Myrosinase: The Enzyme That Decides Whether Your Sulforaphane Supplement Works

Myrosinase is the enzyme that cuts glucoraphanin into sulforaphane. Glucoraphanin is a glucosinolate — a stable sulfur compound broccoli stockpiles — and sulforaphane, the isothiocyanate that does the work downstream, only exists once that cut is made.

The plant keeps the two apart. Immunocytochemical imaging of Arabidopsis and Brassica napus tissue located the enzyme exclusively in specialised idioblasts called myrosin cells, structurally distinct from the cells holding the glucosinolate store⁶. That is plant cell biology rather than a human finding, and it is the structural reason chewing, chopping or crushing starts the reaction: the two are in the same plant, not the same place. In vitro, crystal structures resolve the step itself — ascorbate binds the enzyme's active site and substitutes for its catalytic base during hydrolysis⁷.

The enzyme is not unique to broccoli. Every cruciferous vegetable carries a version of it — kale, cabbage, cauliflower, Brussels sprouts, watercress, mustard, wasabi — but broccoli and its sprouts are the most studied, which is why nearly all the human data below is broccoli-derived.

A supplement inherits the problem the plant created: if the tissue was never broken while the enzyme was alive, or the enzyme died on the way into the capsule, nothing has done the cutting yet.

In one line: myrosinase converts inert glucoraphanin into sulforaphane, and in an intact plant it is stored away from the compound it acts on⁶.

Read next: what sulforaphane is

Heat removes myrosinase activity, and it does so well below boiling. In food-chemistry work on the enzyme extracted from broccoli for study — not a human trial — thermal inactivation ran across roughly 30–60 °C, and the authors described the enzyme as thermolabile⁸.

That is why a heat-processed extract can test rich in precursor and still convert badly: the number on the panel is intact, the machinery that acts on it is not. Drying and extraction steps sit between the field and the capsule, and none of that history appears on a label.

A glucoraphanin-only supplement, with no active myrosinase source on the panel, is not a dead end. It is a lottery. Gut bacteria perform some of the conversion, and how much they perform differs enormously between people.

When 23 adults ate the same 200 g of cooked broccoli, delivering roughly 120 µmol of glucosinolates, 24-hour urinary isothiocyanate output ranged from 1.2 to 34.7 µmol — about a thirty-fold spread on one identical dose³. High excreters averaged 23.4 ± 7.5 µmol against 5.6 ± 2.7 µmol for low excreters (p<0.01), and the same split reappeared when the highest and lowest excreters' faecal bacteria were incubated with glucoraphanin outside the body³.

That is the honest version of "your gut will handle it." Some people's bacteria do. Nothing on a label tells you which group you are in: the missing enzyme does not make a product inert, it makes its output unreadable in advance.

Read next: fresh sprouts versus a supplement

Adding an active myrosinase source alongside glucoraphanin changes the measured result, and four human datasets point the same way.

The anchor study compared preparations that differed by enzyme rather than by format: about 9.4–11.5% of a 100 µmol dose absorbed without the active enzyme, about 40–41.8% with it, three- to four-fold more¹.

Food gives the same signal. In a randomised crossover of 12 adults, 1 g of powdered brown mustard stirred into cooked broccoli raised sulforaphane bioavailability up to roughly eight-fold, with urinary metabolites up about four-fold⁴. A four-subject pilot using air-dried broccoli sprout powder as the enzyme donor moved the same way⁹ — n=4, so it corroborates rather than proves.

The most recent trial tested the supplement version of the question. A randomised, double-blind crossover published in 2026 measured 39.8 ± 3.1% bioavailability when a glucoraphanin-rich broccoli seed extract was taken with mustard-seed myrosinase, against 18.6 ± 3.1% for the same extract alone; first-eight-hour conversion was 25.4 ± 2.7% versus 8.0 ± 2.7%⁵. Three qualifiers travel with those numbers wherever they are quoted: the design was single-dose, so it says nothing about weeks of daily use; the sample was 16 people; and the study was funded by the company that makes the glucoraphanin ingredient it tested, with one author employed there and one a paid consultant⁵. The person-to-person ranges were wide in both arms — 5.3–76.9% with the enzyme and 2.2–38.1% without⁵.

