Human betaine-homocysteine methyltransferase is a zinc metalloenzyme

Stimulation of growth and changes in the hepatic transcriptome by 17β-estradiol in the yellow perch (Perca flavescens)

The effects of dietary 17β-estradiol (E2) on growth and liver transcriptomics were investigated in the yellow perch ( Perca flavescens). After a 3-mo treatment, E2 significantly stimulated an increase in length and weight of juvenile male and female perch relative to control animals. The increase was significantly greater in females compared with males. Separate, unnormalized cDNA libraries were constructed from equal quantities of RNA from 6 male and 6 female livers of E2-treated and control perch, and 3,546 and 3,719 expressed sequence tags (ESTs) were obtained, respectively. To characterize E2-regulated transcripts, EST frequencies between libraries were calculated within contiguous sequences that were assembled from the combined ESTs of both libraries. Frequencies were also determined in EST transcript groupings produced by aligning all of the ESTs from both libraries at the nucleotide level. From these analyses, there were 28 annotated transcripts that were regulated by 75% between libraries and for which there were at least 5 ESTs of the same transcript between libraries. Regulation of a subset ( 14 ) of these transcripts was confirmed by quantitative reverse transcription-polymerase chain reaction (QPCR). Transcripts that were upregulated by E2 included reproduction-related proteins, binding proteins, and proteases and protease inhibitors. While not part of the transcript frequency analysis, QPCR showed significant upregulation of estrogen receptor esr1 and of insulin-like growth factor I (IGF-I) in E2 livers. E2-downregulated transcripts represented a variety of functional categories including components of the respiratory chain, lipid transport and metabolism, glycolysis, amino acid and nitrogen metabolism, binding proteins, a hydrolytic enzyme, and a transcriptional regulator. In perch it appears that exogenous estrogen drastically shifts liver metabolism toward the production of lipoproteins and carbohydrate binding proteins, and that the growth-promoting action may involve an increase in hepatic IGF-I production.

Structure-activity study of new inhibitors of human betaine-homocysteine S-methyltransferase

Betaine-homocysteine S-methyltransferase (BHMT) catalyzes the transfer of a methyl group from betaine to l-homocysteine, yielding dimethylglycine and l-methionine. In this study, we prepared a new series of BHMT inhibitors. The inhibitors were designed to mimic the hypothetical transition state of BHMT substrates and consisted of analogues with NH, N(CH(3)), or N(CH(3))(2) groups separated from the homocysteine sulfur atom by a methylene, ethylene, or a propylene spacer. Only the inhibitor with the N(CH(3)) moiety and ethylene spacer gave moderate inhibition. This result led us to prepare two inhibitors lacking a nitrogen atom in the S-linked alkyl chain: (RS,RS)-5-(3-amino-3-carboxypropylthio)-3-methylpentanoic acid and (RS)-5-(3-amino-3-carboxypropylthio)-3,3-dimethylpentanoic acid. Both of these compounds were highly potent inhibitors of BHMT. The finding that BHMT does not tolerate a true betaine mimic within these inhibitors, especially the nitrogen atom, is surprising and evokes questions about putative conformational changes of BHMT upon the binding of the substrates/products and inhibitors.

Characterization of selenocysteine methyltransferases from Astragalus species with contrasting selenium accumulation capacity

A group of selenium (Se)-hyperaccumulating species belonging to the genus Astragalus are known for their capacity to accumulate up to 0.6% of their foliar dry weight as Se, with most of this Se being in the form of Se-methylselenocysteine (MeSeCys). Here, we report the isolation and molecular characterization of the gene that encodes a putative selenocysteine methyltransferase (SMT) enzyme from the non-accumulator Astragalus drummondii and biochemically compare it with an authentic SMT enzyme from the Se-hyperaccumulator Astragalus bisulcatus, a related species that lives within the same native habitat. The non-accumulator enzyme (AdSMT) shows a high degree of homology with the accumulator enzyme (AbSMT) but lacks the selenocysteine methyltransferase activity in vitro, explaining why little or no detectable levels of MeSeCys accumulation are observed in the non-accumulator plant. The insertion of mutations on the coding region of the non-accumulator AdSMT enzyme to better resemble enzymes that originate from Se accumulator species results in increased selenocysteine methyltransferase activity, but these mutations were not sufficient to fully gain the activity observed in the AbSMT accumulator enzyme. We demonstrate that SMT is localized predominantly within the chloroplast in Astragalus, the principal site of Se assimilation in plants. By using a site-directed mutagenesis approach, we show that an Ala to Thr amino acid mutation at the predicted active site of AbSMT results in a new enzymatic capacity to methylate homocysteine. The mutated AbSMT enzyme exhibited a sixfold higher capacity to methylate selenocysteine, thereby establishing the evolutionary relationship of SMT and homocysteine methyltransferase enzymes in plants.

Effect of zinc supplementation on serum homocysteine in type 2 diabetic patients with microalbuminuria

OBJECTIVES

Elevated homocysteine levels are considered to be an independent risk factor for cardiovascular complications in diabetic patients. The aim of this study was to find out if zinc supplementation improves homocysteine levels, which may exert vascular-protective effects in type 2 diabetes subjects with microalbuminuria.

METHODS

In a randomized, double-blind, controlled, crossover study, 50 type 2 diabetic patients with microalbuminuria were subdivided into two groups and supplemented with 30 mg/d of zinc (group 1) or placebo (group 2) for three months with a four-week wash out period. Serum creatinine, vitamin B(12), folate, fasting plasma glucose, HbA1c, lipid profiles, zinc, homocysteine levels and random urine albumin were measured before and after the first and second phase of the study in all participants.

RESULTS

Mean serum zinc was significantly increased after zinc supplementation (from 76 +/- 16 mug/dl to 93 +/- 20 microg/dl; p < 0.05), while there was no change in the placebo group (75 +/- 16 microg/dl to 75 +/- 15 microg/dl). With zinc supplementation, homocysteine levels reduced significantly (from 13.71 +/- 3.84 mumol/l to 11.79 +/- 3.06 mumol/l; p < 0.05), which did not occur on placebo (from 12.59 +/- 2.13 mumol/l to 13.36 +/- 2.03 mumol/l). Simple regression was used to show a positive correlation between urine albumin excretion and serum homocysteine (r = 0.37, p = 0.023). Vitamin B(12) and folate levels increased significantly in patients who received zinc in comparison to those who received placebo. A negative correlation was observed between homocysteine and vitamin B(12) concentration (r = -0.36, p = 0.025).

CONCLUSION

Zinc supplementation reduced serum homocysteine and increased vitamin B(12) and folate concentrations in type 2 diabetic patients with microalbuminuria.

A study on the differential protein profiles in liver cells of heat stress rats with and without turpentine treatment

BACKGROUND

Heat stress (HS) and related illnesses are a major concern in military, sports, and fire brigadiers. HS results in physiologic responses of increased temperature, heart rate and sweating. In heat stroke, inflammatory response plays an important role and it is evidenced that turpentine (T) induced circulating inflammatory cytokines reduced survival rate and duration at 42 degrees C. Here we report the alteration in the protein expression in liver cells upon HS with and without T treatment using two dimensional gel electrophoresis (2-DE), tryptic in-gel digestion and MALDI-TOF-MS/MS approaches.

RESULTS

The effects of HS and T treatments alone and a combined treatments (T+HS) was performed in Wistar rat models. Proteomic analysis of liver in the HS and T+HS groups were analyzed compared to liver profiles of resting control and T treated groups. The study revealed a total of 25 and 29 differentially expressed proteins in the HS and T+HS groups respectively compared to resting control group. Fourteen proteins showed altered expression upon T treatment compared to resting control group. Proteins that are involved in metabolic and signal transduction pathways, defense, redox regulation, and cytoskeletal restructuring functions were identified. The altered expression of proteins reflected in 2D gels were corroborated by quantitative real time RT-PCR analysis of 8 protein coding genes representing metabolic and regulatory pathways for their expression and normalized with the house keeping gene beta-actin.

