Methylselenocysteine

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Methylselenocysteine (Me-Sec), also called Se-methylselenocysteine (SeMSC), is a form of the amino acid cysteine in which sulfur is replaced by selenium.

Where it’s found
- It occurs in many vegetables, including onions, leeks, garlic, ramps, broccoli, radish, Brussels sprouts, and cabbage.
- It’s also present in milk vetch and in selenized yeast.
- In plants, it’s made by a selenocysteine methyltransferase; adding inorganic selenium to soil increases the gene’s activity. In yeast, it may form through a selenomethyltransferase enzyme or another enzyme’s action.

What it does in plants
- Me-Sec can boost the production of beneficial compounds such as capsaicin in chili peppers and various antioxidants and other metabolites (carotenoids, phenols, glucosinolates, sulforaphane) in Brassica vegetables.

How the body uses it
- The body’s bioavailability of Se-methylselenocysteine is similar to other organic selenium forms.
- It is metabolized first by beta-lyase to methylselenol, then to hydrogen selenide, a key selenium metabolite.

Safety notes
- Health authorities say there aren’t enough human data to set a safe upper limit specifically for Se-methylselenocysteine, so more research is needed on its safety and bioavailability.

Health effects and cancer research
- Se-methylselenocysteine has shown chemopreventive activity in some studies, including blocking cell growth and promoting death (apoptosis) of premalignant mammary cells in culture. It tends to induce apoptosis more effectively and with less toxicity than selenite (a different selenium form).
- In an important cancer trial, selenized yeast (which contains Se-methylselenocysteine and selenomethionine) reduced prostate cancer and overall cancer incidence. However, trials using selenomethionine alone did not show the same benefit, suggesting the combination in selenized yeast may matter.