Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:4.2.1.22 (cystathionine beta-synthase)
965 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The neurologic complications of cystathionine beta-synthase deficiency are thought to be secondary to accumulation of homocyst(e)ine in the CNS. Treatment of this disorder with betaine has been shown to improve the behavior of individuals, to reduce plasma total homocysteine, and to correct secondary abnormalities of serine. To test the hypothesis that homocyst(e)ine accumulates within the CNS and that this can be reduced by treatment with betaine, we measured total homocysteine and related metabolites in the plasma of 10 children with cystathionine beta-synthase deficiency and cerebrospinal fluid of five children before and during betaine therapy. In plasma, betaine significantly lowered total homocysteine (but not to the normal range) and had a variable effect on methionine. In the cerebrospinal fluid, total homocysteine was raised before treatment (mean 1.2 microM) and was significantly reduced by betaine (mean 0.32 microM) but not to the normal range (<0.10 microM). Cerebrospinal fluid methionine was raised before and during treatment, but betaine did not cause a significant further increase. Cerebrospinal fluid serine was significantly reduced before treatment and rose to the normal range with betaine. Cerebrospinal fluid S-adenosylmethionine was normal before treatment and rose significantly with treatment; there were no significant changes in cerebrospinal fluid 5-methyltetrahydrofolate. The demonstration of accumulation of homocysteine within the CNS lends support to the hypothesis that this may be one cause of the neurologic complications of cystathionine beta-synthase deficiency. Betaine is effective in reducing cerebrospinal fluid homocysteine, but concentrations are still significantly raised during treatment.
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PMID:Cerebrospinal fluid and plasma total homocysteine and related metabolites in children with cystathionine beta-synthase deficiency: the effect of treatment. 935 26

Epidemiological studies have provided strong evidence that an elevated plasma homocysteine concentration is an important independent risk factor for cardiovascular disease. We have shown, in the rat, that the kidney is a major site for the removal and subsequent metabolism of plasma homocysteine [Bostom, Brosnan, Hall, Nadeau and Selhub (1995) Atherosclerosis 116, 59-62]. To characterize the role of the kidney in homocysteine metabolism further, we measured the disappearance of homocysteine in isolated renal cortical tubules of the rat. Renal tubules metabolized homocysteine primarily through the transulphuration pathway, producing cystathionine and cysteine (78% of homocysteine disappearance). Methionine production accounted for less than 2% of the disappearance of homocysteine. Cystathionine, and subsequently cysteine, production rates, as well as the rate of disappearance of homocysteine, were sensitive to the level of serine in the incubation medium, as increased serine concentrations permitted higher rates of cystathionine and cysteine production. On the basis of enrichment profiles of cystathionine beta-synthase and cystathionine gamma-lyase, in comparison with marker enzymes of known location, we concluded that cystathionine beta-synthase was enriched in the outer cortex, specifically in cells of the proximal convoluted tubule. Cystathionine gamma-lyase exhibited higher enrichment patterns in the inner cortex and outer medulla, with strong evidence of an enrichment in cells of the proximal straight tubule. These studies indicate that factors that influence the transulphuration of homocysteine may influence the renal clearance of this amino acid.
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PMID:Characterization of homocysteine metabolism in the rat kidney. 935 66