CONCLUSION

The present study has identified a number of differentially expressed proteins in the liver cells of rats subjected to T, HS and T+HS treatments. Most of these proteins are implicated in cell metabolism, as well as adaptive response to incurred oxidative stress and tissue damage due to T+HS and HS effects.

Human betaine-homocysteine methyltransferase (BHMT) and BHMT2: common gene sequence variation and functional characterization

Betaine-homocysteine methyltransferase (BHMT) catalyzes the remethylation of homocysteine. BHMT2 encodes a protein 73% identical in amino acid sequence to BHMT, but the function of BHMT2 remains unclear. We set out to identify and functionally characterize common genetic variation in BHMT and BHMT2. Specifically, we sequenced exons, exon-intron splice junctions and the 5'-flanking regions (5'-FRs) of BHMT and BHMT2 using 240 DNA samples from four ethnic groups. Twenty-five single nucleotide polymorphisms (SNPs), including 4 nonsynonymous SNPs, and 39 SNPs, including 4 nonsynonymous, were observed in BHMT and BHMT2, respectively. BHMT wild type (WT) and variant allozymes were expressed in COS-1 cells. Variant allozymes showed no significant differences from WT in levels of enzyme activity or immunoreactive protein, but there were statistically significant differences in apparent K(m) values. Luciferase reporter gene constructs were created for the three most common BHMT 5'-FR haplotypes, and significant variation was observed in the ability of these constructs to drive transcription. Although BHMT2 mRNA has been observed in human liver and kidney, expression of the protein has not been reported. We were unable to express BHMT2 in mammalian cells, and the protein aggregated after bacterial expression. Furthermore, BHMT2 was rapidly degraded in a rabbit reticulocyte lysate, but it could be stabilized by cotransfection of COS-1 cells with BHMT and, after cotransfection, it coprecipitated with BHMT. These studies have defined common genetic variation in BHMT and BHMT2 and functionally characterized BHMT SNPs. They may also help to explain why BHMT2 has not previously been defined functionally.

Betaine-homocysteine S-methyltransferase-2 is an S-methylmethionine-homocysteine methyltransferase

We demonstrate that purified recombinant human betainehomocysteine methyltransferase-2 (BHMT-2) is a zinc metalloenzyme that uses S-methylmethionine (SMM) as a methyl donor for the methylation of homocysteine. Unlike the highly homologous betaine-homocysteine methyltransferase (BHMT), BHMT-2 cannot use betaine. The K(m) of BHMT-2 for SMM was determined to be 0.94 mm, and it has a turnover number similar to BHMT. Several compounds were tested as inhibitors of recombinant human BHMT and BHMT-2. The SMM-specific methyltransferase activity of BHMT-2 is not inhibited by dimethylglycine and betaine, whereas the former is a potent inhibitor of BHMT. Methionine is a stronger inhibitor of BHMT-2 than BHMT, and S-adenosylmethionine does not inhibit BHMT but is a weak inhibitor of BHMT-2. BHMT can use SMM as a methyl donor with a k(cat)/K(m) that is 5-fold lower than the k(cat)/K(m) for betaine. However, SMM does not inhibit BHMT activity when it is presented to the enzyme at concentrations that are 10-fold greater than the subsaturating amounts of betaine used in the assay. Based on these data, it is our current hypothesis that in vivo most if not all of the SMM-dependent methylation of homocysteine occurs via BHMT-2.

Metal active site elasticity linked to activation of homocysteine in methionine synthases

Enzymes possessing catalytic zinc centers perform a variety of fundamental processes in nature, including methyl transfer to thiols. Cobalamin-independent (MetE) and cobalamin-dependent (MetH) methionine synthases are two such enzyme families. Although they perform the same net reaction, transfer of a methyl group from methyltetrahydrofolate to homocysteine (Hcy) to form methionine, they display markedly different catalytic strategies, modular organization, and active site zinc centers. Here we report crystal structures of zinc-replete MetE and MetH, both in the presence and absence of Hcy. Structural investigation of the catalytic zinc sites of these two methyltransferases reveals an unexpected inversion of zinc geometry upon binding of Hcy and displacement of an endogenous ligand in both enzymes. In both cases a significant movement of the zinc relative to the protein scaffold accompanies inversion. These structures provide new information on the activation of thiols by zinc-containing enzymes and have led us to propose a paradigm for the mechanism of action of the catalytic zinc sites in these and related methyltransferases. Specifically, zinc is mobile in the active sites of MetE and MetH, and its dynamic nature helps facilitate the active site conformational changes necessary for thiol activation and methyl transfer.

Liver betaine-homocysteine S-methyltransferase activity undergoes a redox switch at the active site zinc

Using a redox-inert methyl acceptor, we show that betaine-homocysteine S-methyltransferase (BHMT) requires a thiol reducing agent for activity. Short-term exposure of BHMT to reducing agent-free buffer inactivates the enzyme without causing any loss of its catalytic zinc. Activity can be completely restored by the re-addition of a thiol reducing agent. The catalytic zinc of BHMT is bound by three thiolates and one hydroxyl group. Thiol modification experiments indicate that a disulfide bond is formed between two of the three zinc-binding ligands when BHMT is inactive in a reducing agent-free buffer, and that this disulfide can be readily reduced with the concomitant restoration of activity by re-establishing reducing conditions. Long-term exposure of BHMT to reducing agent-free buffer results in the slow, irreversible loss of its catalytic Zn and a corresponding loss of activity. Experiments using the glutamate-cysteine ligase modifier subunit knockout mice Gclm(-/-), which are severely impaired in glutathione synthesis, show that BHMT activity is reduced about 75% in Gclm(-/-) compared to Gclm(+/+) mice.

Does ageing affect zinc homeostasis and dietary requirements?

Dietary intakes of zinc are lower in the elderly because of reduced energy requirements, and it is not clear whether ageing impacts on adaptive homeostatic mechanisms, namely absorptive efficiency and endogenous losses in the GI tract. Physiological requirements for zinc are unlikely to change significantly, but there are several attributes of ageing that may affect aspects of zinc metabolism (e.g. changes in gut structure and function, disease states, chronic inflammation, epigenetic changes in genes that express zinc-related proteins and drug regimens) that are worthy of further investigation. There is, as yet, no information on the effects of ageing on zinc transporters, and there are no sensitive and specific measures of zinc status, therefore dietary recommendations for zinc have been derived from factorial calculations using information on zinc absorption and loss, and estimates of dietary bioavailability.

Mechanisms of protection by the betaine-homocysteine methyltransferase/betaine system in HepG2 cells and primary mouse hepatocytes

Betaine-homocysteine methyltransferase (BHMT) regulates homocysteine levels in the liver. We previously reported that the alteration of BHMT is associated with alcoholic liver steatosis and injury. In this study, we tested whether BHMT protects hepatocytes from homocysteine-induced injury and lipid accumulation. Both BHMT transfectants of HepG2 cells and primary mouse hepatocytes with suppressed BHMT were generated. Comparisons were made between the cell models with respect to their response to homocysteine treatments. Homocysteine metabolism was impaired in HepG2 cells, and the expression of BHMT in HepG2 cells ameliorated the impairment and stabilized the levels of intracellular homocysteine after the addition of exogenous homocysteine. BHMT expression inhibited homocysteine-induced glucose-regulated protein 78 (GRP78) and C/EBP-homologous protein (CHOP) and homocysteine-induced cell death. A betaine treatment protected primary mouse hepatocytes from a homocysteine-induced increase in GRP78 and cell death but not a tunicamycin-induced increase. Homocysteine induced greater CHOP expression (2.7-fold) in BHMT small interfering RNA (siRNA)-transfected cells than in a control (1.9-fold). Homocysteine-induced cell death was increased by 40% in the siRNA-treated cells in comparison with the control. Apolipoprotein B (apoB) expression was higher and triglycerides and cholesterol were lower in HepG2 expressing BHMT. In primary mouse hepatocytes, homocysteine induced the accumulation of triglycerides and cholesterol, which was reduced in the presence of betaine. Betaine partially reduced homocysteine-induced sterol regulatory element binding protein 1 expression in HepG2 cells and increased S-adenosylmethionine in primary mouse hepatocytes.

