Nutritional interventions to prevent and treat osteoarthritis. Part II: focus on micronutrients and supportive nutraceuticals

Osteoarthritis (OA) is the most common cause of musculoskeletal disability in the elderly, and it places an enormous economic burden on society, which will remain a major health care challenge with an aging population. Management of OA is primarily focused on palliative relief using agents such as nonsteroidal anti-inflammatory drugs (NSAID) and analgesics. However, such an approach is limited by a narrow therapeutic focus that fails to address the progressive and multimodal nature of OA. Given the favorable safety profile of most nutritional interventions, identifying disease-modifying pharmaconutrients capable of improving symptoms and also preventing, slowing, or even reversing the degenerative process in OA should remain an important paradigm in translational and clinical research. The goals of pharmaconutrition for metabolic optimization are to drive biochemical reactions in a desired direction and to meet health condition-specific metabolic demands. Applying advances in nutritional science to musculoskeletal medicine remains challenging, given the fluid and dynamic nature of the field, along with a rapidly developing regulatory climate over manufacturing and commerce requirements. The purpose of this article is to review the available literature on effectiveness and potential mechanism for OA of micronutrient vitamins; minerals; glycosaminoglycans; avocado-soybean unsaponifiable fractions; methylsulfonylmethane; s-adenosylmethionine; undenatured and hydrolyzed collagen preparations; phytoflavonoid compounds found in fruits, vegetables, spices, teas, and nuts; and other nutrients on the horizon. There also is a discussion on the concept of rational polysupplementation via the strategic integration of multiple nutraceuticals with potential complementary mechanisms for improving outcomes in OA. As applied nutritional science evolves, it will be important to stay on the forefront of proteomics, metabolomics, epigenetics, and nutrigenomics, because they hold enormous potential for developing novel therapeutic and prognostic breakthroughs in many areas of medicine, including OA.

Excessive S-adenosyl-L-methionine-dependent methylation increases levels of methanol, formaldehyde and formic acid in rat brain striatal homogenates: possible role in S-adenosyl-L-methionine-induced Parkinson's disease-like disorders

AIMS

Excessive methylation may be a precipitating factor for Parkinson's disease (PD) since S-adenosylmethionine (SAM), the endogenous methyl donor, induces PD-like changes when injected into the rat brain. The hydrolysis of the methyl ester bond of the methylated proteins produces methanol. Since methanol is oxidized into formaldehyde, and formaldehyde into formic acid in the body, we investigated the effects of SAM on the production of methanol, formaldehyde and formic acid in rat brain striatal homogenates and the toxicity of these products in PC12 cells.

MAIN METHODS

Radio-enzymatic and colorimetric assays, cell viability, Western blot.

KEY FINDINGS

SAM increased the formation of methanol, formaldehyde and formic acid in a concentration and time-dependent manner. Concentrations of [3H-methyl]-SAM at 0.17, 0.33, 0.67 and 1.34 nM produced 3.8, 8.0, 18.3 and 34.4 fmol/mg protein/h of [3H] methanol in rat striatal homogenates, respectively. SAM also significantly generated formaldehyde and formic acid in striatal homogenates. Formaldehyde was the most toxic metabolite to differentiated PC12 pheochromocytoma cells in cell culture studies, indicating that formaldehyde formed endogenously may contribute to neuronal damage in excessive methylation conditions. Subtoxic concentration of formaldehyde decreased the expression of tyrosine hydroxylase, the limiting factor in dopamine synthesis. Formaldehyde was more toxic to catecholaminergic PC12 cells than C6 glioma cells, indicating that neurons are more vulnerable to formaldehyde than glia cells.

SIGNIFICANCE

We suggest that excessive carboxylmethylation of proteins might be involved in the SAM-induced PD-like changes and in the aging process via the toxic effects of formaldehyde.

Over-expression of an S-adenosylmethionine-dependent methyltransferase leads to an extended lifespan of Podospora anserina without impairments in vital functions

PaMTH1, a putative methyltransferase previously described to increase in abundance in total protein extracts during aging of Podospora anserina is demonstrated to accumulate in the mitochondrial cell fraction of senescent cultures. The protein is localized in the mitochondrial matrix and displays a methyltransferase activity utilizing flavonoids as substrates. Constitutive over-expression of PaMth1 in P. anserina results in a reduced carbonylation of proteins and an extended lifespan without impairing vital functions suggesting a protecting role of PaMTH1 against oxidative stress.

Role of S-adenosyl-L-methionine in liver health and injury

S-adenosylmethionine (SAMe) has rapidly moved from being a methyl donor to a key metabolite that regulates hepatocyte growth, death, and differentiation. Biosynthesis of SAMe occurs in all mammalian cells as the first step in methionine catabolism in a reaction catalyzed by methionine adenosyltransferase (MAT). Decreased hepatic SAMe biosynthesis is a consequence of all forms of chronic liver injury. In an animal model of chronic liver SAMe deficiency, the liver is predisposed to further injury and develops spontaneous steatohepatitis and hepatocellular carcinoma. However, impaired SAMe metabolism, which occurs in patients with mutations of glycine N-methyltransferase (GNMT), can also lead to liver injury. This suggest that hepatic SAMe level needs to be maintained within a certain range, and deficiency or excess can both lead to abnormality. SAMe treatment in experimental animal models of liver injury shows hepatoprotective properties. Meta-analyses also show it is effective in patients with cholestatic liver diseases. Recent data show that exogenous SAMe can regulate hepatocyte growth and death, independent of its role as a methyl donor. This raises the question of its mechanism of action when used pharmacologically. Indeed, many of its actions can be recapitulated by methylthioadenosine (MTA), a by-product of SAMe that is not a methyl donor. A better understanding of why liver injury occurs when SAMe homeostasis is perturbed and mechanisms of action of pharmacologic doses of SAMe are essential in defining which patients will benefit from its use.

A pathogenic linked mutation in the catalytic core of human cystathionine beta-synthase disrupts allosteric regulation and allows kinetic characterization of a full-length dimer

Cystathionine beta-synthase catalyzes the condensation of serine and homocysteine to yield cystathionine and is the single most common locus of mutations associated with homocystinuria. In this study, we have examined the kinetic consequences of a pair of linked patient mutations, P78R/K102N, that are housed in the catalytic core of the protein and compared it to the effects of the corresponding single mutations. The P78R mutation affords purification of a mixture of higher order oligomers, P78R-I, which resembles the mixed quaternary state associated with wild-type enzyme. However, unlike wild-type enzyme, P78R-I converts over time to P78R-II, which exists predominantly as a full-length dimer. The specific activities of the K102N, P78R-I, and P78R-II mutants in the absence of AdoMet are approximately 3-, 9-, and 3-fold lower than of wild-type enzyme and are stimulated 2.9-, 2.5-, and 1.4-fold respectively by AdoMet. However, when linked, the specific activity of the resulting double mutant is comparable to that of wild-type enzyme but it is unresponsive to AdoMet, revealing that interactions between the two sites modulate the phenotype of the enzyme. Steady-state kinetic analysis for the double mutant reveals a sigmoidal dependence on homocysteine that is not observed with wild-type enzyme, which is ascribed to the mutation at the K102 locus and indicates changes in subunit interactions. Hydrogen-deuterium mass spectrometric analysis reveals that, even in the absence of AdoMet, the double mutant is locked in an activated conformation that is observed for wild-type enzyme in the presence of AdoMet, providing a structural rationale for loss of this allosteric regulation. To our knowledge, this is the first example of mutations in the catalytic core of cystathionine beta-synthase that result in failure of AdoMet-dependent regulation. Furthermore, analysis of individual single mutations has permitted, for the first time, partial kinetic characterization of a full-length dimeric form of human cystathionine beta-synthase.

