Selenium Methylation: Insights into Chemical Reactions and Enzymatic Pathways

Selenium, an essential metalloid, plays a dual role in biological systems: while crucial for maintaining normal biological processes, excessive levels can be toxic. Organisms mitigate selenium toxicity through a biochemical process known as methylation, in which inorganic selenium species are enzymatically converted into less toxic, excretable organic metabolites. This review synthesizes recent biochemical and environmental findings (with an emphasis on the past decade) related to selenium methylation. It outlines the enzymatic mechanisms—particularly involving glutathione reductase, SAM-dependent methyltransferases, and selenocysteine lyase—through which selenite and selenate are reduced and methylated to intermediates such as hydrogen selenide (H2Se), ultimately yielding MMSe, DMSe, and TMSe+. The role of enzymes such as selenocysteine lyase in processing organic selenium and factors affecting the efficiency of these processes, including environmental conditions, are discussed. The role of enzymes such as selenocysteine lyase in metabolizing organic selenium species is also discussed, along with how environmental conditions (e.g., soil composition, redox potential) and genetic variability influence methylation efficiency and selenium speciation. In conclusion, this paper explores selenium methylation in plants, focusing on rice and corn, and how their selenium uptake and metabolism are affected by environmental factors. It examines the conversion of selenium into organic forms like selenomethionine and selenocysteine, and the role of methylation in managing excess selenium. The findings offer insights into selenium chemistry, with implications for food safety, nutrition, and environmental management, addressing key knowledge gaps and enhancing our understanding of selenium’s biological and chemical roles.