CONCLUSION

The BHMT/betaine system directly protects hepatocytes from homocysteine-induced injury but not tunicamycin-induced injury, including an endoplasmic reticulum stress response, lipid accumulation, and cell death. This system also exhibits a more generalized effect on liver lipids by inducing ApoB expression and increasing S-adenosylmethionine/S-adenosylhomocysteine.

Purification and characterization of 4-N-trimethylamino-1-butanol dehydrogenase of Pseudomonas sp. 13CM

A new enzyme, NAD+-dependent 4-N-trimethylamino-1-butanol dehydrogenase from Pseudomonas sp. 13CM, was purified 526-fold to apparent homogeneity in 5 chromatographic steps. The enzyme had a molecular mass of 45 kDa and appeared to be a monomer enzyme. The isoeletric point was found to be 4.8. The optimum temperature was 50 degrees C, and the optimum pHs for the oxidation and reduction reactions were 9.5 and 6.0 respectively. The purified enzyme was further characterized with respect to substrate specificity, kinetic parameters, and amino acid terminal sequence. The Km values for trimethylamino-1-butanol and NAD+ were 0.54 mM and 0.22 mM respectively. In the reduction reaction, the apparent Km values for trimethylaminobutylaldehyde and NADH were 0.67 mM and 0.04 mM, respectively. The enzyme was inhibited by SH reagents, chelating reagents, and heavy metal ions. The N-terminal 12 amino acid residues were sequenced.

Modeling the Metal Center of Cys4 Zinc Proteins

We present here a 67Zn solid-state NMR investigation of several model complexes of zinc coordinated by four sulfurs. The lineshapes were obtained at a variety of magnetic fields from 11.7 T (500 MHz for 1H) to 21.15 T (900 MHz for 1H) and at ambient temperature down to 10 K. The quadrupole coupling constants, Cq's, ranged from 3.25 to 16.7 MHz throughout the series, while the average bond distances only spanned 2.34-2.36 A. Reasonable agreement with experiment was achieved in the molecular orbital calculations using DFT methods and the local density approximation to predict electric field gradients.

Osmotic regulation of betaine homocysteine-S-methyltransferase expression in H4IIE rat hepatoma cells

Cell hydration changes critically affect liver metabolism and gene expression. In the course of gene expression studies using nylon cDNA-arrays we found that hyperosmolarity (405 mosmol/l) suppressed the betaine-homocysteine methyltransferase (Bhmt) mRNA expression in H4IIE rat hepatoma cells. This was confirmed by Northern blot and real-time quantitative RT-PCR analysis, which in addition unraveled a pronounced induction of Bhmt mRNA expression by hypoosmotic (205 mosmol/l) swelling. Osmotic regulation of Bhmt mRNA expression was largely paralleled at the levels of Bhmt protein and enzymatic activity. Like hyperosmotic NaCl, hyperosmotic raffinose but not hyperosmotic urea suppressed Bhmt mRNA expression, suggesting that cell shrinkage rather than increased ionic strength or hyperosmolarity per se is the trigger. Hypoosmolarity increased the expression of a reporter gene driven by the entire human BHMT promoter, whereas destabilization of BHMT mRNA was observed under hyperosmotic conditions. Osmosensitivity of Bhmt mRNA expression was impaired by inhibitors of tyrosine kinases and cyclic nucleotide-dependent kinases. The osmotic regulation of BHMT may be part of a cell volume-regulatory response and additionally lead to metabolic alterations that depend on the availability of betaine-derived methyl groups.

Biochemical and molecular characterization of the homocysteine S-methyltransferase from broccoli (Brassica oleracea var. italica)

Plants are known for their unique ability to synthesize methionine from S-methylmethionine (SMM) and homocysteine using the enzyme SMM: homocysteine S-methyltransferase (HMT) in the SMM cycle. Two cDNAs exhibiting HMT activity were cloned from broccoli and functionally expressed in E. coli. One cDNA, that encodes an enzyme with high substrate specificity for homocysteine, was designated as BoHMT1. The other cDNA was the BoSMT gene that we previously characterized and encodes a selenocysteine methyltransferase (Lyi, S.M., Heller, L.I., Rutzke, M., Welch, R.M., Kochian, L.V., Li, L., 2005. Molecular and biochemical characterization of the selenocysteine Se-methyltransferase gene and Se-methylselenocysteine synthesis in broccoli. Plant Physiol. 138, 409-420). Both exist as single gene sequences in the broccoli genome. While BoSMT expression was extremely low or undetectable in broccoli plants unless the plants were exposed to selenium, the BoHMT1 mRNA accumulated in most tissues of the plant except older leaves. In contrast to BoSMT whose expression was dramatically upregulated by treating plants with selenate, the transcript levels of BoHMT1 were not markedly affected in plants exposed to selenium. BoHMT1 expression responded significantly to changes in plant sulfur status. However, its expression was not dramatically affected in plants treated with methionine, SMM, homocysteine, or the heavy metal, cadmium. The differences in the substrate specificity and gene expression in response to changes in plant sulfur and selenium status between BoHMT1 and BoSMT suggest that the enzymes encoded by these two genes play distinct roles in sulfur and selenium metabolism in broccoli.

Does s-methyl methanethiosulfonate trap the thiol-disulfide state of proteins?

S-Methyl methanethiosulfonate (MMTS) is a reagent used to trap the natural thiol-disulfide state of the proteins. The efficiency of trapping mixed disulfides in vivo has been found to be higher for MMTS than for the more commonly used N-ethylmaleimide. MMTS has also been used for studying protein S-nitrosylation and the role of catalytic and structural cysteines on enzyme activities. However, the treatment of a protein with MMTS can potentially generate additional protein disulfide bonds. These results indicate that in vitro MMTS is able to form both intramolecular and intermolecular protein disulfide bonds in addition to dithiomethane adducts.

Zinc-promoted alkyl transfer: a new role for zinc

The roles of zinc in biology are often thought to be limited to activating water, as in hydrolytic enzymes, and conferring structure, as in the zinc finger proteins. Over the past 15 years, it has been shown that there are many zinc-containing proteins that have 'structural-like' zinc sites with multiple cysteine ligands but in which the site promotes the alkylation of a zinc-bound thiolate. Recent work continues to extend the range of proteins showing zinc-promoted alkytransfer activity, and has refined the structural details of these sites. Of particular interest are recent crystal structures suggesting that in most cases the endogenous ligand that is displaced when the substrate thiol bind is an endogenous amino acid and not water, as had been previously thought. Despite extensive study, it remains unclear whether these enzymes function via an associative mechanism (direct alkylation of a zinc-bound thiolate) or a dissociate mechanism (nucleophilic attack by a free thiolate that has dissociated from the zinc).

S-alkylated homocysteine derivatives: new inhibitors of human betaine-homocysteine S-methyltransferase

A series of S-alkylated derivatives of homocysteine were synthesized and characterized as inhibitors of human recombinant betaine-homocysteine S-methyltransferase (BHMT). Some of these compounds inhibit BHMT with IC50 values in the nanomolar range. BHMT is very sensitive to the structure of substituents on the sulfur atom of homocysteine. The S-carboxybutyl and S-carboxypentyl derivatives make the most potent inhibitors, and an additional sulfur atom in the alkyl chain is well tolerated. The respective (R,S)-5-(3-amino-3-carboxy-propylsulfanyl)-pentanoic, (R,S)-6-(3-amino-3-carboxy-propylsulfanyl)-hexanoic, and (R,S)-2-amino-4-(2-carboxymethylsulfanyl-ethylsulfanyl)-butyric acids are very potent inhibitors and are the strongest ever reported. We determined that (R,S)-5-(3-amino-3-carboxy-propylsulfanyl)-pentanoic acid displays competitive inhibition with respect to betaine binding with a Kappi of 12 nM. Some of these compounds are currently being tested in mice to study the influence of BHMT on the metabolism of sulfur amino acids in vivo.

