Myrosinase: The Enzyme That Activates Sulforaphane in Broccoli

Anyone who looks into broccoli and sulforaphane sooner or later comes across an unwieldy term: myrosinase. This enzyme is the actual key, because without it the sought-after plant compound simply stays inactive. In this guide we explain clearly, from a biochemical standpoint, what myrosinase is, how activation takes place, and why heat, processing and product quality make the decisive difference here. Once you understand how the enzyme and its starting material interact, you'll make noticeably better-informed decisions with both fresh vegetables and food supplements, and see through more than a few marketing claims much more easily.

What is myrosinase?

Myrosinase is a plant's own enzyme found in cruciferous vegetables such as broccoli, rocket, cress, cabbage and mustard. Chemically, it belongs to the group of thioglucosidases, enzymes that break a sugar-sulphur bond. Its biological role in the plant is a kind of defence system: when the plant tissue is damaged, myrosinase sets off a reaction that releases pungent, sometimes irritant compounds, deterring animals that might otherwise eat it.

What's fascinating about this: in the intact plant, the enzyme and its starting material are kept spatially separate. The mustard oil glycoside glucoraphanin is stored in certain cell compartments, while the myrosinase sits elsewhere. Only when the cell structure is destroyed do the two come into contact. This very separation is the reason why a whole, undamaged broccoli contains practically no sulforaphane.

The name itself hints at the function: it derives from mustard oil (sometimes referred to in Latin as myron), which is formed during the reaction. In the scientific literature, myrosinase therefore also appears under names such as thioglucoside glucohydrolase. For everyday purposes, a simple rule of thumb suffices: myrosinase is the biochemical scissors that cuts the sugar away from the mustard oil glycoside, which is what releases the pungent, reactive mustard oils in the first place.

Glucoraphanin, myrosinase and sulforaphane: the reaction step by step

To understand what myrosinase does, it helps to look at the three building blocks involved. Glucoraphanin is the inactive storage form, what's known as a mustard oil glycoside (glucosinolate). Myrosinase is the enzyme that unlocks this storage form. Sulforaphane is the reactive end product, an isothiocyanate that contributes to the typical pungent note of mustard or cress.

The process can be described in several steps. First, the plant tissue is mechanically damaged, for example by cutting, chewing or blending. This causes glucoraphanin and myrosinase to leave their separate compartments and come into contact. The myrosinase then cleaves the sugar group from the glucoraphanin. Sulforaphane finally forms from the resulting intermediate product. In simplified terms: glucoraphanin plus active myrosinase yields sulforaphane. If either factor is missing, the end product does not form, or only in trace amounts. You can read in detail how sulforaphane is further classified biochemically in the article Sulforaphane from Broccoli.

Why heat stops the activation

Myrosinase is a protein and therefore sensitive to heat. During prolonged boiling, frying or heavy steaming, the enzyme loses its three-dimensional structure and, with it, its function. It becomes denatured and can no longer cleave glucoraphanin. This explains an often surprising phenomenon: heavily cooked broccoli can still contain plenty of glucoraphanin, yet deliver noticeably less sulforaphane, because the activating enzyme is missing.

Interestingly, glucoraphanin itself is more heat-stable than the enzyme. So the storage form withstands quite a lot, while the myrosinase drops out earlier. In practice, this means: anyone who wants to get as much as possible out of the reaction works in an enzyme-gentle way. Eating it raw, brief and gentle steaming, or chopping it and then letting it sit briefly before heating, are common approaches. The human body also has a backup: some gut bacteria have their own enzymatic capabilities and can convert glucoraphanin to a certain extent, even once the plant's own myrosinase has already been destroyed. However, this bacterial conversion varies from person to person and is less efficient than direct activation by the plant enzyme.

What else influences sulforaphane yield

Heat is the best-known factor, but not the only one. The temperature during chopping, the waiting time after cutting, and a second plant protein also play a part. Besides myrosinase, some cruciferous vegetables contain what's known as an epithiospecifier protein. When it's active, the reaction can shift towards a different breakdown product, a nitrile, instead of sulforaphane. This side pathway is influenced by certain conditions, which is why processing and temperature control also help decide which end product predominates.

The brief resting time after chopping is also practically relevant. Anyone who cuts raw broccoli or sprouts and lets them sit for a few minutes before briefly heating them gives the myrosinase time to do its work before the heat stops the enzyme. The acidity of the surrounding environment and the presence of water also influence how efficiently the conversion proceeds. You don't need to control these details down to the last degree, but they explain why the sheer amount of glucoraphanin alone never tells the whole story.