S-Adenosylmethionine in protozoan parasites: Functions, synthesis and regulation

S-adenosylmethionine is one of the most frequently used enzymatic substrates in all living organisms. It plays a role in all biological methyl transfer reactions in as much as it is a donor of propylamine groups in the synthesis of the polyamines spermidine and spermine, it participates in the trans-sulphuration pathway to cysteine one of the three amino acids involved in glutathione and trypanothione synthesis in trypanosomatids and finally it is a source of the 5-deoxyadenosyl radicals, which are involved in many reductive metabolic processes, biodegradative pathways, tRNA modification and DNA repair. This mini-review is an update of the progress on the S-adenosylmethionine synthesis in different representative protozoan parasites responsible for many of the most devastating so-called tropical diseases that have an enormous impact on global health.

Flipping off the riboswitch: RNA structures that control gene expression

Riboswitches are metabolite-sensing RNA structures that have been discovered in regulatory regions of messenger RNA (mRNA). They have the remarkable ability to shut off the transcription or translation of their own mRNAs in response to binding a specific metabolite. In other words, riboswitches regulate their own genes using RNA instead of protein. Three new crystal structures reveal how S-adenosylmethionine and thiamine pyrophosphate riboswitches accomplish this task.

S-adenosylmethionine and radical-based catalysis

S-adenosylmethionine is the major methyl donor in all living organisms, but it is also involved in many other reactions occurring through radical-based catalysis. The structure and function of some of these enzymes, including those involved in the synthesis of the molybdenum cofactors, biotin, lipoate, will be discussed.

Suppression of TNF-α production by S-adenosylmethionine in human mononuclear leukocytes is not mediated by polyamines

Endotoxin-induced cytokine production is an important mechanism in the development of several types of liver damage. Methionine, some of its precursors and metabolites were reported to have protective effects against such injury. The aim of this study was to investigate whether methionine, its precursors or metabolites [phosphatidylcholine, choline, betaine, S-adenosylmethionine (SAM)] have a modulating effect on tumor necrosis factor alpha (TNF-alpha) production by endotoxin-stimulated human mononuclear leukocytes and whether SAM-dependent polyamines (spermidine, spermine) are mediators of SAM-induced inhibition of TNF-alpha synthesis. Methionine and betaine had a moderate stimulatory effect on TNF-alpha production, whereas phosphatidylcholine (ID(50) 5.4 mM), SAM (ID(50) 131 microM), spermidine (ID(50) 4.5 microM) and spermine (ID(50) 3.9 microM) had a predominantly inhibitory effect. Putrescine did not alter TNF-alpha release. Inhibitors of polyamine synthesis that blocked either putrescine (difluoromethylornithine) or spermine (CGP48664A) production did not affect TNF-alpha synthesis. Endotoxin stimulation of leukocytes did not alter the intracellular levels of polyamines. In addition, supplementation with SAM did not change the intracellular concentration of either polyamine measured. We conclude that phosphatidylcholine-induced immunosuppression is not caused by methionine and polyamines are not involved in SAM-induced inhibition of TNF-alpha production. The limitation of TNF-alpha release by spermidine is specific and is not due to its conversion into spermine.

Vitamin B12 and methionine synthesis: a critical review. Is nature's most beautiful cofactor misunderstood?

The mechanism by which Vitamin B12 prevents demyelination of nerve tissue is still not known. The evidence indicates that the critical site of B12 function in nerve tissue is in the enzyme, methionine synthase, in a system which requires S-adenosylmethionine. In recent years it has been recognized that S-adenosylmethionine gives rise to the deoxyadenosyl radical which catalyzes many reactions including the rearrangement of lysine to beta-lysine. Evidence is reviewed which suggests that there is an analogy between the two systems and that S-adenosyl methionine may catalyze a rearrangement of homocysteine on methionine synthase giving rise to iso- or beta-methionine. The rearranged product is readily degraded to CH3-SH, providing a mechanism for removing toxic homocysteine.

A mutation inactivating the methyltransferase gene in avirulent Madrid E strain of Rickettsia prowazekii reverted to wild type in the virulent revertant strain Evir

Rickettsia prowazekii Madrid E (E) strain is an effective vaccine, but can revert to virulent status when passaged in animals. The aim of this study is to identify the reverse mutation that may determine the virulence of R. prowazekii by comparing the genetic structures of E strain and its virulent revertant Evir strain. We determined that the gene (Rp028/Rp027) encoding the methyltransferase was mutated by frameshift in avirulent E strain but not in virulent revertant Evir strain and wild type virulent Breinl strain. We conclude that the mutation in the E strain gene reverts to wild type in the virulent revertant Evir strain. Whether the mutation plays an essential role in the attenuation of E strain needs to be further investigated.

Role of methionine adenosyltransferase and S-adenosylmethionine in alcohol-associated liver cancer

Two genes (MAT1A and MAT2A) encode for the essential enzyme methionine adenosyltransferase (MAT), which catalyzes the biosynthesis of S-adenosylmethionine (SAMe), the principal methyl donor and, in the liver, a precursor of glutathione. MAT1A is expressed mostly in the liver, whereas MAT2A is widely distributed. MAT2A is induced in the liver during periods of rapid growth and dedifferentiation. In human hepatocellular carcinoma (HCC) MAT1A is replaced by MAT2A. This is important pathogenetically because MAT2A expression is associated with lower SAMe levels and faster growth, whereas exogenous SAMe treatment inhibits growth. Rats fed ethanol intragastrically for 9 weeks also exhibit a relative switch in hepatic MAT expression, decreased SAMe levels, hypomethylation of c-myc, increased c-myc expression, and increased DNA strand break accumulation. Patients with alcoholic liver disease have decreased hepatic MAT activity owing to both decreased MAT1A expression and inactivation of the MAT1A-encoded isoenzymes, culminating in decreased SAMe biosynthesis. Consequences of chronic hepatic SAMe depletion have been examined in the MAT1A knockout mouse model. In this model, the liver is more susceptible to injury. In addition, spontaneous steatohepatitis develops by 8 months, and HCC develops by 18 months. Accumulating evidence shows that, in addition to being a methyl donor, SAMe controls hepatocyte growth response and death response. Whereas transient SAMe depletion is necessary for the liver to regenerate, chronic hepatic SAMe depletion may lead to malignant transformation. It is interesting that SAMe is antiapoptotic in normal hepatocytes, but proapoptotic in liver cancer cells. This should make SAMe an attractive agent for both chemoprevention and treatment of HCC.

