Glucosinolate

These natural chemicals most likely contribute to plant defence against pests and diseases, and impart a characteristic bitter flavor property to cruciferous vegetables.

[1][3] Glucosinolates constitute a natural class of organic compounds that contain sulfur and nitrogen and are derived from glucose and an amino acid.

The essence of glucosinolate chemistry is their ability to convert into an isothiocyanate (a "mustard oil") upon hydrolysis of the thioglucoside bond by the enzyme myrosinase.

[6] Care must be taken when discussing these compounds since some older publications do not make it clear whether they refer to the anion alone, its corresponding acid or the potassium salt.

They are biosynthesized from amino acids: so-called aliphatic glucosinolates derived from mainly methionine, but also alanine, leucine, isoleucine, or valine.

[4] Full details of the sequence of reactions that converts individual amino acids into the corresponding glucosinolate have been studied in the cress Arabidopsis thaliana.

The sulfur atom is incorporated from glutathione (GSH) and the sugar component is added to the resulting thiol derivative by a glycosyltransferase before the final sulfonation step.

[11] The remaining molecule then quickly converts to an isothiocyanate, a nitrile, or a thiocyanate; these are the active substances that serve as defense for the plant.

[12] In the chemical reaction illustrated above, the red curved arrows in the left side of figure are simplified compared to reality, as the role of the enzyme myrosinase is not shown.

In contrast, the reaction illustrated by red curved arrows at the right side of the figure, depicting the rearrangement of atoms resulting in the isothiocyanate, is expected to be non-enzymatic.

[17] The isothiocyanates formed from glucosinolates are under laboratory research to assess the expression and activation of enzymes that metabolize xenobiotics, such as carcinogens.