Effects of diabetes and insulin on betaine-homocysteine S-methyltransferase expression in rat liver

Elevation of plasma homocysteine levels has been recognized as an independent risk factor for the development of cardiovascular disease, a major complication of diabetes. Plasma homocysteine reflects a balance between its synthesis via S-adenosyl-L-methionine-dependent methylation reactions and its removal through the transmethylation and the transsulfuration pathways. Betaine-homocysteine methyltransferase (BHMT, EC 2.1.1.5) is one of the enzymes involved in the remethylation pathway. BHMT, a major zinc metalloenzyme in the liver, catalyzes the transfer of methyl groups from betaine to homocysteine to form dimethylglycine and methionine. We have previously shown that plasma homocysteine levels and the transsulfuration pathway are affected by diabetes. In the present study, we found increased BHMT activity and mRNA levels in livers from streptozotocin-diabetic rats. In the rat hepatoma cell line (H4IIE cells), glucocorticoids (triamcinolone) increased the level and rate of BHMT mRNA synthesis. In the same cell line, insulin decreased the abundance of BHMT mRNA and the rate of de novo mRNA transcription of the gene. Thus the decreased plasma homocysteine in various models of diabetes could be due to enhanced homocysteine removal brought about by a combination of increased transsulfuration of homocysteine to cysteine and increased remethylation of homocysteine to methionine by BHMT.

Conformation-dependent inactivation of human betaine-homocysteine S-methyltransferase by hydrogen peroxide in vitro

Betaine-homocysteine S-methyltransferase (BHMT) transfers a methyl group from betaine to Hcy to form DMG (dimethylglycine) and Met. The reaction is ordered Bi Bi; Hcy is the first substrate to bind and Met is the last product off. Using intrinsic tryptophan fluorescence [Castro, Gratson, Evans, Jiracek, Collinsova, Ludwig and Garrow (2004) Biochemistry 43, 5341-5351], it was shown that BHMT exists in three steady-state conformations: enzyme alone, enzyme plus occupancy at the first substrate-binding site (Hcy or Met), or enzyme plus occupancy at both substrate-binding sites (Hcy plus betaine, or Hcy plus DMG). Betaine or DMG alone do not bind to the enzyme, indicating that the conformational change associated with Hcy binding creates the betaine-binding site. CBHcy [S-(d-carboxybutyl)-D,L-homocysteine] is a bisubstrate analogue that causes BHMT to adopt the same conformation as the ternary complexes. We report that BHMT is susceptible to conformation-dependent oxidative inactivation. Two oxidants, MMTS (methyl methanethiosulphonate) and hydrogen peroxide (H2O2), cause a loss of the enzyme's catalytic Zn (Zn2+ ion) and a correlative loss of activity. Addition of 2-mercaptoethanol and exogenous Zn after MMTS treatment restores activity, but oxidation due to H2O2 is irreversible. CD and glutaraldehyde cross-linking indicate that H2O2 treatment causes small perturbations in secondary structure but no change in quaternary structure. Oxidation is attenuated when both binding sites are occupied by CBHcy, but Met alone has no effect. Partial digestion of ligand-free BHMT with trypsin produces two large peptides, excising a seven-residue peptide within loop L2. CBHcy but not Met binding slows down proteolysis by trypsin. These findings suggest that L2 is involved in the conformational change associated with occupancy at the betaine-binding site and that this conformational change and/or occupancy at both ligand-binding sites protect the enzyme from oxidative inactivation.

Modulation of inhibitory glycine receptors in cultured embryonic mouse hippocampal neurons by zinc, thiol containing redox agents and carnosine

Modulation of inhibitory glycine receptors by zinc (Zn(2+)) and endogenous redox agents such as glutathione may alter inhibition in the mammalian brain. Despite the abundance of Zn(2+) in the hippocampus and its ability to modulate glycine receptors, few studies have examined Zn(2+) modulation of hippocampal glycine receptors. Whether redox agents modulate hippocampal glycine receptors also remains unknown. This study examined Zn(2+) and redox modulation of glycine receptor-mediated currents in cultured embryonic mouse hippocampal neurons using whole-cell recordings. Zn(2+) concentrations below 10 microM potentiated currents elicited by low glycine, beta-alanine, and taurine concentrations by 300-400%. Zn(2+) concentrations above 300 microM produced nearly complete inhibition. Potentiating Zn(2+) concentrations shifted the dose-response curves for the three agonists to the left and decreased the Hill coefficient for glycine and beta-alanine but not taurine. Inhibiting Zn(2+) concentrations shifted the dose-response curves for glycine and beta-alanine to the right but reduced the maximum taurine response. Histidine residues may participate in potentiation because diethyl pyrocarbonate and pH 5.4 diminished Zn(2+) enhancement of glycine currents. pH 5.4 diminished Zn(2+) block of glycine currents, but diethyl pyrocarbonate did not. These findings indicate that separate sites mediate Zn(2+) potentiation and inhibition. The redox agents glutathione, dithiothreitol, tris(2-carboxyethyl)phosphine, and 5,5'-dithiobis(2-nitrobenzoic acid) did not alter glycine currents by a redox mechanism. However, glutathione and dithiothreitol interfered with the effects of Zn(2+) on glycine currents by chelating it. Carnosine had similar effects. Thus, Zn(2+) and thiol containing redox agents that chelate Zn(2+) modulate hippocampal glycine receptors with the mechanism of Zn(2+) modulation being agonist dependent.

Rat liver betaine-homocysteine S-methyltransferase equilibrium unfolding: insights into intermediate structure through tryptophan substitutions

Equilibrium folding of rat liver BHMT (betaine-homocysteine methyltransferase), a TIM (triosephosphate isomerase)-barrel tetrameric protein, has been studied using urea as denaturant. A combination of activity measurements, tryptophan fluorescence, CD and sedimentation-velocity studies suggested a multiphasic process including two intermediates, a tetramer (I4) and a monomer (J). Analysis of denaturation curves for single- and six-tryptophan mutants indicated that the main changes leading to the tetrameric intermediate are related to alterations in the helix alpha4 of the barrel, as well as in the dimerization arm. Further dissociation to intermediate J included changes in the loop connecting the C-terminal alpha-helix of contact between dimers, disruption of helix alpha4, and initial alterations in helix alpha7 of the barrel, as well as in the dimerization arm. Evolution of the monomeric intermediate continued through additional perturbations in helix alpha7 of the barrel and the C-terminal loop. Our data highlight the essential role of the C-terminal helix in dimer-dimer binding through its contribution to the increased stability shown by BHMT as compared with other TIM barrel proteins. The results are discussed in the light of the high sequence conservation shown by betaine-homocysteine methyltransferases and the knowledge available for other TIM-barrel proteins.

Genetic variation of folate-mediated one-carbon transfer pathway predicts susceptibility to choline deficiency in humans

Choline is a required nutrient, and some humans deplete quickly when fed a low-choline diet, whereas others do not. Endogenous choline synthesis can spare some of the dietary requirement and requires one-carbon groups derived from folate metabolism. We examined whether major genetic variants of folate metabolism modify susceptibility of humans to choline deficiency. Fifty-four adult men and women were fed diets containing adequate choline and folate, followed by a diet containing almost no choline, with or without added folate, until they were clinically judged to be choline-deficient, or for up to 42 days. Criteria for clinical choline deficiency were a more than five times increase in serum creatine kinase activity or a >28% increase of liver fat after consuming the low-choline diet that resolved when choline was returned to the diet. Choline deficiency was observed in more than half of the participants, usually within less than a month. Individuals who were carriers of the very common 5,10-methylenetetrahydrofolate dehydrogenase-1958A gene allele were more likely than noncarriers to develop signs of choline deficiency (odds ratio, 7.0; 95% confidence interval, 2.0-25; P < 0.01) on the low-choline diet unless they were also treated with a folic acid supplement. The effects of the C677T and A1298C polymorphisms of the 5,10-methylene tetrahydrofolate reductase gene and the A80C polymorphism of the reduced folate carrier 1 gene were not statistically significant. The most remarkable finding was the strong association in premenopausal women of the 5,10-methylenetetrahydrofolate dehydrogenase-1958A gene allele polymorphism with 15 times increased susceptibility to developing organ dysfunction on a low-choline diet.