An enzyme on the label is not the same as a completed conversion, because the reaction has more than one exit. In recombinant-protein work — plant biochemistry in vitro, not a human study — an epithiospecifier protein isolated from broccoli redirected myrosinase-catalysed hydrolysis of glucoraphanin toward sulforaphane nitrile in place of sulforaphane, and that protein's activity was inversely correlated with how much sulforaphane formed¹⁰.

That is a mechanism, not a measurement of any capsule. No human trial has quantified how much of a finished supplement's glucoraphanin takes the nitrile exit. Treat it as the reason the story is not simply "more enzyme, more sulforaphane" — not as a product advantage, and not as something any panel discloses.

Enzyme-inclusive and glucoraphanin-only formulas differ on one line of the panel, and that line decides which step still has to happen inside you. No trial has run these approaches against each other, so the rows below are read from separate studies rather than one comparison¹¹.

Read next: how to choose a sulforaphane supplement

A supplement label answers the enzyme question in four lines, if you know which four to read.

Is an enzyme source named at all? Front-of-pack wording is not the test. The Supplement Facts panel either carries an ingredient line that supplies the converting enzyme, or it does not.

Does it say active? "Contains myrosinase" and "contains active myrosinase" are different statements, and only the second one describes an enzyme that can still work. Processing adjectives on the front of a pack describe a method nobody outside the factory can check.

Is the enzyme on its own ingredient line? An enzyme source listed separately from the glucoraphanin source is a stated fact with an amount attached. An enzyme implied by the word "sprout" somewhere in a blend is not.

Is there a serving size to hold the numbers against? Milligram figures mean nothing until you know how many capsules deliver them.

Worked example, read off one panel. Sproutly's Supplement Facts panel lists 700 mg broccoli seed extract standardised to 13% glucoraphanin, 200 mg broccoli sprout extract carrying active myrosinase, 100 mg mustard seed extract and 50 mg ascorbic acid, in a two-capsule serving, 30 servings per pouch. The sprout extract is the line the panel assigns the active enzyme to, so the conversion step has a named source rather than an implied one. Mustard seed is listed separately and the panel states no mechanistic role for it; what can be said is that mustard seed was the enzyme donor in the 12-person crossover and in the industry-funded, single-dose 2026 trial of 16^4,5. The panel also claims "Up to 30mg sulforaphane per serving" — the label's own claim, not a bioavailability measurement.

And the limit worth saying out loud: no peer-reviewed analytical survey compares label claims against measured content across commercial products¹⁴. A panel tells you what a formula intends. It does not tell you what a capsule delivered.

A myrosinase claim on a panel is a statement of intent, and four things are missing from it.

Activity is not quantified. Enzyme activity has units, and panels in this category almost never declare them: "active" tells you the ingredient is present, not how much work it can do. No peer-reviewed survey measures what any panel here claims¹⁴.

Nothing has been compared head to head. No trial has run an enzyme-inclusive formula against a pre-converted one¹¹, so any ranking between them is reasoning rather than evidence.

Single-dose trials do not describe weeks. The 2026 crossover measured one dose in 16 people and was industry-funded⁵. Daily use over weeks was not tested, by that trial or any other on this page.

Individual response stays wide even when the label is right. Bioavailability ranged from 5.3% to 76.9% inside the enzyme arm of the single-dose, industry-funded trial of 16⁵, and the thirty-fold spread among people relying on their bacteria was measured separately³.

Myrosinase is the step, not the extra. A formula that supplies the precursor and no enzyme hands the conversion to bacteria that differ enormously between people; a formula naming an active enzyme source on its own ingredient line has done the part it can control. That is the criterion to carry to any shelf: find the enzyme line, check it says active, check it sits apart from the glucoraphanin line, and check the serving size the amounts refer to. Everything past that — activity units, head-to-head comparisons, what happens across weeks — has not been measured, and a label implying otherwise is ahead of the evidence.