Cystathionine beta-synthase (CBS) catalyzes the irreversible, serine-dependent conversion of homocysteine to cystathionine via a transsulfuration pathway. CBS deficiency not only is the leading cause of homocystinuria, an inherited genetic disorder, but may contribute to cardiovascular disease as well. We isolated three new isoforms of human CBS mRNA from a human liver cDNA library. We designate these CBS mRNAs as CBS 3, CBS 4, and CBS 5, and the CBS mRNAs reported previously by Kraus et al. (1993) (Hum. Mol. Genet. 2, 1933-1938) and Kruger and Cox (1994) (Proc. Natl. Acad. Sci. USA 91, 6614-6618) as CBS 1 and CBS 2, respectively. Sequence analyses show that the only difference among the five CBS mRNAs is at the beginning of the 5'-untranslated region. Tissue distribution studies reveal that liver and pancreas have the highest amounts of CBS mRNAs. CBS mRNA is present in all regions of the brain tested. We also report the differential distribution of CBS mRNA isoforms in tissues, showing that pancreas contains all five CBS isoforms and the liver has four CBS mRNA isoforms, CBS 1-4. The kidney contains only CBS 1 and CBS 2. In human fetal tissues, CBS 2 is present in the liver and kidney. PCR-based quantitative analyses of CBS mRNA isoforms in human liver demonstrate that CBS 1 and CBS 2 are the major species, with CBS 2 being more abundant, while CBS 3-5 are the minor species. Furthermore, results from our human liver cDNA screening and primer extension experiments show that each of the five CBS transcripts begins with a different exon, suggesting that CBS gene transcription might be regulated by more than one promoter.
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PMID:Identification and tissue distribution of human cystathionine beta-synthase mRNA isoforms. 946 25

We have screened a rat brain library to identify proteins which interact with the 5'-end of huntingtin (amino acids 1-171), including the polyglutamine tract, in the yeast two-hybrid system. We detected an interaction with cystathionine beta-synthase (CBS) [L-serine hydrolyase (adding homocysteine), EC 4.2.1.22], which was confirmed in vitro using His-tagged CBS expressed in Escherichia coli , which was able to specifically bind both rat and human full-length huntingtin. Neither normal nor expanded polyglutamine repeat alone interacted with CBS in the yeast two-hybrid system and nor did constructs containing SBMA or DRPLA with normal or expanded polyglutamine tracts. CBS therefore appears to bind specifically to huntingtin. CBS deficiency is associated with homocystinuria, which is known to affect various physiological systems, including the central nervous system. Homocysteine, one of the substrates of CBS, is known to accumulate in homocystinuria and is metabolized to homocysteate and homocysteine sulphinate, both known to be powerful excitotoxic amino acids. It has been suggested that Huntington's disease involves the action of excitotoxic amino acids and this interaction with CBS may suggest a mechanism for such excitotoxic damage.
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PMID:Huntingtin interacts with cystathionine beta-synthase. 946 92

Cystathionine beta-synthase (CBS) catalyzes the condensation of homocysteine and serine to cystathionine-an irreversible step in the eukaryotic transsulfuration pathway. The native enzyme is a homotetramer or multimer of 63-kDa (551 amino acids) subunits and is activated by S-adenosyl-l-methionine (AdoMet) or by partial cleavage with trypsin. Amino-terminal analysis of the early products of trypsinolysis demonstrated that the first cleavages occur at Lys 30, 36, and 39. The enzyme still retains the subunit organization as a tetramer or multimer composed of 58-kDa subunits. Analysis by electrospray ionization mass spectrometry showed that further trypsin treatment cleaves CBS in its COOH-terminal region at Arg 413 to yield 45-kDa subunits. This 45-kDa active core is the portion of CBS most conserved with the evolutionarily related enzymes isolated from plants, yeast, and bacteria. The active core of CBS forms a dimer of approximately 85 kDa. The dimer is about twice as active as the tetramer. It binds both pyridoxal 5'-phosphate and heme cofactors but is no longer activated by AdoMet. Further analysis suggests that the dissociation of CBS to dimers causes a decrease in enzyme thermostability and a threefold increase in affinity toward the sulfhydryl-containing substrate-homocysteine. We found that the COOH-terminal region, residues 414-551, is essential for maintaining the tetrameric structure and AdoMet activation of the enzyme. The inability of the active core to form multimeric aggregates has facilitated its crystallization and X-ray diffraction studies.
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PMID:Trypsin cleavage of human cystathionine beta-synthase into an evolutionarily conserved active core: structural and functional consequences. 967 31