Autoregulation of the gene for cystathionine gamma-synthase in Arabidopsis: post-transcriptional regulation induced by S-adenosylmethionine

Cystathionine gamma-synthase (CGS) catalyses the first committed step of methionine biosynthesis in higher plants. CGS is encoded by the CGS1 gene in Arabidopsis. Stability of CGS1 mRNA is down-regulated in response to methionine application and the exon 1-coding region of CGS1 itself is necessary and sufficient for this regulation. mto1 (for methionine overaccumulation) mutants of Arabidopsis, which carry single-amino-acid sequence alterations within CGS1 exon 1, are deficient in this regulation and overaccumulate methionine. Since CGS1 exon 1 acts in cis during this regulation, we have proposed a model that the regulation occurs during translation of CGS1 mRNA when the nascent polypeptide of CGS and its mRNA are in close proximity. In fact, application of the translation inhibitor cycloheximide abolished this regulation in vivo. This model predicts that the regulation can be reproduced in an in vitro translation system. Studies using the in vitro translation system of wheatgerm extract have indicated that S-adenosylmethionine, a direct metabolite of methionine, is the effector of this regulation. A 5'-truncated RNA species, which is a probable degradation intermediate of CGS1 mRNA in vivo, was also detected in vitro, suggesting that the wheatgerm in vitro translation system reflects the in vivo regulation.

S-Adenosyl-L-methionine-dependent restriction enzymes

Restriction-modification (R-M) enzymes are classified into type I, II, III, and IV, based on their recognition sequence, subunit composition, cleavage position, and cofactor requirements. While the role of S-Adenosyl-L-methionine (AdoMet) as the methyl group donor in the methylation reaction is undisputed, its requirement in DNA cleavage reaction has been subject to intense study. AdoMet is a prerequisite for the DNA cleavage by most type I enzymes known so far, with the exception of R.EcoR124I. A number of new type II restriction enzymes belonging to the type IIB and IIG family were found to show AdoMet dependence for their cleavage reaction. The type III enzymes have been found to require AdoMet for their restriction function. AdoMet functions as an allosteric effector of the DNA cleavage reaction and has been shown to bring about conformational changes in the protein upon binding.

S-adenosylmethionine and Pneumocystis

Pneumocystis is a parasitic fungus causing pneumonia in immunosuppressed mammals and S-adenosylmethionine a key intermediary metabolite for all cells. Other than a species of Rickettsia bacteria and an aberrant strain of the protozoan Amoeba proteus, Pneumocystis is the only cell known unable to synthesize AdoMet; it must extract this key compound from its host. This was discovered using a culture system and confirmed by observing depletion of AdoMet in the plasma of infected animals. Depletion also occurs in patients with Pneumocystis pneumonia (PcP), a phenomenon suggested as a basis for a method for diagnosis and evaluation of response to therapy. Preliminary data indicate that deliberate reduction of host lung AdoMet by nicotine treatment is therapeutic in the rat model of Pneumocystis pneumonia.

Differential protein expression analysis of Leishmania major reveals novel roles for methionine adenosyltransferase and S-adenosylmethionine in methotrexate resistance

Leishmania is a trypanosomatid parasite causing serious disease and displaying resistance to various drugs. Here, we present comparative proteomic analyses of Leishmania major parasites that have been either shocked with or selected in vitro for high level resistance to the model antifolate drug methotrexate. Numerous differentially expressed proteins were identified by these experiments. Some were associated with the stress response, whereas others were found to be overexpressed due to genetic linkage to primary resistance mediators present on DNA amplicons. Several proteins not previously associated with resistance were also identified. The role of one of these, methionine adenosyltransferase, was confirmed by gene transfection and metabolite analysis. After a single exposure to low levels of methotrexate, L. major methionine adenosyltransferase transfectants could grow at high concentrations of the drug. Methotrexate resistance was also correlated to increased cellular S-adenosylmethionine levels. The folate and S-adenosylmethionine regeneration pathways are intimately connected, which may provide a basis for this novel resistance phenotype. This thorough comparative proteomic analysis highlights the variety of responses required for drug resistance to be achieved.

Involvement of S-adenosylmethionine in G1 cell-cycle regulation in Saccharomyces cerevisiae

S-adenosyl-l-methionine (AdoMet) is a molecule central to general metabolism, serving as a principal methyl donor for methylation of various cellular constituents. The alteration in the availability of AdoMet has profound effect on cell growth. A mutant allele of Saccharomyces cerevisiae gene SAH1 encoding S-adenosyl-l-homocysteine (AdoHcy) hydrolase, was isolated as a mutation that suppressed the Ca(2+)-sensitive phenotypes of the zds1Delta strain, such as the Ca(2+)-induced, Swe1p- and Cln2p-mediated G(2) cell-cycle arrest, and polarized bud growth. The mutation (sah1-1) led the cells to accumulate AdoMet besides AdoHcy, the substrate of Sah1p. The cells treated with exogenous AdoMet and AdoHcy had markedly decreased levels of SWE1 and CLN2 mRNA, providing the basis for the suppression of the Ca(2+) sensitivity by the sah1-1 mutation. Exogenous AdoMet transiently led the cells to G(1) cell-cycle delay whereas AdoHcy caused growth inhibition irrelevant to the cell cycle. The effect of AdoMet in inducing the cell-cycle delay was exerted in a manner independent of Met4p, an overall transcriptional activator for MET genes. Our observation provides an insight into the role played by AdoMet in cell cycle regulation.

Impaired liver regeneration in mice lacking methionine adenosyltransferase 1A

Methionine adenosyltransferase (MAT) is an essential enzyme because it catalyzes the formation of S-adenosylmethionine (SAMe), the principal biological methyl donor. Of the two genes that encode MAT, MAT1A is mainly expressed in adult liver and MAT2A is expressed in all extrahepatic tissues. Mice lacking MAT1A have reduced hepatic SAMe content and spontaneously develop hepatocellular carcinoma. The current study examined the influence of chronic hepatic SAMe deficiency on liver regeneration. Despite having higher baseline hepatic staining for proliferating cell nuclear antigen, MAT1A knockout mice had impaired liver regeneration after partial hepatectomy (PH) as determined by bromodeoxyuridine incorporation. This can be explained by an inability to up-regulate cyclin D1 after PH in the knockout mice. Upstream signaling pathways involved in cyclin D1 activation include nuclear factor kappaB (NFkappaB), the c-Jun-N-terminal kinase (JNK), extracellular signal-regulated kinases (ERKs), and signal transducer and activator of transcription-3 (STAT-3). At baseline, JNK and ERK are more activated in the knockouts whereas NFkappaB and STAT-3 are similar to wild-type mice. Following PH, early activation of these pathways occurred, but although they remained increased in wild-type mice, c-jun and ERK phosphorylation fell progressively in the knockouts. Hepatic SAMe levels fell progressively following PH in wild-type mice but remained unchanged in the knockouts. In culture, MAT1A knockout hepatocytes have higher baseline DNA synthesis but failed to respond to the mitogenic effect of hepatocyte growth factor. Taken together, our findings define a critical role for SAMe in ERK signaling and cyclin D1 regulation during regeneration and suggest chronic hepatic SAMe depletion results in loss of responsiveness to mitogenic signals.