Myrosinase in different cruciferous vegetables

Not every cruciferous vegetable brings the same amount of enzyme or starting material. Broccoli sprouts are especially rich in glucoraphanin and simultaneously supply active myrosinase, which is why they're considered one of the most productive sources. Mature broccoli usually contains less glucoraphanin per gram, but is practical to eat in larger quantities. Cress, rocket, mustard and various types of cabbage each contribute their own glucosinolates and their own myrosinase, so different mustard oils can result.

This diversity is also why combining different cruciferous vegetables in the kitchen makes sense. Fresh cress sprinkled over steamed broccoli, or a little mustard in a dressing, brings active myrosinase back into play, even if the enzyme in the cooked vegetable has already been denatured. This lets you support the natural reaction with simple means, without needing a complicated preparation plan.

What this means for broccoli supplements

This is exactly where myrosinase becomes relevant for food supplements. Many broccoli extracts are offered standardised to their glucoraphanin content. That makes sense, but on its own it says nothing about how much sulforaphane can actually form. Without active myrosinase, the glucoraphanin remains in its inactive storage form.

Two routes to forming sulforaphane

There are basically two approaches. In the first, the product already supplies ready-formed, stabilised sulforaphane. In the second, the product combines a glucoraphanin-rich broccoli extract with an active myrosinase source, often from broccoli sprouts or mustard seeds, so that the conversion can take place in the digestive tract. Both variants have their place; what matters is the manufacturer's transparency. Another point is the delivery format: enteric-coated capsules or a protected formulation are meant to help sensitive components survive the journey through the acidic stomach as well as possible. Because sulforaphane itself is sensitive to heat and moisture, production, storage and shelf life also play a role. A high-quality product provides traceable information on these points, rather than just advertising with the biggest possible number on the packaging.

How to recognise a well-thought-out product

Look for clear information on glucoraphanin content and, where present, sulforaphane content, as well as details on a myrosinase source or active enzymes. A bare glucoraphanin figure with no mention at all of activation is a signal to look more closely. We've put together which other criteria count when buying in the guide Buying Sulforaphane: What to Look For.

Myrosinase in everyday life: practical context

For everyday eating, the knowledge about myrosinase is simple to put to use. Fresh, raw broccoli, broccoli sprouts, rocket and cress all bring their own active myrosinase along with them. Anyone who prefers cooked broccoli can add a little mustard powder, fresh cress or raw vegetables to supply an enzyme source. This isn't a must, just a way to get more out of the natural reaction. Broccoli and other cruciferous vegetables are in any case part of a plant-forward, varied diet, as also described in our overview of antioxidant-rich foods.

Important context: myrosinase, glucoraphanin and sulforaphane are food components, or compounds found in food supplements. They do not replace a varied diet or medical treatment. If you have health-related questions, take medication, are pregnant or breastfeeding, it's best to discuss taking any product with your doctor beforehand.

Frequently asked questions (FAQ)

What exactly does myrosinase do?

Myrosinase is a plant enzyme that splits the mustard oil glycoside glucoraphanin. Sulforaphane only forms as a result of this cleavage. Without active myrosinase, the glucoraphanin stays in its inactive storage form and is barely converted into sulforaphane.

Why does sulforaphane only form when chewing or cutting?

In the intact plant, glucoraphanin and myrosinase are kept spatially apart. Only when the cell structure is destroyed by cutting, blending or chewing do the enzyme and the starting material come into contact, and the conversion to sulforaphane begins.

Does cooking destroy myrosinase?

Myrosinase is a heat-sensitive protein. Prolonged boiling or intense heating denatures the enzyme, so it can no longer split glucoraphanin. Glucoraphanin itself is more stable, which is why heavily cooked broccoli usually delivers less sulforaphane.

Do I need myrosinase with broccoli capsules too?

That depends on the product. If a product contains only glucoraphanin, an active myrosinase source can support the formation of sulforaphane. Some products already supply ready-formed sulforaphane or combine the extract with an enzyme source. The manufacturer's information will tell you which applies.

Can the body convert glucoraphanin even without plant myrosinase?

Some gut bacteria have their own enzymes and can convert glucoraphanin to a certain extent, even when the plant's myrosinase has been destroyed. However, this bacterial conversion varies from person to person and is generally less efficient than direct activation by plant myrosinase.