Homocysteine and redox signaling

Homocysteine is a thiol-containing amino acid that has gained notoriety because its elevation in the plasma is correlated with complex and multifactorial diseases, including cardiovascular diseases, neurodegenerative diseases, and neural tube defects. Homocysteine is redox-active, and its toxic effects have been frequently attributed to direct or indirect perturbation of redox homeostasis. Although the literature on the pathophysiology of elevated homocysteine is rather extensive, a very wide range of concentrations of this amino acid has been used in these studies ranging from normal to pathophysiological to unphysiological. It is clear that homocysteine induces varied responses that are specific to cell type and that cells, depending on their origin, display a wide range of sensitivity to homocysteine. In this review, we focus on the redox signaling pathways that have been connected to homocysteine in vascular (endothelial and smooth muscle) cells and in neuronal cells. We also discuss redox regulation of the key enzymes involved in homocysteine clearance: methionine synthase, betaine-homocysteine methyltranferase, and cystathionine beta-synthase.

A nuclear-magnetic-resonance-based assay for betaine-homocysteine methyltransferase activity

Betaine-homocysteine methyltransferase (BHMT) activity can be measured directly and kinetically by (1)H-nuclear magnetic resonance spectroscopy. The disappearance of substrates and the formation of products are monitored simultaneously. Alternative substrates, separately and when mixed with glycine betaine, can also be monitored. Each assay can be completed in 1h. Using 2mM glycine betaine and homocysteine as substrates in 20 mM phosphate buffer (pH 7.5) and measuring the production of N,N-dimethylglycine, the CV is 6.3% (n=6) and the detection limit is 6 nkatal. An endpoint assay for BHMT activity was also developed, by measuring the N,N-dimethylglycine produced after incubation with 2 mM glycine betaine and homocysteine (CV=5.3%, n = 6) with a detection limit of 2 nkatal. These assays were used to show that the natural betaines trigonelline, proline betaine, arsenobetaine, and l-carnitine are neither substrates nor significant inhibitors of rat liver BHMT, that the thetins dimethylthetin and dimethylsulfoniopropionate are substrates and inhibit methyl transfer from glycine betaine, and that the K(m) for glycine betaine is 0.19+/-0.03 mM with a V(max) of 17+/-0.7 nMol min(-1) mg(-1).

Homocysteine, glycine betaine, and N,N-dimethylglycine in patients attending a lipid clinic

We recruited nondiabetic subjects (n = 158) attending a lipid disorders clinic, a subset of whom (n = 46) had established cardiovascular disease. Glycine betaine, N,N-dimethylglycine, and carnitine were measured in fasting plasma and urine samples. The concentrations and excretions were related to known cardiovascular risk factors in multivariate regression models. The relationships between homocysteine and plasma and urinary glycine betaine were highly significant (P < .002), comparable with the known relationships with folate and plasma creatinine. The regression coefficient for plasma glycine betaine was consistently approximately -0.1 in 5 different regression models (3 best-subsets and forward and backward stepwise regression models) for predicting homocysteine using 23 variables. Plasma glycine betaine was higher in males than in females, and the difference was associated with a difference in percentage of body fat. Its concentration included a constant factor of approximately 20 micromol/L that was independent of any of the variables investigated here. In the total group, body fat, homocysteine, and carnitine were significant predictors of plasma glycine betaine. Carnitine, an important betaine that is involved in lipid metabolism positively correlated with both homocysteine and glycine betaine. In our sample, the urinary excretion of glycine betaine was outside the reference range in 14 of the 158 subjects and the betaine fractional clearances were above the reference range in 23 subjects. Fractional clearance correlated strongly with plasma homocysteine (r = 0.50), and this relationship may be stronger in patients with known vascular disease. Urinary loss of glycine betaine may contribute to hyperhomocysteinemia and the development of cardiovascular disease.

In vitro modification of betaine-homocysteine S-methyltransferase by tissue-type transglutaminase

Transglutaminases catalyze the cross-linking and amine incorporation of proteins, and are implicated in various biological phenomena. To elucidate the physiological roles of transglutaminase at the molecular level, we need to identify its physiological protein substrates and clarify the relationship between transglutaminase modification of protein substrates and biological responses. Here we examined whether betaine-homocysteine S-methyltransferase (BHMT: EC 2.1.1.5) can be a substrate of tissue-type transglutaminase by in vitro experiments using porcine liver BHMT and guinea pig liver transglutarninase. Guinea pig liver transglutaminase incorporated 5-(biotinamido) pentylamine and [3H] histamine into BHMT in a time-dependent manner. Putrescine and spermidine also seemed to be incorporated into BHMT by transglutaminase. In the absence of the primary amines, BHMT subunits were cross-linked intra- and intermolecularly. BHMT activity was decreased significantly through the cross-linking by transglutaminase. Histamine incorporation slightly reduced the BHMT activity. Peptide fragments of BHMT containing the glutamine residues reactive for transglutaminase reaction were isolated through biotin labelling, proteinase digestion, biotin-avidin a affinity separation, and reverse phase HPLC. The results of amino acid sequence analyses of these peptides and sequence homology alignment with other mammalian liver BHMT subunits showed that these reactive glutamine residues were located in the region near the carboxyl terminal of porcine BHMT subunit. These results suggested that the liver BHMT can be modified by tissue-type transglutaminase and its activity is regulated repressively by the modification, especially by the cross-linking. This regulatory reaction might be involved in the regulation of homocysteine metabolism in the liver.

Betaine in human nutrition

Betaine is distributed widely in animals, plants, and microorganisms, and rich dietary sources include seafood, especially marine invertebrates ( approximately 1%); wheat germ or bran ( approximately 1%); and spinach ( approximately 0.7%). The principal physiologic role of betaine is as an osmolyte and methyl donor (transmethylation). As an osmolyte, betaine protects cells, proteins, and enzymes from environmental stress (eg, low water, high salinity, or extreme temperature). As a methyl donor, betaine participates in the methionine cycle-primarily in the human liver and kidneys. Inadequate dietary intake of methyl groups leads to hypomethylation in many important pathways, including 1) disturbed hepatic protein (methionine) metabolism as determined by elevated plasma homocysteine concentrations and decreased S-adenosylmethionine concentrations, and 2) inadequate hepatic fat metabolism, which leads to steatosis (fatty accumulation) and subsequent plasma dyslipidemia. This alteration in liver metabolism may contribute to various diseases, including coronary, cerebral, hepatic, and vascular diseases. Betaine has been shown to protect internal organs, improve vascular risk factors, and enhance performance. Databases of betaine content in food are being developed for correlation with population health studies. The growing body of evidence shows that betaine is an important nutrient for the prevention of chronic disease.