S-adenosyl-L-methionine is an effector in the posttranscriptional autoregulation of the cystathionine gamma-synthase gene in Arabidopsis

Cystathionine gamma-synthase, the first committed enzyme of methionine biosynthesis in higher plants, is encoded by the CGS1 gene in Arabidopsis thaliana. We have shown previously that the stability of the CGS1 mRNA is negatively regulated in response to methionine application [Chiba, Y., Ishikawa, M., Kijima, F., Tyson, R. H., Kim, J., Yamamoto, A., Nambara, E., Leustek, T., Wallsgrove, R. M. & Naito, S. (1999) Science 286, 1371-1374]. To determine whether methionine itself is the effector of the CGS1 exon 1-mediated posttranscriptional regulation, we carried out transfection experiments. The results suggested that, rather than methionine, S-adenosyl-L-methionine (AdoMet), or one of its metabolites, acts as the effector of this regulation. To further identify the actual effector, we exploited the wheat germ in vitro translation system. The effects of various metabolites and analogs of AdoMet were tested by using RNA carrying a CGS1 exon 1-reporter fusion. These tests identified AdoMet as the effector of this regulation. S-adenosyl-L-ethionine, an analog of AdoMet, also had effector activity. A. thaliana mto1 mutants, which are deficient in this regulation, showed a much reduced response to AdoMet in vitro, with a leaky allele showing a less reduced response. RNA translated in vitro in the presence of AdoMet contained a 5'-truncated RNA species, similar to the one that we previously suggested was an in vivo degradation intermediate of CGS1 mRNA. Together, the results show that the basic reactions of CGS1 exon 1-mediated posttranscriptional regulation occur in the wheat germ in vitro translation system, and that AdoMet acts as the effector.

L-methionine availability regulates expression of the methionine adenosyltransferase 2A gene in human hepatocarcinoma cells: role of S-adenosylmethionine

In mammals, methionine adenosyltransferase (MAT), the enzyme responsible for S-adenosylmethionine (AdoMet) synthesis, is encoded by two genes, MAT1A and MAT2A. In liver, MAT1A expression is associated with high AdoMet levels and a differentiated phenotype, whereas MAT2A expression is associated with lower AdoMet levels and a dedifferentiated phenotype. In the current study, we examined regulation of MAT2A gene expression by l-methionine availability using HepG2 cells. In l-methionine-deficient cells, MAT2A gene expression is rapidly induced, and methionine adenosyltransferase activity is increased. Restoration of l-methionine rapidly down-regulates MAT2A mRNA levels; for this effect, l-methionine needs to be converted into AdoMet. This novel action of AdoMet is not mediated through a methyl transfer reaction. MAT2A gene expression was also regulated by 5'-methylthioadenosine, but this was dependent on 5'-methylthioadenosine conversion to methionine through the salvage pathway. The transcription rate of the MAT2A gene remained unchanged during l-methionine starvation; however, its mRNA half-life was significantly increased (from 100 min to more than 3 h). The effect of l-methionine withdrawal on MAT2A mRNA stabilization requires both gene transcription and protein synthesis. We conclude that MAT2A gene expression is modulated as an adaptive response of the cell to l-methionine availability through its conversion to AdoMet.

Role of S-adenosylmethionine in two experimental models of pancreatitis

Severe necrotizing pancreatitis occurs in young female mice fed a choline-deficient and ethionine-supplemented (CDE) diet. Although the mechanism of the pancreatitis is unknown, one consequence of this diet is depletion of hepatic S-adenosylmethionine (SAM). SAM formation is catalyzed by methionine adenosyltransferases (MATs), which are encoded by liver-specific (MAT1A) and non-liver-specific (MAT2A) genes. In this work, we examined changes in pancreatic SAM homeostasis in mice receiving the CDE diet and the effect of SAM treatment. We found that both MAT forms are expressed in normal pancreas and pancreatic acini. After 48 h of the CDE diet, SAM levels decreased 50% and MAT1A-encoded protein disappeared via post-translational mechanisms, whereas MAT2A-encoded protein increased via pretranslational mechanisms. CDE-fed mice exhibited extensive necrosis, edema, and acute pancreatic inflammatory infiltration, which were prevented by SAM treatment. However, old female mice consuming the CDE diet that do not develop pancreatitis showed a similar fall in pancreatic SAM level. SAM was also protective in cerulein-induced pancreatitis in the rat, but the protection was limited. Although the pancreatic SAM level fell by more than 80% in the MAT1A knockout mice, no pancreatitis developed. This study thus provides several novel findings. First, the so-called liver-specific MAT1A is highly expressed in the normal pancreas and pancreatic acini. Second, the CDE diet causes dramatic changes in the expression of MAT isozymes by different mechanisms. Third, in contrast to the situation in the liver, where absence of MAT1A and decreased hepatic SAM level can lead to spontaneous tissue injury, in the pancreas the roles of SAM and MAT1A appear more complex and remain to be defined.

S-Adenosylmethionine: molecular, biological, and clinical aspects--an introduction

In clinical research, a novel approach has emerged: some of the essential nutrients are being used to treat pathologic conditions. Many of these nutrients, including methionine, must first be activated in the liver or in other tissues before they can exert their key functions. However, this activating process is impaired in disease states and, as a consequence, nutritional requirements change. For instance, for methionine to act as the main cellular methyl donor, it must first be activated to S-adenosylmethionine (SAMe; also known as ademethionine). SAMe is required and is of fundamental importance for the metabolism of nucleic acids and polyamines, the structure and function of membranes, and as a precursor of glutathione. These processes are often seriously altered in various pathologic states addressed in this symposium, but they cannot be restored by simply administering methionine. For instance, in liver disease associated with impairment of the enzyme that activates methionine to SAMe, supplementation with methionine is useless and may even become toxic as it accumulates because it is not used. Accordingly, one must correct the lack of SAMe by bypassing the deficiency in enzyme activation; this is done by providing the product of the defective reaction, namely SAMe. Under these pathologic conditions, SAMe becomes crucial for the functioning of the cell. Thus SAMe, which is found in all living organisms, becomes the essential nutrient instead of methionine. This symposium reviewed the biological and corresponding molecular aspects of SAMe metabolism and the clinical consequences of its deficiency or supplementation in various tissues.

S-Adenosyl-L-methionine (SAMe): from the bench to the bedside--molecular basis of a pleiotrophic molecule

S-Adenosyl-L-methionine (SAMe), a metabolite present in all living cells, plays a central role in cellular biochemistry as a precursor to methylation, aminopropylation, and transsulfuration pathways. As such, SAMe has been studied extensively since its chemical structure was first described in 1952. Decades of research on the biochemical and molecular roles of SAMe in cellular metabolism have provided an extensive foundation for its use in clinical studies, including those on depression, dementia, vacuolar myelopathy, liver disease, and osteoarthritis. This article provides an overview of the biochemical, molecular, and therapeutic effects of this pleiotrophic molecule.

S-Adenosylmethionine revisited: its essential role in the regulation of liver function

Dietary methionine is mainly metabolized in the liver where it is converted into S-adenosylmethionine (AdoMet), the main biologic methyl donor. This reaction is catalyzed by methionine adenosyltransferase I/III (MAT I/III), the product of MAT1A gene, which is exclusively expressed in this organ. It was first observed that serum methionine levels were elevated in experimental models of liver damage and in liver cirrhosis in human beings. Results of further studies showed that this pathological alteration was due to reduced MAT1A gene expression and MAT I/III enzyme inactivation associated with liver injury. Synthesis of AdoMet is essential to all cells in the organism, but it is in the liver where most of the methylation reactions take place. The central role played by AdoMet in cellular function, together with the observation that AdoMet administration reduces liver damage caused by different agents and improves survival of alcohol-dependent patients with cirrhosis, led us to propose that alterations in methionine metabolism could play a role in the onset of liver disease and not just be a consequence of it. In the present work, we review the recent findings that support this hypothesis and highlight the mechanisms behind the hepatoprotective role of AdoMet.

