title: Characterization of O-acetyl-L-serine sulfhydrylase purified from an alkaliphilic bacterium.
abstract: O-Acetyl-L-serine sulfhydrylase (EC 4.2.99.8) activity was shown to be very high compared with O-acetyl-L-homoserine sulfhydrylase (EC 4.2.99.10) activity and L-cystathionine cleaving activities, in an extract of cells of an alkaliphilic bacterium grown in a synthetic medium. The synthesis of the first enzyme was repressed by approximately 55% by both L-cystine and L-djenkolic acid added to the medium at a concentration of 0.5 mM, but L-methionine (1 mM) and S-adenosyl-L-methionine (0.5 mM) affected it to lesser extents. Its enzyme activity was inhibited by 25% and 12% by methionine (10 mM) and S-adenosylmethionine (5 mM), respectively. The enzyme was purified from the extract through ammonium sulfate fractionation, heat treatment, and chromatography on columns of DEAE-cellulose, Sephacryl S-300, and Octyl Sepharose CL-4B with a recovery of 21%. Polyacrylamide gel electrophoresis with sodium dodecylsulfate of the preparation obtained finally showed its homogeneity and the molecular mass of 37,000 Da for dissociated subunits. Gel filtration of the enzyme on a Sephacryl S-300 column showed an approximate molecular mass of 72,000 Da, suggesting that the enzyme was comprised of two identical subunits. The enzyme catalyzed the beta-replacement reaction with O-acetylserine as a substrate, and showed no reactivity to other O-substituted amino acids tested. The reaction proceeded best at 40 degrees C (when tested at pH 7.5), and at pH 6.5 (at 40 degrees C). The enzyme kept 90% its activity after incubation at 65 degrees C (at pH 7.5) for 30 min, and more than 90% after 30 min incubation at pHs 7-12 at 30 degrees C. The enzyme had a Km of 4 mM for O-acetyl-L-serine and a Vmax of 37.0 micromol/min/mg of protein, a very low value compared with those of other organisms. However, the content of the enzyme in the extract was calculated to be approximately 3.5% total protein. Sensitivity of the enzyme to carbonyl reagents was very low, although it was shown to have pyridoxal 5'-phosphate as a cofactor by examination of its absorption spectrum. Sulfhydryl reagents tested showed no inhibition. The novelty of this enzyme among analogous sulfhydrylases purified from other organisms was discussed.
authors.affiliation: United Graduate School of Agricultural Sciences, Faculty of Agriculture, Gifu University, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Sugihara
authors.forename: S
authors.initials: S
authors.lastname: Yamagata
authors.forename: Y
authors.initials: Y
authors.lastname: Mizuno
authors.forename: T
authors.initials: T
authors.lastname: Ezaki
mesh_terms: D000603:Amino Acids, Sulfur / Q000096:biosynthesis; D001419:Bacteria / Q000201:enzymology*; D002384:Catalysis; D003547:Cysteine Synthase / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism; D004795:Enzyme Stability; D020868:Gene Silencing; D006863:Hydrogen-Ion Concentration; D008970:Molecular Weight; D011487:Protein Conformation; D013379:Substrate Specificity; D013455:Sulfur / Q000378:metabolism; D013696:Temperature
chemical_list: D000603:Amino Acids, Sulfur; D013455:Sulfur; D003547:Cysteine Synthase