Oligomerization is required for betaine-homocysteine S-methyltransferase function

Betaine-homocysteine methyltransferase (BHMT) is a member of a family (Pfam 02574) of zinc- and thiol/selenol-dependent methyltransferases. All family members purified to date are monomers, except BHMT, which is an oligomer. We have studied how C-terminal truncation or mutagenic replacement of residues within or associated with the unique dimerization arm of this enzyme affects oligomerization and function. Two C-terminal truncation mutants, S325 and D371, do not express well in Escherichia coli and are inactive. Residues within the dimerization arm (H338, R346, W352, R361, P362, Y363, N364, and P365) and one that forms a hydrogen bond to the arm (E266) were changed to alanine. All mutants maintained a normal or near-normal ability to bind zinc. E266A, R361A, P362A, Y363A, N364A, and P365A displayed near-normal catalytic activity, but H338A had only 10% of the wild-type enzyme activity. Like the wild-type enzyme, most mutants eluted as tetramers from gel filtration columns and formed discrete bands on SDS-PAGE gels following glutaraldehyde crosslinking. Mutants R346A and W352A had negligible activity, eluted as dimers, and displayed aberrant crosslinking properties. These data indicate that unlike other Pfam 02574 members, oligomerization of BHMT is required for function.

Crystal Structure of Rat Liver Betaine Homocysteine S-Methyltransferase Reveals New Oligomerization Features and Conformational Changes Upon Substrate Binding

Betaine homocysteine S-methyltransferase (BHMT) is one of the two enzymes known to methylate homocysteine to generate methionine in the liver. It presents a Zn(2+) atom linked to three essential Cys residues. The crystal structure of rat liver BHMT has been solved at 2.5A resolution, using crystals with P2(1) symmetry and 45% solvent content in the cell. The asymmetric unit contains the whole functional tetramer showing point symmetry 222. The overall fold of the subunit consists mostly of a (alpha/beta)(8) barrel, as for human BHMT. From the end of the barrel, the polypeptide chain extends away and makes many interactions with a different subunit, forming tight dimers. The most remarkable structural feature of rat liver BHMT is the presence of a helix including residues 381-407, at the C terminus of the chain, which bind together the dimers AB to CD. A strong ion-pair and more than 60 hydrophobic interactions keep this helix stacked to the segment 316-349 from the opposite subunit. Moreover, the crystal structure of free rat liver BHMT clearly shows that Tyr160 is the fourth ligand coordinated to Zn, which is replaced by Hcy upon binding. Two residues essential for substrate recognition, Phe76 and Tyr77, are provided by a conformational change in a partially disordered loop (L2). The crucial role of these residues is highlighted by site-directed mutagenesis.

Effects of betaine in a murine model of mild cystathionine-beta-synthase deficiency

Cystathionine-beta-synthase (CBS) is required for transsulfuration of homocysteine, an amino acid implicated in vascular disease. We studied homocysteine metabolism in mice with mild hyperhomocysteinemia due to a heterozygous disruption of the Cbs gene. Mice were fed diets supplemented with betaine or dimethylsulfonioacetate (DMSA); betaine and DMSA provide methyl groups for an alternate pathway of homocysteine metabolism, remethylation by betaine:homocysteine methyltransferase (BHMT). On control diets, heterozygous mice had 50% higher plasma homocysteine than did wild-type mice. Betaine and DMSA had similar effects in both genotype groups: liver betaine increased dramatically, while plasma homocysteine decreased by 40% to 50%. With increasing betaine supplementation, homocysteine decreased by 75%. Plasma homocysteine and BHMT activity both showed a strong negative correlation with liver betaine. Homocysteinemia in mice is sensitive to a disruption of Cbs and to methyl donor intake. Because betaine leads to a greater flux through BHMT and lowers homocysteine, betaine supplementation may be beneficial in mild hyperhomocysteinemia.

Dissecting the Catalytic Mechanism of Betaine−HomocysteineS-Methyltransferase by Use of Intrinsic Tryptophan Fluorescence and Site-Directed Mutagenesis†

Betaine-homocysteine S-methyltransferase (BHMT) is a zinc-dependent enzyme that catalyzes the transfer of a methyl group from glycine betaine (Bet) to homocysteine (Hcy) to form dimethylglycine (DMG) and methionine (Met). Previous studies in other laboratories have indicated that catalysis proceeds through the formation of a ternary complex, with a transition state mimicked by the inhibitor S-(delta-carboxybutyl)-l-homocysteine (CBHcy). Using changes in intrinsic tryptophan fluorescence to determine the affinity of human BHMT for substrates, products, or CBHcy, we now demonstrate that the enzyme-substrate complex reaches its transition state through an ordered bi-bi mechanism in which Hcy is the first substrate to bind and Met is the last product released. Hcy, Met, and CBHcy bind to the enzyme to form binary complexes with K(d) values of 7.9, 6.9, and 0.28 microM, respectively. Binary complexes with Bet and DMG cannot be detected with fluorescence as a probe, but Bet and DMG bind tightly to BHMT-Hcy to form ternary complexes with K(d) values of 1.1 and 0.73 microM, respectively. Mutation of each of the seven tryptophan residues in human BHMT provides evidence that the enzyme undergoes two distinct conformational changes that are reflected in the fluorescence of the enzyme. The first is induced when Hcy binds, and the second, when Bet binds. As predicted by the crystal structure of BHMT, the amino acids Trp44 and Tyr160 are involved in binding Bet, and Glu159 in binding Hcy. Replacing these residues by site-directed mutagenesis significantly reduces the catalytic efficiency (V(max)/K(m)) of the enzyme. Replacing Tyr77 with Phe abolishes enzyme activity.

Betaine analogues alter homocysteine metabolism in rats

Glycine betaine supplementation lowers homocysteine levels in homocystinuria and in chronic renal failure patients through methylation catalysed by betaine-homocysteine methyltransferase (BHMT). The aim of this study was to determine the effect of glycine betaine analogues on homocysteine metabolism in Lewis rats. Glycine betaine, proline betaine, trigonelline, dimethylsulfoniopropionate (DMSP) or dimethylthetin (1.5 mmoles) was subcutaneously administered to rats fed a low betaine diet. The effect of each betaine on total plasma homocysteine and urinary and plasma betaine concentrations was monitored for 24h following administration. Baseline plasma homocysteine was 8.5 +/- micromol/l (S.E.M., n=44) and compared to controls concentrations decreased following glycine betaine (0.8+/-0.4 micromol/l, P = 0.064), DMSP (1.0+/-0.5 micromol/l, P = 0.041) and dimethylthetin (1.5 +/- 0.7micromol/l, P = 0.033) treatment, while concentrations increased following proline betaine (2.24 +/-0.7micromol/l, P = 0.002) and trigonelline (1.6 +/-0.3 micromol/l, P < 0.001) treatment. The effect of glycine betaine, DMSP and dimethylthetin on circulating homocysteine concentrations was thought to be mediated by BHMT in vivo. This hypothesis was supported by the finding that circulating glycine betaine concentrations increased following DMSP and dimethylthetin treatment. Proline betaine and trigonelline appeared to be poor BHMT substrates, being largely excreted in the urine unchanged, yet increased circulating homocysteine levels. This suggests they are inhibitors of BHMT. Urinary excretion of glycine betaine increased following treatment with all betaines, suggesting that the resorption of glycine betaine in the kidney was inhibited. The study shows that glycine betaine analogues have multiple effects on homocysteine metabolism (250).

Vitamin B-12 deficiency induces anomalies of base substitution and methylation in the DNA of rat colonic epithelium

Derangements of one-carbon metabolism can directly affect the integrity of the genome by producing inappropriate uracil insertion into DNA and by altering patterns of DNA methylation. Vitamin B-12, a one-carbon nutrient, serves as a cofactor in the synthesis of precursors of biological methylation and in nucleotide synthesis. We therefore examined whether vitamin B-12 deficiency can induce these molecular anomalies in the colonic mucosa of rats. Weanling male Sprague-Dawley rats (n = 30) were divided into 2 groups and fed either a vitamin B-12-deficient diet or a similar diet containing adequate amounts of the vitamin. Rats from each group were killed at 6 and 10 wk. Uracil misincorporation into DNA was measured by GC/MS and genomic DNA methylation was measured by LC/MS. Plasma vitamin B-12 concentrations in deficient rats were below detectable limits at 6 and 10 wk; in control rats, concentrations were 0.46 +/- 0.07 and 0.42 +/- 0.10 nmol/L at those times. Although the colon total folate concentration did not differ between the groups, the proportion that was methylfolate was marginally greater in the deficient rats at 10 wk (P = 0.05) compared with control, consistent with the "methylfolate trap" that develops during vitamin B-12 deficiency. After 10 wk, the colonic DNA of the deficient rats displayed a 35% decrease in genomic methylation and a 105% increase in uracil incorporation (P < 0.05). This vitamin B-12-deficient diet, which was of insufficient severity to cause anemia or illness, created aberrations in both base substitution and methylation of colonic DNA, which might increase susceptibility to carcinogenesis.