The last steps of cysteine synthesis in plants involve two consecutive enzymes. The first enzyme, serine acetyltransferase, catalyses the acetylation of L-serine in the presence of acetyl-CoA to form O-acetylserine. The second enzyme, O-acetylserine (thiol) lyase, converts O-acetylserine to L-cysteine in the presence of sulfide. We have, in the present work, over-produced in Escherichia coli harboring various type of plasmids, either a plant serine acetyltransferase or this enzyme with a plant O-acetylserine (thiol) lyase. The free recombinant serine acetyltransferase (subunit mass of 34 kDa) exhibited a high propensity to form high-molecular-mass aggregates and was found to be highly unstable in solution. However, these aggregates were prevented in the presence of O-acetylserine (thiol) lyase (subunit mass of 36 kDa). Under these conditions homotetrameric serine acetyltransferase associated with two molecules of homodimeric O-acetylserine (thiol) lyase to form a bienzyme complex (molecular mass approximately 300 kDa) called cysteine synthase containing 4 mol pyridoxal 5'-phosphate/mol complex. O-Acetylserine triggered the dissociation of the bienzyme complex, whereas sulfide counteracted the action of O-acetylserine. Protein-protein interactions within the bienzyme complex strongly modified the kinetic properties of plant serine acetyltransferase: there was a transition from a typical Michaelis-Menten model to a model displaying positive kinetic co-operativity with respect to serine and acetyl-CoA. On the other hand, the formation of the bienzyme complex resulted in a very dramatic decrease in the catalytic efficiency of bound O-acetylserine (thiol) lyase. The latter enzyme behaved as if it were a structural and/or regulatory subunit of serine acetyltransferase. Our results also indicated that bound serine acetyltransferase produces a build-up of O-acetylserine along the reaction path and that the full capacity for cysteine synthesis can only be achieved in the presence of a large excess of free O-acetylserine (thiol) lyase. These findings contradict the widely held belief that such a bienzyme complex is required to channel the metabolite intermediate O-acetylserine.
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PMID:Interactions between serine acetyltransferase and O-acetylserine (thiol) lyase in higher plants--structural and kinetic properties of the free and bound enzymes. 969 24

Human cystathionine beta-synthase catalyzes the first step in the catabolic removal of the toxic metabolite, homocysteine. It is unique in being dependent on both pyridoxal phosphate (PLP) and heme for activity. The reaction involves condensation of serine and homocysteine to give cystathionine. Although the role of PLP can be rationalized in analogy with other PLP-dependent enzymes that catalyze beta-replacement reactions, the role of the heme is unknown. In this study, we have purified and characterized the recombinant human enzyme and have examined the effect of heme oxidation state on enzyme activity. We find that under reducing conditions, generated by addition of titanium citrate, the enzyme exhibits a 1.7-fold lower activity than under oxidizing conditions. Reoxidation of the ferrous enzyme with ferricyanide results in alleviation of inhibition. This redox-linked change in enzyme activity correlates with changes in heme oxidation state monitored by UV-visible spectroscopy. Dithiothreitol, which does not reduce the enzyme-bound heme, does not perturb enzyme activity. These studies provide the first evidence for redox-linked regulation of cystathionine beta-synthase which is heme-dependent.
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PMID:Evidence for heme-mediated redox regulation of human cystathionine beta-synthase activity. 973 78

Cystathionine beta-synthase [CBS; l-serine hydro-lyase (adding homocysteine), EC 4.2.1.22] catalyzes the first committed step of transsulfuration and is the enzyme deficient in classical homocystinuria. In this report, we describe the molecular cloning and the complete nucleotide sequence of the human CBS gene. We report a total of 28,046 nucleotides of sequence, which, in addition to the CBS gene, contains approximately 5 kb of the 5' flanking region. The human CBS gene contains 23 exons ranging from 42 to 209 bp. The 5' UTR is formed by 1 of 5 alternatively used exons and 1 invariably present exon, while the 3' UTR is encoded by exons 16 and 17. We also describe the identification of two alternatively used promoter regions that are GC rich (approximately 80%) and contain numerous putative binding sites for Sp1, Ap1, Ap2, and c-myb, but lack the classical TATA box. The CBS locus contains an unusually high number of Alu repeats, which may predispose this gene to deleterious rearrangements. Additionally, we report on a number of DNA sequence repeats that are polymorphic in North American and European Caucasians.
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PMID:The human cystathionine beta-synthase (CBS) gene: complete sequence, alternative splicing, and polymorphisms. 979 Jul 50

