Enzymic synthesis of selenocysteine in rat liver

We have investigated selenocysteine (2-amino-3hydroselenopropionic acid) synthesis with cystathionine ß- synthase (EC 4.2.1.22)and cystathionine -lyase (EC 4.4.1.1) of rat liver.When selenohomocysteine and serine were in- cubated with cystathionine /3-synthase, selenocystathionine was formed at a rate of 69% of that of cystathionine synthesis.Cystathionine -lyase catalyzed , elimination of seleno- cystathionine to yield -ketobutyrate, selenocysteine, and NH3.The reaction rate was about 3 times higher than that of cystathionine elimination.Cystathionine /3-synthase, however, did not catalyze direct formation of selenocysteine from serine and H2Se.Thus, selenocysteine is synthesized from selenohomocysteine and serine through selenocystathionine by cou- pling of cystathionine /3-synthase and cystathionine -lyase reactions.We confirmed this synthetic pathway also with a mixture of both enzymes and with a homogenate of rat liver.Various selenium-containing amino acids occur in nature and play important physiological roles (Shrift, 1973; Stadtman, 1980).Several microbial and mammalian proteins, in par- ticular enzymes, contain selenium as an essential component (Stadtman, 1980).Three of them have been shown to contain a selenocysteine (2-amino-3-hydroselenopropionic acid) residue