Methiin

Methiin, also known as S‑methyl‑L‑cysteine sulfoxide (SMCSO) and by its CAS number 528‑72‑5, is a non‑proteinogenic amino‑sulfoxide that serves as a major flavor precursor in several Allium species, such as garlic (Allium sativum) and onion (Allium cepa). Chemically it is the sulfoxide derivative of S‑methylcysteine, with the molecular formula C4H9NO2S and a molecular weight of 139.18 g·mol⁻¹. In its neutral form the compound exists as a chiral sulfoxide, typically isolated as a mixture of the (R) and (S) enantiomers.

Biosynthesis of methiin proceeds from the essential amino acid methionine. A methyltransferase catalyzes the S‑methylation of cysteine to give S‑methylcysteine, which is subsequently oxidized by a flavin‑dependent sulfoxidase to the sulfoxide. The pathway is conserved across many members of the Alliaceae and Brassicaceae families, contributing to the characteristic pungency of these vegetables.

When plant tissue is disrupted, the enzyme alliinase hydrolyzes methiin, releasing methanesulfenic acid, which rapidly condenses to form various volatile thiosulfinates, thiosulfonates and polysulfides. These products are responsible for the sharp aroma of fresh garlic and for the formation of more stable, less odorous compounds such as S‑allyl‑cysteine and its derivatives during cooking.

Beyond its sensory role, methiin has attracted interest for potential health effects. In vitro studies suggest antioxidant activity and the ability to modulate phase‑II detoxifying enzymes, while limited animal data indicate possible cardioprotective and anti‑inflammatory actions. Human dietary exposure occurs primarily through consumption of raw or minimally processed Allium vegetables, where methiin concentrations can range from 0.5 to 5 mg g⁻¹ of fresh weight. Further clinical research is required to clarify its bioavailability and physiological impact.