Structural characterization of the zinc site in protein farnesyltransferase

X-ray absorption spectroscopy has been used to determine the structure of the Zn site in protein farnesyltransferase. Extended X-ray absorption fine structure (EXAFS) data are consistent with a Zn site that is ligated to three low-Z (oxygen or nitrogen) ligands and one cysteine sulfur, as predicted from the crystal structures that are available for farnesyltransferase. However, in contrast with the crystallographic results the EXAFS data do not show evidence for significant distortions in the Zn-ligand distances. The average Zn-(N/O) and Zn-S distances are 2.04 and 2.31 A, respectively. Addition of a farnesyl diphosphate analogue causes no detectable change in the structure of the Zn site. However, addition of peptide substrate causes a change in ligation from ZnS(N/O)(3) to ZnS(2)(N/O)(2), consistent with ligation of the C-terminal cysteine to the Zn. There is no significant change in Zn-ligand distances when a substrate binds, demonstrating that the Zn remains four-coordinate. Addition of both peptide and farnesyl diphosphate to give the product complex causes the Zn to return to ZnS(N/O)(3) ligation, indicating that the product thioether is not tightly coordinated to the Zn. These spectroscopic experiments provide insight into the catalytic mechanism of FTase.

Betaine homocysteine methyltransferase: gene cloning and expression analysis in rat liver cirrhosis

It has been known for over half a century that homocysteine levels are elevated in liver cirrhosis, but the basis for it is not fully understood. Using differential display, we identified betaine homocysteine methyltransferase (BHMT) as a gene down-regulated in rat liver cirrhosis and most likely involved in this dysregulation. A partial BHMT clone was isolated by screening of a cDNA library with the differential display fragment. The full-length gene was generated by primer extension of cDNA. Expression levels of BHMT in cirrhotic livers of bile duct ligated rats were compared to controls by Northern and Western blotting as well as by enzyme activity measurements. BHMT mRNA levels were reduced to 29+/-23% in established liver cirrhosis induced by bile duct ligation (BDL) as compared to controls. Enzyme assays in crude liver homogenates showed a similar reduction in BHMT activity in bile duct ligated rat livers. By Western blotting, BHMT could be detected in crude liver homogenates of control animals, but was reduced to below the limit of detection in cirrhotic livers. In conclusion, these findings establish a reduced BHMT enzyme activity in cirrhotic rat livers, which may explain the elevated plasma homocysteine levels in cirrhosis.

Concentrations of choline-containing compounds and betaine in common foods

Choline is important for normal membrane function, acetylcholine synthesis and methyl group metabolism; the choline requirement for humans is 550 mg/d for men (Adequate Intake). Betaine, a choline derivative, is important because of its role in the donation of methyl groups to homocysteine to form methionine. In tissues and foods, there are multiple choline compounds that contribute to total choline concentration (choline, glycerophosphocholine, phosphocholine, phosphatidylcholine and sphingomyelin). In this study, we collected representative food samples and analyzed the choline concentration of 145 common foods using liquid chromatography-mass spectrometry. Foods with the highest total choline concentration (mg/100 g) were: beef liver (418), chicken liver (290), eggs (251), wheat germ (152), bacon (125), dried soybeans (116) and pork (103). The foods with the highest betaine concentration (mg/100 g) were: wheat bran (1339), wheat germ (1241), spinach (645), pretzels (237), shrimp (218) and wheat bread (201). A number of epidemiologic studies have examined the relationship between dietary folic acid and cancer or heart disease. It may be helpful to also consider choline intake as a confounding factor because folate and choline methyl donation can be interchangeable.

Investigations of a common genetic variant in betaine-homocysteine methyltransferase (BHMT) in coronary artery disease

Hyperhomocysteinemia, a risk factor for cardiovascular disease, can be caused by genetic mutations in enzymes of homocysteine metabolism. Homocysteine remethylation to methionine is catalyzed by folate-dependent methionine synthase, or by betaine-homocysteine methyltransferase (BHMT), which utilizes betaine as the methyl donor. Since genetic variants in folate-dependent remethylation have been reported to increase risk for cardiovascular disease and other common disorders, we screened BHMT for sequence changes that might alter risk for coronary artery disease (CAD). A variant in exon 6-R239Q-was identified. The frequency of this change was examined in 504 individuals who had undergone coronary angiography and were stratified into controls (those with no or mild disease) and cases (those with significant [>50% reduction in luminal diameter stenosis] 1-, 2-, 3-vessel disease). Although this variant did not affect plasma homocysteine, the QQ genotype was present in higher frequency in those with no or mild disease, compared with those with significant disease (11 vs. 6%), suggesting that it may decrease risk of CAD; a statistically-significant decrease was seen in the older subjects (13 vs. 7%). Multivariate analysis for the entire group revealed an odds ratio of 0.48 (95% CI: 0.21-1.06) for the QQ genotype; this association was similar in the younger (OR=0.36; 95% CI: 0.09-1.41) and older subjects (OR=0.42; 95% CI: 0.15-1.18). Our study suggests that the Q allele of the R239Q mutation may decrease the risk of CAD and that this variant warrants additional investigation of its relationship with the development of CAD as well as other homocysteine-dependent disorders.

Active-site-mutagenesis study of rat liver betaine-homocysteine S-methyltransferase

A site-directed-mutagenesis study of putative active-site residues in rat liver betaine-homocysteine S-methyltransferase has been carried out. Identification of these amino acids was based on data derived from a structural model of the enzyme. No alterations in the CD spectra or the gel-filtration chromatography elution pattern were observed with the mutants, thus suggesting no modification in the secondary structure content or in the association state of the proteins. All the mutants obtained showed a reduction of the enzyme activity, the most dramatic effect being that of Glu(159), followed by Tyr(77) and Asp(26). Changes in affinity for either of the substrates, homocysteine or betaine, were detected when substitutions were performed of Glu(21), Asp(26), Phe(74) and Cys(186). Interestingly, Asp(26), postulated to be involved in homocysteine binding, has a strong effect on affinity for betaine. The relevance of these results is discussed in the light of very recent structural data obtained for the human enzyme.

Homocysteine-betaine interactions in a murine model of 5,10-methylenetetrahydrofolate reductase deficiency

Hyperhomocysteinemia, a proposed risk factor for cardiovascular disease, is also observed in other common disorders. The most frequent genetic cause of hyperhomocysteinemia is a mutated methylenetetrahydrofolate reductase (MTHFR), predominantly when folate status is impaired. MTHFR synthesizes a major methyl donor for homocysteine remethylation to methionine. We administered the alternate choline-derived methyl donor, betaine, to wild-type mice and to littermates with mild or severe hyperhomocysteinemia due to hetero- or homozygosity for a disruption of the Mthfr gene. On control diets, plasma homocysteine and liver choline metabolite levels were strongly dependent on the Mthfr genotype. Betaine supplementation decreased homocysteine in all three genotypes, restored liver betaine and phosphocholine pools, and prevented severe steatosis in Mthfr-deficient mice. Increasing betaine intake did not further decrease homocysteine. In humans with cardiovascular disease, we found a significant negative correlation between plasma betaine and homocysteine concentrations. Our results emphasize the strong interrelationship between homocysteine, folate, and choline metabolism. Hyperhomocysteinemic Mthfr-compromised mice appear to be much more sensitive to changes of choline/betaine intake than do wild-type animals. Hyperhomocysteinemia, in the range of that associated with folate deficiency or with homozygosity for the 677T MTHFR variant, may be associated with disturbed choline metabolism.