1. Fahey JW, et al. 2015. PLoS One. Crossover, n=5 core subjects, extended to n=22 for the supplement comparison. Glucoraphanin-rich preparations lacking active myrosinase delivered about 9.4–11.5% of a 100 µmol dose as absorbed sulforaphane; preparations containing active myrosinase delivered about 40–41.8%, described by the authors as three- to four-fold more bioavailable. PMID 26524341 · https://doi.org/10.1371/journal.pone.0140963
2. Vermeulen M, et al. 2008. J Agric Food Chem. Human feeding study, raw versus cooked broccoli. Sulforaphane bioavailability 37% raw versus 3.4% cooked; peak plasma time 1.6 h versus 6 h. PMID 18950181 · https://doi.org/10.1021/jf801989e
3. Li F, Hullar MAJ, Beresford SAA, Lampe JW. 2011. Br J Nutr. Feeding study plus ex vivo faecal incubation, n=23 (10 selected as high and low excreters). 24-hour urinary isothiocyanate output 1.2–34.7 µmol after 200 g cooked broccoli (about 120 µmol glucosinolates); high versus low excreters 23.4 ± 7.5 versus 5.6 ± 2.7 µmol (p<0.01); ex vivo glucoraphanin degradation 13.3 ± 5.9% versus 5.6 ± 4.8% over 48 h (p=0.05). PMID 21342607 · https://doi.org/10.1017/S0007114511000274
4. Okunade O, et al. 2018. Mol Nutr Food Res. Randomised crossover, n=12 healthy adults. 200 g cooked broccoli plus 1 g powdered brown mustard raised sulforaphane bioavailability up to about eight-fold versus cooked broccoli alone, with urinary SFN-NAC about four-fold higher. PMID 29806738 · https://doi.org/10.1002/mnfr.201700980
5. Mastaloudis A, Holcomb L, Fahey JW, et al. 2026. Sci Rep. Randomised, double-blind, single-dose crossover, n=16 (9 female, 7 male). Bioavailability 39.8 ± 3.1% (range 5.3–76.9%) for a glucoraphanin-rich broccoli seed extract taken with mustard-seed myrosinase versus 18.6 ± 3.1% (range 2.2–38.1%) for the extract alone; first-eight-hour conversion 25.4 ± 2.7% versus 8.0 ± 2.7%. PMID 41692762 · https://doi.org/10.1038/s41598-026-39389-4
6. Andréasson E, et al. 2001. Plant Physiology. Immunocytochemical and confocal localisation in Arabidopsis and Brassica napus tissue. PMID 11743118 · https://doi.org/10.1104/pp.010334
7. Burmeister WP, et al. 2000. J Biol Chem. X-ray crystallography — in vitro structural biochemistry. PMID 10978344 · https://doi.org/10.1074/jbc.M006796200
8. Ludikhuyze L, et al. 1999. J Agric Food Chem. Enzyme-kinetics study on myrosinase extracted from broccoli. Thermal inactivation proceeds across 30–60 °C. PMID 10552454 · https://doi.org/10.1021/jf980964y
9. Cramer JM, Jeffery EH. 2011. Nutrition and Cancer. Pilot, n=4. Air-dried broccoli sprout powder used as a myrosinase donor. https://doi.org/10.1080/01635581.2011.523495
10. Matusheski NV, et al. 2006. J Agric Food Chem. Recombinant-protein plant biochemistry, in vitro. PMID 16536577 · https://doi.org/10.1021/jf0525277
11. Evidence gap. No human trial was located that compares a pre-converted or stabilised sulforaphane product head to head against a glucoraphanin-plus-myrosinase product.
12. Egner PA, et al. 2011. Cancer Prev Res. Randomised crossover, n=50. About 70% of dose recovered in 24-hour urine from a pre-hydrolysed sulforaphane beverage versus about 5% from a glucoraphanin-rich, myrosinase-poor beverage. PMID 21372038 · https://doi.org/10.1158/1940-6207.CAPR-10-0296
13. Clarke JD, et al. 2011. Pharmacological Research. Crossover, n=12. Fresh broccoli sprouts produced 7-fold higher peak plasma sulforaphane and 5-fold higher 24-hour urinary excretion than a broccoli supplement. PMID 21816223 · https://doi.org/10.1016/j.phrs.2011.07.005
14. Evidence gap. No peer-reviewed analytical survey comparing label claims against measured content across commercial products was located.