High homocysteine and thrombosis without connective tissue disorders are associated with a novel class of cystathionine beta-synthase (CBS) mutations

Cystathionine beta-synthase (CBS) is a crucial regulator of plasma levels of the thrombogenic amino acid homocysteine (Hcy). Homocystinuria due to CBS deficiency confers a dramatically increased risk of thrombosis. Early diagnosis usually occurs after the observation of ectopia lentis, mental retardation, or characteristic skeletal abnormalities. Homocystinurics with this phenotype typically carry mutations in the catalytic region of the protein that abolish CBS activity. We describe a novel class of missense mutations consisting of I435T, P422L, and S466L that are located in the non-catalytic C-terminal region of CBS that yield enzymes that are catalytically active but deficient in their response to S-adenosylmethionine (AdoMet). The P422L and S466L mutations were found in patients suffering premature thrombosis and homocystinuric levels of Hcy but lacking any of the connective tissue disorders typical of homocystinuria due to CBS deficiency. The P422L and S466L mutants demonstrated a level of CBS activity comparable to that of the AdoMet stimulated wild-type CBS but could not be further induced by the addition of AdoMet. In terms of temperature stability, oligomeric organization, and heme saturation the I435T, P422L, and S466L mutants are indistinguishable from wild-type CBS. Our findings illustrate the importance of AdoMet for the regulation of Hcy metabolism and are consistent with the possibility that the characteristic connective tissue disturbances observed in homocystinuria due to CBS deficiency may not be due to elevated Hcy.

Bacteriophage T4 Dam DNA-[N6-adenine]methyltransferase. Kinetic evidence for a catalytically essential conformational change in the ternary complex

We carried out a steady state kinetic analysis of the bacteriophage T4 DNA-[N6-adenine]methyltransferase (T4 Dam) mediated methyl group transfer reaction from S-adenosyl-l-methionine (AdoMet) to Ade in the palindromic recognition sequence, GATC, of a 20-mer oligonucleotide duplex. Product inhibition patterns were consistent with a steady state-ordered bi-bi mechanism in which the order of substrate binding and product (methylated DNA, DNA(Me) and S-adenosyl-l-homocysteine, AdoHcy) release was AdoMet downward arrow DNA downward arrow DNA(Me) upward arrow AdoHcy upward arrow. A strong reduction in the rate of methylation was observed at high concentrations of the substrate 20-mer DNA duplex. In contrast, increasing substrate AdoMet concentration led to stimulation in the reaction rate with no evidence of saturation. We propose the following model. Free T4 Dam (initially in conformational form E) randomly interacts with substrates AdoMet and DNA to form a ternary T4 Dam-AdoMet-DNA complex in which T4 Dam has isomerized to conformational state F, which is specifically adapted for catalysis. After the chemical step of methyl group transfer from AdoMet to DNA, product DNA(Me) dissociates relatively rapidly (k(off) = 1.7 x s(-1)) from the complex. In contrast, dissociation of product AdoHcy proceeds relatively slowly (k(off) = 0.018 x s(-1)), indicating that its release is the rate-limiting step, consistent with kcat = 0.015 x s(-1). After AdoHcy release, the enzyme remains in the F conformational form and is able to preferentially bind AdoMet (unlike form E, which randomly binds AdoMet and DNA), and the AdoMet-F binary complex then binds DNA to start another methylation cycle. We also propose an alternative pathway in which the release of AdoHcy is coordinated with the binding of AdoMet in a single concerted event, while T4 Dam remains in the isomerized form F. The resulting AdoMet-F binary complex then binds DNA, and another methylation reaction ensues. This route is preferred at high AdoMet concentrations.

S-Adenosylmethionine: a control switch that regulates liver function

Genome sequence analysis reveals that all organisms synthesize S-adenosylmethionine (AdoMet) and that a large fraction of all genes is AdoMet-dependent methyltransferases. AdoMet-dependent methylation has been shown to be central to many biological processes. Up to 85% of all methylation reactions and as much as 48% of methionine metabolism occur in the liver, which indicates the crucial importance of this organ in the regulation of blood methionine. Of the two mammalian genes (MAT1A, MAT2A) that encode methionine adenosyltransferase (MAT, the enzyme that makes AdoMet), MAT1A is specifically expressed in adult liver. It now appears that growth factors, cytokines, and hormones regulate liver MAT mRNA levels and enzyme activity and that AdoMet should not be viewed only as an intermediate metabolite in methionine catabolism, but also as an intracellular control switch that regulates essential hepatic functions such as regeneration, differentiation, and the sensitivity of this organ to injury. The aim of this review is to integrate these recent findings linking AdoMet with liver growth, differentiation, and injury into a comprehensive model. With the availability of AdoMet as a nutritional supplement and evidence of its beneficial role in various liver diseases, this review offers insight into its mechanism of action.

A dual role for substrate S-adenosyl-L-methionine in the methylation reaction with bacteriophage T4 Dam DNA-[N6-adenine]-methyltransferase

The fluorescence of 2-aminopurine ((2)A)-substituted duplexes (contained in the GATC target site) was investigated by titration with T4 Dam DNA-(N6-adenine)-methyltransferase. With an unmethylated target ((2)A/A duplex) or its methylated derivative ((2)A/(m)A duplex), T4 Dam produced up to a 50-fold increase in fluorescence, consistent with (2)A being flipped out of the DNA helix. Though neither S-adenosyl-L-homocysteine nor sinefungin had any significant effect, addition of substrate S-adenosyl-L-methionine (AdoMet) sharply reduced the Dam-induced fluorescence with these complexes. In contrast, AdoMet had no effect on the fluorescence increase produced with an (2)A/(2)A double-substituted duplex. Since the (2)A/(m)A duplex cannot be methylated, the AdoMet-induced decrease in fluorescence cannot be due to methylation per se. We propose that T4 Dam alone randomly binds to the asymmetric (2)A/A and (2)A/(m)A duplexes, and that AdoMet induces an allosteric T4 Dam conformational change that promotes reorientation of the enzyme to the strand containing the native base. Thus, AdoMet increases enzyme binding-specificity, in addition to serving as the methyl donor. The results of pre-steady-state methylation kinetics are consistent with this model.

Role of polyamines and ethylene as modulators of plant senescence

Under optimal conditions of growth, senescence, a terminal phase of development, sets in after a certain physiological age. It is a dynamic and closely regulated developmental process which involves an array of changes at both physiological and biochemical levels including gene expression. A large number of biotic and abiotic factors accelerate the process. Convincing evidence suggests the involvement of polyamines (PAs) and ethylene in this process. Although the biosynthetic pathways of both PAs and ethylene are interrelated, S-adenosylmethionine (SAM) being a common precursor, their physiological functions are distinct and at times antagonistic, particularly during leaf and flower senescence and also during fruit ripening. This provides an effective means for regulation of their biosynthesis and also to understand the mechanism by which the balance between the two can be established for manipulating the senescence process. The present article deals with current advances in the knowledge of the interrelationship between ethylene and PAs during senescence which may open up new vistas of investigation for the future.

S-Adenosylmethionine

S-Adenosyl-Lmethionine (SAM) is an important molecule in normal cell function and survival. SAM is utilized by three key metabolic pathways: transmethylation; transsulfuration; and polyamine synthesis. In transmethylation reactions, the methyl group of SAM is donated to a large variety of acceptor substrates including DNA, phospholipids and proteins. Thus, interference of these reactions can affect a wide spectrum of processes ranging from gene expression to membrane fluidity. In transsulfuration, the sulfur atom of the SAM is converted via a series of enzymatic steps to cysteine, a precursor of taurine and glutathione, a major cellular anti-oxidant. Polyamines are required for normal cell growth. Given the importance of SAM in tissue function, it is not surprising that this molecule is being investigated as a possible therapeutic agent for the treatment of various clinical disorders.

