Specificity and some other properties of liver serine sulphhydrase: Evidence for its identity with cystathionine β-synthase

Characterization of Transsulfuration and Cysteine Biosynthetic Pathways in the Protozoan Hemoflagellate, Trypanosoma cruzi

“…4) with a K m of 1.0 Ϯ 0.2 mM for L-serine and a K m of 0.9 Ϯ 0.3 mM for L-homocysteine. Similar to yeast and mammalian CBS (23,34), rTcCBS also showed SS activity, which forms cysteine from serine and sodium sulfide. The SS activity was 4.58 Ϯ 0.85 units/mg of protein with a K m of 1.1 Ϯ 0.2 mM for L-serine and a K m of 3.1 Ϯ 0.2 mM for sodium sulfide.…”

“…rTcCBS also catalyzed the production of hydrogen sulfide from L-cysteine and ␤-mercaptoethanol with a K m of 2.4 Ϯ 0.5 mM for L-cysteine and a K m of 26 Ϯ 9 mM for ␤-mercaptoethanol. This hydrogen sulfideforming activity was relatively insensitive to 10 mM aminooxyacetic acid (only 50% inhibition) and 1 mM hydroxylamine (only 20% inhibition), which are known to inhibit mammalian CBS (55-100% inhibition at 0.1 mM) (23). Unlike human CBS, rTc-CBS was not activated by the presence of 1 mM S-adenosylmethionine and did not contain detectable amounts of heme (data not shown).…”

“…This suggests that trypanosomal CBS functions as CS in E. coli in vivo. Although the native and recombinant yeast and mammalian CBS showed CS and CBS activities in vitro (23,34), this is, in the literature, the first demonstration that a eukaryotic CBS physiologically functions as CS in prokaryotes. Sequencing of six randomly chosen cDNA clones obtained by rescue, and two additional CBS cDNA clones obtained by hybridization revealed that all of these clones contained an insert with a 1155-bp ORF, encoding a protein of 384 amino acids with a calculated molecular mass of 42.0 kDa.…”

“…The enzymatic activity of serine sulfhydrylase (SS) was measured in the same way as described for CS assay except that serine was used instead of OAS. The rate of cysteine desulfhydration in ␤-replacement reaction was monitored by measuring hydrogen sulfide production (23) in the mixture of 50 mM Tris-HCl, pH 8.0, 10 mM L-cysteine, 20 mM ␤-mercaptoethanol, 0.1 mM pyridoxal phosphate, 0.075 mM lead acetate. For all assays using a recombinant CBS, bovine serum albumin was included at 1 mg/ml.…”

Beta-cyanoalanine synthase: purification and characterization.

“…Cyanoalanine synthase catalyzes slow isotopic a-H exchange in cysteine and in end-product amino acids; the rates of a-H exchange in nonreacted (excess) cysteine are markedly increased in the presence of an adequate cosubstrate; no exchange is observed of H atoms in f3-position. In recent years we made comparative studies of some vitamin-B6-dependent lyases exclusively catalyzing replacement reactions of l-substituent in cysteine, serine, and related analogs (8,9,18,19,25). As a rule, these lyases, e.g., cysteine lyase (EC 4.4.1.10), serine sulfhydrase, and the identical or closely similar cystathionine j3-synthase (EC 4.2.1.22), use aliphatic thiols as cosubstrates (replacing agents).…”