Purification of properties of Saccharomyces cerevisiae cystathionine β‐synthase

Cystathionine β‐synthase (β‐CTSase), which catalyses cystathionine synthesis from serine and homocysteine, was purified to homogeneity from Saccharomyces cerevisiae. The molecular mass of the enzyme was estimated to be 235 kDa by gel filtration and 55 kDa by sodium dodecyl sulphate–polyacrylamide gel electrophoresis, indicating that it is a homotetramer. The N‐terminal amino acid sequence of the enzyme perfectly coincided with that deduced from the nucleotide sequence of CYS4, except for the absence of initiation methionine. The purified β‐CTSase catalysed cysteine synthesis from serine (or O‐acetylserine) and H₂S. From this finding, we discuss the multifunctional nature and evolutionary divergence of S‐metabolizing enzymes.