An endogenous proteinacious inhibitor for S-adenosyl-L-methionine-dependent transmethylation reactions; identification of S-adenosylhomocystein as an integral part

A proteinacious inhibitor with a molecular weight of 1,600 Da which inhibits S-adenosyl-L-methionine-dependent transmethylation reactions was purified from porcine liver to homogeneity by procedures including boiling, Sephadex G-25 column chromatography and repeated HPLC. Employing both Nuclear Magnetic Resonance (NMR) and Fast Atom Bombardment-Mass (FAB-Mass) spectroscopy, S-adenosylhomocysteine was conclusively identified as an integral part of the inhibitor. The purified S-adenosylhomocysteine was competitive with S-adenosyl-L-methionine with Ki value of 6.3x10(-6) M towards protein methylase II.

Role of SAM-dependent thiol methylation in the renal toxicity of several solvents in mice

The role of S-adenosylmethionine (SAM)-dependent thiol methylation in the nephrotoxicity of seven industrial solvents was studied in mice. The seven following solvents were utilized: bromobenzene (BB), styrene (STY), tetrachloroethylene (TTCE), trichloroethylene (TCE), 1,1-dichloroethylene (DCE), 1,2-dichloroethane (DCA) and hexachlorobutadiene (HCB). The experimental model comprised mice pretreated with periodate oxidized adenosine (ADOX) (100 micromol kg(-1) i.p.) 30 min before injection of solvents. In the first 4 h after ADOX treatment, the SAM levels were about fourfold higher than controls for the liver and kidney. The S-adenosylhomocysteine (SAH) levels were increased by factors of 11 and 14 and the SAM/SAH ratios were decreased by factors of 3 and 10 for the liver and kidney, respectively. These results show that ADOX treatment probably induces an inhibition of methyltransferase SAM-dependent in the liver and kidney and thus decreases the methylation capabilities. A single oral administration of BB (500 or 800 mg kg(-1)), TTCE (3500 or 4000 mg kg(-1)), TCE (3000 or 3500 mg kg(-1)) or STY (400 or 600 mg kg(-1)) did not induce renal toxicity, evaluated by the percentage of damaged tubules compared to controls. On the other hand, the three solvents DCE, HCB and DCA were nephrotoxic and the percentage of damaged tubules observed for each solvent was significantly different from the value of <1.8% for controls: 19% and 40% for DCE (130 and 200 mg kg(-1)), 50% and 46% for HCB (80 and 100 mg kg(-1)) and 5.1% and 7.6% for DCA (1000 and 1500 mg kg(-1)). The ADOX treatment in the mice did not modify the renal toxicity of the seven solvents. Thus, their renal toxicity, when it existed, was probably independent of the SAM-dependent thiolmethyltransferase activity in the mice. The results of this study are discussed from two viewpoints. The first concerns the general considerations on inhibition of thiol methyltransferase activities in mice and the second is related to the different solvents that are evoked individually.

S-adenosyl-L-methionine: role in phosphatidylcholine synthesis and in vitro effects on the ethanol-induced alterations of lipid metabolism

Hepatic lipid metabolism is extremely modified by excessive ethanol consumption, but the cellular mechanisms of such alterations are still largely unexplored. S-Adenosyl-L-methionine (SAMe) is known as an important methylating agent and as a precursor of glutathione and it has been shown to prevent some of the toxic effects of ethanol in the liver. We therefore studied the effects of ethanol on cholesterol synthesis in a human hepatomal cell line (HepG2), the kinetics of SAMe, and its putative protective effects on the alterations of lipid metabolism induced by toxic concentrations of alcohol. Incubation of HepG2 cells with [3H]SAMe resulted in a progressive increase in the labelling of phosphatidylcholine and of its two intermediates during synthesis starting from phosphatidylethanolamine. This process is enzymatic, since it does not take place in heat-inactivated cells. Also, ethanol induced an increase in cholesterol and triglycerides syntheses and a decrease in phospholipid labelling. These alterations were not prevented by SAMe 10(-4) M, indicating that the protective effects of the drug are related to other mechanisms of action such as reduced formation of collagen, restoration of glutathione levels, and normalization of membrane functions.

S-adenosylmethionine synthesis: molecular mechanisms and clinical implications

Methionine adenosyltransferase (MAT) is an ubiquitous enzyme that catalyzes the synthesis of S-adenosylmethionine from methionine and ATP. In mammals, there are two genes coding for MAT, one expressed exclusively in the liver and a second enzyme present in all tissues. Molecular studies indicate that liver MAT exists in two forms: as a homodimer and as a homotetramer of the same oligomeric subunit. The liver-specific isoenzymes are inhibited in human liver cirrhosis, and this is the cause of the abnormal metabolism of methionine in these subjects.

Changes in rat liver gene expression induced by thioacetamide: protective role of S-adenosyl-L-methionine by a glutathione-dependent mechanism

Chronic liver damage induced by thioacetamide (TAM) was accompanied by changes in the expression of genes related to growth (beta-actin) and function (albumin and haptoglobin) of the liver. Their messenger RNA (mRNA) levels increased during the first days after TAM administration, but 4 to 7 days after prolonged treatment with this drug, liver gene expression was considerable decreased. TAM-induced changes in albumin and beta-actin mRNA levels were prevented by cotreatment with S-adenosyl-L-methionine (SAM). We have investigated the possible involvement of glutathione in the protective mechanism of SAM. Firstly, we found that TAM treatment in the rat induced changes in liver glutathione disulfide (GSSG) levels, with a concomitant increase in the glutathione reductase enzymatic activity, these changes being abolished when animals were cotreated with TAM and SAM. Secondly, when rats were pretreated with buthionine sulfoximine (BSO), a glutathione synthesis inhibitor, before thioacetamide administration, the beneficial effect of SAM on liver gene expression was completely abolished. These results were confirmed by assaying the alanine transaminase serum activity, a parameter of liver injury. TAM-treated animals had increases in this serum enzyme, this effect being partially blocked by SAM. However, in BSO-pretreated rats, the protective effect of SAM was impaired. Taking together all these results, we propose a glutathione-dependent mechanism in the SAM protection against TAM hepatotoxicity in the rat.

Functional analysis of Met4, a yeast transcriptional activator responsive to S-adenosylmethionine

Transcription of the genes necessary for sulfur amino acid biosynthesis in Saccharomyces cerevisiae is dependent on Met4, a transcriptional activator that belongs to the basic region-leucine zipper protein family. In this report, we show that one mechanism permitting the repression of the sulfur network by S-adenosylmethionine (AdoMet) involves inhibition of the transcriptional activation function of Met4. Using a wide array of deleted LexA-Met4 fusion proteins as well as various Gal4-Met4 hybrids, we identify the functional domains of Met4 and characterize their relationship. Met4 appears to contain only one activation domain, located in its N-terminal part. We demonstrate that this activation domain functions in a constitutive manner and that AdoMet responsiveness requires a distinct region of Met4. Furthermore, we show that when fused to a heterologous activation domain, this inhibitory region confers inhibition by AdoMet. Met4 contains another distinct functional domain that appears to function as an antagonist of the inhibitory region when intracellular AdoMet is low. On the basis of the presented results, a model for intramolecular regulation of Met4 is proposed.