Cystathionine beta-synthase (CBS), a pyridoxal 5'-phosphate (PLP) dependent enzyme, catalyzes the condensation of serine and homocysteine to form cystathionine. Mammalian CBS was recently shown to be a heme protein. While the role of heme in CBS is unknown, catalysis by CBS can be explained solely by participation of PLP in the reaction mechanism. In this study, treatment of CBS with sodium borohydride selectively reduced the Schiff base but did not affect the heme. Purification and sequencing of the PLP-cross-linked peptide from a trypsin digest of the reduced enzyme revealed the evolutionarily conserved Lys119 to be the residue forming the Schiff base. Serine and hydroxylamine form an alpha-aminoacrylate and an oxime with PLP in CBS, respectively. The sulfhydryl-containing substrate, homocysteine, disturbs the heme environment but does not interact with PLP. In contrast to other PLP-dependent enzymes, CBS emits no PLP-related fluorescence when excited at 296 or 330 nm. PLP but not heme dissociates from the enzyme in the presence of hydroxylamine. The dissociation of PLP is a multistage process involving a short approximately 500 s lag phase, followed by a rapid inactivation and a slower PLP-oxime formation. PLP-free CBS exhibits a decrease of secondary structure as well as loss of CBS activity that can be only partially restored by PLP. This study constitutes the first comprehensive investigation of PLP interaction with a heme protein.
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PMID:Binding of pyridoxal 5'-phosphate to the heme protein human cystathionine beta-synthase. 1005 42

Cystathionine beta-synthase is an unusual enzyme that requires the cofactors heme and pyridoxal phosphate (PLP) to catalyze the condensation of homocysteine and serine to generate cystathionine. This transsulfuration reaction represents one of two major cellular routes for detoxification of homocysteine, which is a risk factor for atherosclerosis. While the beta-replacement reaction catalyzed by this enzyme suggests a role for the pyridoxal phosphate, the role of the heme is uncertain. In this study we have examined the effect of changing one of the ligands to the heme on the activity of the enzyme. Binding of carbon monooxide results in the displacement of a thiolate ligand to the ferrous heme, and is accompanied by complete loss of cystathionine beta-synthase activity. Furthermore, inhibition by CO is competitive with respect to homocysteine, providing the first indication that the homocysteine binding site is in the proximity of heme. Binding of both CO and cyanide to ferrous cystathionine beta-synthase occurs in two distinct isotherms and indicates that the hemes are nonequivalent. We have employed fluorescence spectroscopy to characterize the bound PLP and its interaction with serine. PLP bound to cystathionine beta-synthase is weakly fluorescent and exists as a mixture of the protonated and unprotonated tautomers. Reaction with hydroxylamine releases the oxime and greatly enhances the associated fluorescence. Binding of serine is accompanied by a shift to the unprotonated tautomer of the external aldimine as well as the appearance of a new fluorescent species at approximately 400 nm that could be due to the aminoacrylate or to a gemdiamine intermediate. These data provide the first characterization of the PLP bound to cystathionine beta-synthase. Treatment of cystathionine beta-synthase with hydroxylamine releases two PLPs after 1 day and results in complete loss of activity. Incubation for an additional 3-4 days results in the release of two more PLPs. These data lead us to revise the PLP stoichiometry to 4 per tetramer, and to the conclusion that the heme and PLP sites in cystathionine beta-synthase are nonequivalent.
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PMID:Characterization of the heme and pyridoxal phosphate cofactors of human cystathionine beta-synthase reveals nonequivalent active sites. 1005 44


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