SCHEME 1.

Proposed fate of metabolically available selenium (‘Se’) in the hepatocyte. Hepatocytes have several sources of selenium that are immediately available for further metabolism. Synthesis of sec-tRNA^[ser]sec ① competes for selenium with methylation reactions ② that produce excretory metabolites. Synthesis of Sepp1 ③ for export to the plasma competes for sec-tRNA^[ser]sec with synthesis of intracellular selenoproteins ④. Liver selenoproteins turn over ⑤ to release selenium. The asterisks on steps ① and ③ indicate the path of selenium favored under conditions of selenium deficiency.