Effects of arginine, S-adenosylmethionine and polyamines on nerve regeneration

INTRODUCTION

Axon growth and axon regeneration are complex processes requiring an adequate supply of certain metabolic precursors and nutrients.

MATERIAL AND METHODS

This article reviews the studies examining some of the processes of protein modification fundamental to both nerve regeneration and to the continuous and adequate supply of specific factors such as arginine, S-adenosylmethionine and polyamines.

RESULTS

The process of arginylation notably increases following nerve injury and during subsequent regeneration of the nerve, with the most likely function of arginine-modification of nerve proteins being the degradation of proteins damaged through injury. It appears that defective methyl group metabolism may be one of the leading causes of demyelination, as suggested by the observation of reduced cerebrospinal fluid concentrations of s-adenosylmethionine (SAMe) and 5-methyltetrahydrofolate, the key metabolites in methylation processes, in patients with a reduction in myelination of corticospinal tracts. Polyamine synthesis, which depends strongly on the availability of both SAMe and arginine, markedly increases in neurons soon after an injury. This "polyamine-response" has been found to be essential for the survival of the parent neurons after injury to their axons. Polyamines probably exert their effects through involvement in DNA, RNA and protein synthesis, or through post-translational modifications that are indicated as the most relevant events of the "axon reaction."

CONCLUSIONS

Nerve regeneration requires the presence of arginine, s-adenosylmethionine, and polyamines. Further studies are needed to explore the mechanisms involved in these processes.

Comparison of sulfur amino acid utilization for GSH synthesis between HepG2 cells and cultured rat hepatocytes

HepG2 cells are widely used as a model of human hepatocytes for studies of drug metabolism and toxicity. However, GSH metabolism in HepG2 cells is poorly characterized. This report describes the utilization of sulfur amino acids for GSH synthesis in HepG2 cells. In contrast to primary cultures of rat hepatocytes, which rely mostly on methionine for GSH synthesis, HepG2 cells use cystine. Their inability to utilize methionine for GSH synthesis was not due to lack of methionine uptake or low cellular ATP levels, but rather to the lack of S-adenosyl-methionine synthetase activity. When HepG2 cells were cultured overnight in medium containing cystine as the only sulfur amino acid, addition of glutamate or acivicin had minimal to no effect on cell GSH; however, addition of threonine significantly depleted cell GSH. When cystine (0.18 mM) uptake was measured, glutamate (2.5 mM), which inhibited cystine uptake in cultured rat hepatocytes, had a minimal effect in HepG2 cells. Instead, threonine (20 mM) strongly inhibited the apparent uptake of cystine by HepG2 cells. Strong inhibition by threonine of apparent cystine uptake was actually due to inhibition of cysteine uptake, which resulted from GSH-cystine mixed disulfide exchange. Radio-HPLC confirmed this. After incubating cells with [35S]cystine (0.18 mM) for 10 min, the total counts inside the cell matched the counts in the uptake medium in the form of GSH-cysteine mixed disulfide. Finally, HepG2 cells took up cysteine by both Na(+)-dependent and -independent mechanisms. The former exhibited high affinity and low capacity, whereas the latter exhibited the opposite. At a physiologic concentration of cysteine (10 microM), 68% of cysteine uptake occurred via the Na(+)-dependent system and 32% via system L1.

A new function of S-adenosylmethionine: the ribosyl moiety of AdoMet is the precursor of the cyclopentenediol moiety of the tRNA wobble base queuine

Queuosine (Q) [7-(((4,5-cis-dihydroxy-2-cyclopenten-1-yl)amino)methyl)-7-deaz agu anosine] usually occurs in the first position of the anticodon of tRNAs specifying the amino acids asparagine, aspartate, histidine, and tyrosine. The hypermodified nucleoside is found in eubacteria and eucaryotes. Q is synthesized de novo exclusively in eubacteria; for eucaryotes the compound is a nutrient factor. In Escherichia coli the Q precursor (oQ), carrying a 2,3-epoxy-4,5-dihydroxycyclopentane ring, is formed from tRNA precursors containing 7-(aminomethyl)-7-deazaguanine (preQ1) by the queA gene product. A genomic queA mutant accumulating preQ1 tRNA was constructed. The QueA enzyme was overexpressed as a fusion protein with the glutathione S-transferase from Schistosoma japonicum and purified to homogeneity by affinity and anion-exchange chromatography. The enzyme QueA synthesizes oQ from preQ1 in a single S-adenosylmethionine- (AdoMet-) requiring step, indicating that the ribosyl moiety of AdoMet is transferred and isomerized to the epoxycyclopentane residue of oQ. The identity of oQ was verified by HPLC and directly combined HPLC/mass spectrometry. The formation of oQ was reconstituted in vitro, applying a synthetic RNA. A 17-nucleotide microhelix (corresponding to the anticodon stem and loop of tRNA(Tyr) from E. coli) is sufficient to act as the RNA substrate for oQ synthesis. We propose that QueA is an S-adenosylmethionine:tRNA ribosyltransferase-isomerase.

[S-adenosyl-L-methionine (SAMe) and its use in hepatology]

S-adenosyl-L-methionine (SAMe), a molecule naturally present in several body tissues and fluids, is produced, by SAMe synthetase, from ATP and methionine. SAMe has a fundamental role, as methyl group donor, in transmethylation reactions in which the synthesis of membrane phospholipids (especially phosphatidylcholine) is mandatory for the maintenance of membrane fluidity. Another metabolic pathway involving SAMe, transsulphuration, is initiated with the release of -CH3 from the molecule and the formation of S-Adenosyl-homocysteine and then homocysteine and cysteine, a precursor of glutathione the main cellular antioxidant, responsible of detoxification of various compounds and xenobiotics. At last SAMe is implicated in aminopropylation process for the polyamine synthesis. The development of stable double salt of p-toluene sulphonic acid and sulphuric acid of SAMe enables the clinical use of the drug, as a therapeutical agent, for the treatment of a number of liver dysfunctions. In various animal and human models, including controlled trials, it has been demonstrated that SAMe can ameliorate some biochemical parameters and pruritus in cholestasis induced by a range of compounds (i.e. oestrogens, lithocolate, etc) and in intrahepatic cholestasis superimposed to chronic liver disease. Concerning alcohol toxicity, SAMe prevents, in ethanol fed baboons, depletion of glutathione levels, normalizes the mitochondrial enzymes and improves the histological hepatic lesions. In human healthy volunteers it has been recently demonstrated that SAMe, after ethanol ingestion, significantly lowers plasma concentration of ethanol and acetaldehyde as well. Finally, SAMe has been proposed, instead of N-acetylcysteine, as precursor of glutathione, in patients who present late after ingestion of an overdose of paracetamol.

