The essential trace element selenium primarily exerts significant physiological functions as the selenocysteine (Sec) residue in selenoproteins. The formation of Sec depends on selenophosphate, which is synthesized by selenophosphate synthetase (SPS) using ATP, selenide, and water as substrates. To ensure that trace selenium compounds are specifically metabolized without interference from the more abundant sulfur compounds, strict enzymatic regulation is considered crucial. It has been suggested that there are selenium carrier proteins that bind with selenide and directly supply physiological selenium substrates to SPS. In animals, we conducted a detailed analysis of selenocysteine lyase, which acts specifically on _L-Sec to release selenium, and clarified the selenium transfer reaction mediated by the active site cysteine residue. Additionally, in bacteria, we demonstrated that thioredoxin is important for the reduction of selenite in the biosynthesis of selenoproteins.