Cystationine synthesis in yeast: an alternative pathway for homocysteine biosynthesis.

Cystathionine synthesis from O-acetylhomoserine and cysteine has been demonstrated in yeast extracts for the first time. The activity is less than that of O-acetylhomoserine sulfhydrylase, but it is higher than that reported for homoserine O-transacetylase and therefore should not be growth limiting. Cystathionine synthase seems to share the regulatory properties of the sulfhydrylase, and both activities are missing from the methionine auxotroph Saccharomyces cerevisiae EY9, suggesting that both reactions are catalyzed by the same enzyme. However, cystathionine synthase activity was lost during purification of the sulfhydrylase, suggesting that the two reactions may be catalyzed by separate enzymes. Since previous studies have shown that yeast extracts can catalyze the cleavage of cystathionine to homocysteine, our results show the existence of two complete alternate pathways for homocysteine biosynthesis in yeast. Which of these is the major physiological pathway remains to be determined.