Indole-N-methylated beta-carbolinium ions as potential brain-bioactivated neurotoxins

N-Methyl-4-phenylpyridinium ion (MPP+), a highly toxic metabolite produced in the brain from a street drug contaminant, is selectively taken up by nigrostriatal dopaminergic neurons and accumulated intraneuronally in mitochondria. There it inhibits respiration, causes neuronal death and, in primates, provokes a parkinsonian condition. It has been suggested that endogenously generated or activated agents resembling MPP+ may contribute to the development of Parkinson's disease. We report here that simple beta-carbolines derived from tryptophan or related open chain indoles, when specifically methyl-substituted on both (2[beta] and 9[indole]) available nitrogens, display mitochondrial inhibitory potencies and neurotoxic effects in vitro (PC12 cultures) and in vivo (striatal microdialysis) which approach or even surpass MPP+. These results take on physiological significance with our finding that brain enzyme activity catalyzes S-adenosylmethionine-dependent methylations of the beta- and indole-nitrogens in beta-carbolines that have been detected in vivo. The unusual 9[indole]-N-methyl transfer, previously unrecognized in animals, apparently requires prior methylation of the 2[beta]-nitrogen. Sequential di-N-methylation of endogenous or xenobiotic beta-carbolines to form unique, neurotoxic 2,9-N,N'-dimethyl-beta-carbolinium ions may serve as a brain bioactivation route in chronic neurodegenerative conditions such as Parkinson's disease.

The pathogenesis of homocysteinemia: interruption of the coordinate regulation by S-adenosylmethionine of the remethylation and transsulfuration of homocysteine

A unified, biochemical hypothesis is proposed to explain the pathogenesis of homocysteinemia. This hypothesis is based on the existence of coordinate regulation by S-adenosylmethionine (SAM) of the partitioning of homocysteine between de novo methionine synthesis and catabolism through cystathionine synthesis. This coordination, which serves to modulate the cellular concentration of homocysteine based on the requirements for methionine, is impaired in homocysteinemia. This hypothesis is evaluated in the context of the conditions known to be associated with homocysteinemia, including enzymatic defects and vitamin deficiencies. The novelty of the hypothesis is the assertion that impairment of one homocysteine metabolic pathway must lead to the impairment of the other homocysteine metabolic pathway to cause homocysteinemia. This extends the simplistic view that a block of only one of the pathways is sufficient to cause homocysteinemia.

Role of S-adenosyl-L-methionine in the treatment of intrahepatic cholestasis

Recent studies have established the clinical efficacy of S-adenosyl-L-methionine (SAMe) in the treatment of cholestasis associated with hepatic diseases, pregnancy and the administration of estrogen-containing oral contraceptives. In 4 clinical trials involving a total of 639 patients with cholestasis due to acute or chronic liver disease, SAMe in an intravenous dose of 800 mg/day or an oral regimen of 1.6 g/day for 2 weeks was superior to placebo in relieving the symptom of pruritus and in restoring serum total bilirubin and serum alkaline phosphatase towards normal. The drug is also effective in intrahepatic cholestasis of pregnancy (ICP), with intravenous administration of 800 mg/day for 2 weeks producing a substantial reduction in pruritus and an improvement in abnormal liver function indices. Moreover, SAMe treatment decreases the incidence of premature labour. SAMe appears to be the first safe and effective approach to the treatment of this syndrome, and also protects against the adverse hepatic effects of small doses of estrogen in patients with a history of ICP by normalising liver biochemistry and the oversaturated biliary lipid composition of the gallbladder bile. In animal models, SAMe reverses the pathological liver changes induced by xenobiotics such as taurolithocholate and alpha-naphthyl-isothiocyanate (ANIT) and the antipsychotic chlorpromazine. Several cooperative mechanisms appear to underlie the anticholestatic action of SAMe, the most important being the restoration of normal hepatocyte membrane fluidity and Na+, K+ ATPase activity, through a reversal of the reduction in phospholipid methylation produced by hepatotoxic agents. In addition, SAMe may act by promoting trans-sulphuration pathway reactions and consequently improving the detoxifying capacity of this metabolic system.

S-adenosylmethionine-dependent enzyme activation

S-Adenosylmethionine (SAM)-dependent activations of pyruvate formate-lyase, lysine 2,3-aminomutase and cobalamin-dependent methionine synthase are discussed. In each case, cleavage of SAM is accompanied by the formation of a catalytically active enzyme. The chemistry of activation of these three enzymes falls into three distinct classes: generation of an essential enzyme radical (pyruvate formate-lyase), formation of a catalytically active 5'-deoxyadenosyl radical (lysine 2,3-aminomutase) and reductive methylation to form a required methylcobalamin complex (methionine synthase).

The role of formate and S-adenosylmethionine in the reversal of nitrous oxide inhibition of formate oxidation in the rat

Studies have been performed in rats in order to test whether methionine reverses the inhibition of formate oxidation produced by nitrous oxide by virtue of the conversion of methionine to formate. At a dose of methionine (100 mg/kg, 671 mumol/kg) that completely reverses the nitrous oxide inhibition of formate oxidation no significant conversion of the methyl group, carboxyl, or backbone of methionine to formate was apparent. No increases in hepatic formate levels were seen after the administration of 671 mumol/kg methionine or ethionine, and formate treatment did not alter the rate of 14CO2 formed after methionine was administered labeled in the methyl, carboxyl, or backbone position. The reversal of nitrous oxide inhibition of formate oxidation was found to correlate temporally with either S-adenosylmethionine levels after methionine administration or S-adenosylethionine levels following ethionine treatment. After methionine or ethionine administration, elevated hepatic steady state levels of tetrahydrofolate were observed and were coincident with elevated S-adenosylmethionine or S-adenosylethionine. Since formate oxidation rates are dependent on the hepatic tetrahydrofolate level, the mechanism of methionine reversal of nitrous oxide inhibition appears to be related to effects of hepatic S-adenosylmethionine which are important in maintaining and regulating tetrahydrofolate, rather than formate generation from methionine.

S-adenosylmethionine may not be essential for signal transduction during bacterial chemotaxis

We previously showed that a mutant strain of Salmonella typhimurium completely deficient in both the chemoreceptor methylating (CheR) and demethylating (CheB) enzymes can still exhibit chemotaxis to aspartate and other attractants (J. Stock, A. Borczuk, F. Chiou, and J. E. B. Burchenal, Proc. Natl. Acad. Sci. USA 82:8364-8368, 1985). We used this cheR cheB mutant to examine the possibility of an additional requirement for S-adenosylmethionine in chemotaxis besides its role in chemoreceptor methylation. A metE mutation was transduced into a cheR cheB double mutant, and the cells were starved for methionine. Despite the fact that intracellular S-adenosylmethionine dropped from approximately 100 microM to less than 0.2 microM, chemotaxis was largely unaffected. In contrast, a corresponding cheR+ cheB+ metE mutant completely lost its chemotaxis ability after being starved for methionine. We conclude from this observation that the primary requirement for S-adenosylmethionine during bacterial chemotaxis is in the methylation of receptor proteins.

S-adenosyl-L-methionine: its effect on aminolevulinate dehydratase and glutathione in acute ethanol intoxication

The effect of disulfiram and S-adenosyl-L-methionine (SAM) administration to acute ethanol intoxicated mice on the hepatic glutathione (GSH) concentration and aminolevulinic and dehydratase (ALA-D) activity was investigated. It was found that both GSH levels and ALA-D activity were decreased, and evidence suggested that the toxic action of ethanol was due to its conversion into acetaldehyde. Administration of SAM reverses the effects of acute alcohol abuse by increasing liver GSH availability. In vitro, hepatic ALA-D activity was not modified by ethanol; instead it was non-competitively inhibited by acetaldehyde. This inhibition was efficiently reversed by GSH and cysteine (CySH). Therefore, a mechanism for the action of ethanol on ALA-D, based on the inhibitory effect of acetaldehyde, is proposed.