Biosynthesis of Se-methylselenocysteine and S-methylcysteine in Astragalus bisulcatus

Biosynthesis of Se-methylselenocysteine and S-methylcysteine in Astragalus bisulcatus has been investigated using radiotracer methods. The results of feeding L-serine-U- I4 C, DL-serine-1- 14 C, L-cysteine-U- 14 C, and L-methionine-methyl- 14 C to 2-month-old seedlings indicate that serine and cysteine are precursors of the three carbon chains, and that methionine methyl group is a precursor of the methyl group of the Se- and S-methylcysteines. It is suggested that in the biosynthesis of Se-methylselenocysteine from serine, Se-cysteine is formed first, and that Se-methylselenocysteine is formed from Se-cysteine by methylation. The incorporation of radioactivity into S-methylcysteine suggests a similar mechanism for its biosynthesis.