauliflower	100 g	Lower but significant	Variable glucosinolate profiles; not all yield SFN specifically
Stabilised SFN supplement	Per label	~50–400 µmol/serving (variable)	Prefer verified glucoraphanin + active myrosinase co-formulations

Preparation tip: Add raw mustard seed powder (~1 tsp) to cooked or microwaved broccoli — provides exogenous myrosinase to recover SFN production from intact surviving glucoraphanin.

Clinical Evidence

Trial	Design	Intervention	Key Result	GRADE
Qidong AFB1 / Air Pollution (Egner et al., Cancer Prev Res 2014)	RCT; n=291; 12 wk; Jiangsu Province, China	Broccoli sprout beverage (glucoraphanin + myrosinase → SFN in vivo)	Urinary benzene mercapturic acid ↑61%; acrolein mercapturic acid ↑23% vs. placebo. AFB1-DNA adduct biomarkers significantly reduced. Direct evidence of enhanced Phase II carcinogen detoxification.	Moderate
ASD Phase II Trial (Singh et al., PNAS 2014)	RCT, DB-PC; n=40; 44 wk	SFN 50–150 µmol/day (broccoli sprout extract)	ABC social withdrawal (p<0.001) and SRS (p=0.017) improved vs. placebo at 18 wks. Changes largely reversed post-discontinuation. Not replicated in larger Curtin 2022 trial (n=54, JAMA Netw Open — primary endpoint not met).	Low
NAFLD RCTs (multiple groups; 2016–2023)	Small RCTs, n=30–80; 12–24 wk	Broccoli sprout extracts ~150–400 mg SFN equivalent/day	Consistent directional effect: ↓ALT, ↓AST, ↓γ-GT; improved liver stiffness (Fibroscan) and ultrasound steatosis grading. Small samples, short follow-up.	Low
H. pylori Pilot (Fahey et al., 2002)	Pilot clinical + murine	Broccoli sprout 70 g/day	Eradication of H. pylori in mouse gastric mucosa; clinical pilot: ↓H. pylori density and ↓IL-8 in gastric biopsies.	Low

Evidence principle: The strongest clinical evidence is for carcinogen detoxification (Qidong, GRADE Moderate) using objective urinary biomarkers that directly confirm mechanistic predictions. Cancer chemoprevention and NAFLD endpoints remain low-evidence due to small samples and short follow-up. The ASD signal requires adequately powered replication before clinical application. Sulforaphane is not a drug substitute for any condition.

Deficiency & Excess

Sulforaphane is not an essential nutrient — no recognised deficiency syndrome exists. The table covers suboptimal intake and high-dose supplemental risks.

Status	Signs & Features	Marker	Threshold / Comment
Low dietary intake	No clinical deficiency; theoretical ↓Phase II enzyme capacity, ↓GSH pool, ↑oxidative burden in high-exposure individuals	Urinary N-acetyl-Cys-SFN (research only)	No established threshold; typical Western diet provides minimal SFN
Adequate / optimal intake	Upregulated Nrf2 target gene expression; ↑intracellular GSH; ↑Phase II enzyme activity	Urinary SFN metabolites; NQO1 induction (research)	Sprout-based doses achieving plasma Cmax ~1–3 µM
High-dose supplementation (>400 µmol/day)	GI discomfort, nausea at very high doses; no major safety signals in Phase I/II trials to date	Clinical monitoring	CYP1A2 induction possible at prolonged high doses — may reduce efficacy of clozapine, theophylline, duloxetine
Iodine-deficient individuals (high dose)	Theoretical goitrogenic effect: isothiocyanate competes with iodine uptake in thyroid	TSH, free T4	Not clinically significant at normal dietary doses; only relevant with concurrent iodine deficiency

Connections

Modulates Liver Modulates Immune System Targets NF-κB Acts in Hepatocyte

References

Fahey JW, Talalay P. Antioxidant functions of sulforaphane: a potent inducer of Phase II detoxication enzymes. Food Chem Toxicol. 1999;37(9-10):973-9. doi:10.1016/s0278-6915(99)00082-4 · PubMed 10541453
Kensler TW, Egner PA, Agyeman AS, et al. Keap1-Nrf2 signaling: a target for cancer prevention by sulforaphane. Top Curr Chem. 2013;329:163-77. doi:10.1007/128_2012_339 · PubMed 22752583
Egner PA, Chen JG, Zarth AT, et al. Rapid and sustainable detoxication of airborne pollutants by broccoli sprout beverage: results of a randomized clinical trial in China. Cancer Prev Res. 2014;7(8):813-23. doi:10.1158/1940-6207.CAPR-14-0103
Singh K, Connors SL, Macklin EA, et al. Sulforaphane treatment of autism spectrum disorder (ASD). Proc Natl Acad Sci USA. 2014;111(43):15550-5. doi:10.1073/pnas.1416940111
Berg JM, Tymoczko JL, Stryer L. Biochemistry. 9th ed. W.H. Freeman; 2019. Chapter on oxidative stress signalling and Nrf2 pathway.