Glucosinolates are a group of over 130 naturally occurring compounds found in certain plants that contain sulfur and are derived from glucose and amino acids. These compounds are responsible for the pungent flavor in vegetables like mustard and horseradish. On their own, glucosinolates are biologically inactive and water-soluble. They must undergo a chemical transformation to become active compounds, a process initiated when the plant material is damaged.
Glucosinolates in Plants
Glucosinolates are primarily found in cruciferous vegetables, which are part of the Brassica genus. This group includes commonly consumed vegetables such as:
- Broccoli
- Brussels sprouts
- Cabbage
- Kale
- Cauliflower
- Bok choy
The compounds are also present in the seeds of these plants. The specific types and amounts of glucosinolates can vary significantly between different plant species and even within different parts of the same plant, such as the roots versus the leaves.
Within the plant, glucosinolates function as a component of the defense system, helping to protect the plant against threats like pests and diseases. When the plant tissue is damaged, this activation releases compounds that can be toxic to pests, deterring them from causing further damage. The system serves as a natural defense mechanism for the plant’s survival.
Biological Activation Process
The conversion of inactive glucosinolates into biologically active compounds is an enzymatic process. In an intact plant cell, glucosinolates are stored in a separate compartment from an enzyme called myrosinase. When the plant’s tissues are damaged, such as through chopping during food preparation or chewing, the cell walls are broken down. This rupture allows the glucosinolates and myrosinase to mix.
Once they come into contact, myrosinase catalyzes a hydrolysis reaction, which breaks down the glucosinolates. This reaction transforms the stable glucosinolates into highly reactive compounds. The primary and most studied of these breakdown products are isothiocyanates. The specific type of isothiocyanate produced depends on the structure of the original glucosinolate molecule.
For example, broccoli and broccoli sprouts are rich in a glucosinolate called glucoraphanin. When glucoraphanin is hydrolyzed by myrosinase, it is converted into the isothiocyanate known as sulforaphane. Sulforaphane is one of the most extensively researched isothiocyanates.
Health-Promoting Compounds
The isothiocyanates formed from glucosinolate breakdown are recognized for their influence on various cellular processes. One of their primary activities is supporting the body’s detoxification pathways. They interact with proteins that regulate the expression of genes involved in detoxification, particularly Phase II enzymes. These enzymes help to neutralize and prepare harmful substances for excretion from the body.
Isothiocyanates also exhibit antioxidant properties. They can help to neutralize free radicals, which are unstable molecules that can cause damage to cells through a process called oxidative stress. By reducing the number of free radicals, these compounds help protect cellular structures, including DNA and proteins, from damage.
Beyond their antioxidant role, these compounds have been observed to possess anti-inflammatory mechanisms. They can influence signaling pathways within cells that are involved in the body’s inflammatory response. By modulating these pathways, isothiocyanates can help to regulate inflammation.
Factors Influencing Bioavailability
The amount of active isothiocyanates available to the body is not solely dependent on the quantity of cruciferous vegetables consumed. Food preparation methods significantly influence the final yield of these compounds. The myrosinase enzyme is sensitive to heat, and cooking methods like boiling or microwaving at high temperatures can deactivate it, preventing the formation of isothiocyanates.
To preserve the activity of myrosinase, gentler cooking techniques are more effective. Lightly steaming vegetables for a few minutes allows the plant tissue to soften while minimizing the heat-related destruction of the enzyme. Chopping vegetables and letting them sit for a period before cooking can also allow the enzyme time to convert glucosinolates into isothiocyanates before heat is applied.
Even if the myrosinase in the plant is destroyed by cooking, the body has another way to activate glucosinolates. Certain species of bacteria in the human gut microbiota can produce their own myrosinase-like enzymes. These gut bacteria can facilitate the conversion of ingested glucosinolates into isothiocyanates, although this process is considered less efficient than the plant’s own enzymatic conversion.