Combining combinatorial chemistry and affinity chromatography: highly selective inhibitors of human betaine: homocysteine S-methyltransferase

A new method to find novel protein targets for ligands of interest is proposed. The principle of this approach is based on affinity chromatography and combinatorial chemistry. The proteins within a crude rat liver homogenate were allowed to interact with a combinatorial library of phosphinic pseudopeptides immobilized on affinity columns. Betaine: homocysteine S-methyltransferase (BHMT) was one of the proteins that was retained and subsequently eluted from these supports. The phosphinic pseudopeptides, which served as immobilized ligands for the isolation of rat BHMT, were then tested for their ability to inhibit human recombinant BHMT in solution. The most potent inhibitor also behaved as a selective ligand for the affinity purification of BHMT from a complex media. Further optimization uncovered Val-Phe-psi[PO(2-)-CH(2)]-Leu-His-NH(2) as a potent BHMT inhibitor that has an IC(50) of about 1 microM.

Synthesis of betaine-homocysteine S-methyltransferase is continuously enhanced in fatty livers of thyroidectomized chickens

We examined thyroidectomized chickens in terms of plasma lipid concentration and protein expression within the liver. Although the body weight of thyroidectomized chickens was remarkably low due to growth retardation, the livers were enlarged and fatty compared to those of sham-operated chickens. An increase in phospholipid, triglyceride, and total cholesterol levels within the blood plasma of thyroidectomized chickens was observed, clearly reflecting increased lipid synthesis within the liver. Overexpression of some proteins, for example, 29- and 45-kDa proteins, was observed in thyroidectomized chicken livers by means of electrophoresis. A peptide map was made for the protein that exhibited the greatest degree of overexpression. One of them demonstrated a molecular mass of 45 kDa and an isoelectric point (pI) between 7.5 and 8.0, depending on its form. Partial N-terminal amino acid sequences were determined from three random peptides of this protein. The amino acid sequence of this protein showed a high degree of homology with the betaine-homocysteine S-methyltransferase (BHMT, EC 2.1.1.5) of some mammalian species. We identified this protein as chicken BHMT because, in addition to its sequence homology with mammalian BHMT, there were similarities were also observed between this 45-kDa protein and mammalian BHMT with respect to molecular mass and isoelectric behavior. In the liver, 10 d after thyroidectomy, the synthesis of hepatic BHMT had already been enhanced, and the high expression was maintained at 50 d of age. Generally, BHMT catalyzes the transfer of a methyl group from betaine to L-homocysteine. In addition, it seems that this enzyme is also closely related to lipid metabolism in the liver; in this study expression of BHMT in the liver corresponded to plasma lipid levels. Moreover, hypothyroidism may be directly or indirectly related to overexpression of BHMT. Due to similarities between the BHMT of chickens and mammalian species, the chicken model might provide a useful means by which to study BHMT, its role in lipid metabolism, and methods of targeting the expression of BHMT. Another 29-kDa protein was unidentified in the homology search.

Pharmacokinetics of oral betaine in healthy subjects and patients with homocystinuria

AIMS

Large oral doses of betaine have proved effective in lowering plasma homocysteine in severe hyperhomocysteinaemia. The pharmacokinetic characteristics and metabolism of betaine in humans have not been assessed and drug monitoring for betaine therapy is not available. We studied the pharmacokinetics of betaine and its metabolite dimethylglycine (DMG) in healthy subjects and in three patients with homocystinuria.

METHODS

Twelve male volunteers underwent an open-label study. After one single administration of 50 mg betaine kg-1 body weight and during continuous intake of twice daily 50 mg kg-1 body weight, serial blood samples and 24 h urines were collected to determine betaine and DMG plasma concentrations and urinary excretion, respectively. Patients were evaluated after one single dose of betaine.

RESULTS

We found rapid absorption (t(1/2),abs 00.28 h, s.d. 0.17) and distribution (t(1/2), lambda1 00.59 h, s.d. 0.22) of betaine. A Cmax of 0.94 mmol l-1 (s.d. 0.19) was reached after tmax 00.90 h (s.d. 0.33). The elimination half life t(1/2), z was 14.38 h (s.d. 7.17). After repeated dosage, t(1/2), lambda1 (01.77 h, s.d. 0.75) and t(1/2), z (41.17 h, s.d. 13.50) increased significantly (95% CI 0.73, 01.64 h and 19.90, 33.70 h, respectively), whereas absorption remained unchanged. DMG concentrations increased significantly after betaine administration and accumulation occurred to the same extent as with betaine. Renal clearance was low and urinary excretion of betaine was equivalent to 4% of the ingested dose. Distribution and elimination kinetics in homocystinuric patients appeared to be accelerated.

CONCLUSIONS

Betaine plasma concentrations change rapidly after ingestion. Elimination half-life increased during continuous dosing over 5 days. Betaine is mainly eliminated by metabolism. More pharmacokinetic and pharmacodynamic studies in hyperhomocysteinaemic patients are needed to refine the current treatment with betaine.

Genetic effects of methylation diets

DNA methylation at cytosines in CpG dinucleotides can lead to changes in gene expression and function without altering the primary sequence of the DNA. Methylation can be affected by dietary levels of methyl-donor components, such as folic acid. This may be an important mechanism for environmentally induced changes in gene expression. Recent literature supports a role for DNA-methylation changes in a number of adult-onset disorders and during development. These changes may be significant for better understanding certain birth defects (e.g., neural tube defects) and the long-term consequences of early environmental influences on gene expression (metabolic programming). Optimal "methylation diets" should be investigated as part of the prevention and treatment of all these conditions, as well as in disorders such as Rett syndrome, whose primary defects may lie in DNA methylation-dependent gene regulation.

Betaine-homocysteine methyltransferase: zinc in a distorted barrel

Betaine-homocysteine methyl transferase (BHMT) catalyzes the synthesis of methionine from betaine and homocysteine (Hcy), utilizing a zinc ion to activate Hcy. BHMT is a key liver enzyme that is important for homocysteine homeostasis. X-ray structures of human BHMT in its oxidized (Zn-free) and reduced (Zn-replete) forms, the latter in complex with the bisubstrate analog, S(delta-carboxybutyl)-L-homocysteine, were determined at resolutions of 2.15 A and 2.05 A. BHMT is a (beta/alpha)(8) barrel that is distorted to construct the substrate and metal binding sites. The zinc binding sequences G-V/L-N-C and G-G-C-C are at the C termini of strands beta6 and beta8. Oxidation to the Cys217-Cys299 disulfide and expulsion of Zn are accompanied by local rearrangements. The structures identify Hcy binding fingerprints and provide a prototype for the homocysteine S-methyltransferase family.

Maternal methyl supplements in mice affect epigenetic variation and DNA methylation of offspring

This study was designed to determine if maternal dietary methyl supplements increase DNA methylation and methylation-dependent epigenetic phenotypes in mammalian offspring. Female mice of two strains were fed two levels of dietary methyl supplement or control diet prior to and during pregnancy. Offspring of these mice vary in phenotype, which is epigenetically determined and affects health and 2-y survival. Phenotype and DNA methylation of a long terminal repeat (LTR) controlling expression of the agouti gene were assayed in the resulting offspring. Methyl supplements increase the level of DNA methylation in the agouti LTR and change the phenotype of offspring in the healthy, longer-lived direction. This shows that methyl supplements have strong effects on DNA methylation and phenotype and are likely to affect long-term health. Optimum dietary supplements for the health and longevity of offspring should be intensively investigated. This should lead to public policy guidance that teaches optimal, rather than minimal, dose levels of maternal supplements.