A thin-layer chromatographic method for the quantitative separation and estimation of S-adenosylmethionine and S-adenosylethionine in rat liver

Alterations in epidermal growth factor binding to hepatic membranes following dietary exposure of rats to known hepatocarcinogens

Administration of the chemical carcinogen 2-acetylaminofluorene (2-AAF) has previously been shown to lower hepatic epidermal growth factor (EGF) binding levels during chemically induced hepatocarcinogenesis. To further characterize the specificity of this response, EGF binding levels for liver microsomes were determined after a 3-week administration of subacute doses of 2-AAF and five other known hepatocarcinogens: 3'-methyl-4-dimethylaminoazobenzene (3'Me-DAB), 2-AAF, aflatoxin B1 (AFB1), thioacetamide (TA), ethionine, benzidine (Benz), as well as four non-hepatocarcinogens: fluorene, p-aminoazobenzene, 4-acetylaminofluorene (4-AAF), and 3-methylcholanthrene. Five of six of the hepatocarcinogens tested (3'Me-DAB, 2-AAF, TA, AFB1 and Benz) caused significant lowering of EGF binding levels, and one of the four non-hepatocarcinogens (4-AAF) caused significant lowering of EGF binding levels. Paired feeding studies indicated that the decreases in EGF binding levels were not a result of differences in net diet consumption. These findings show that decreases in EGF binding capacity are caused by a diverse group of known hepatocarcinogenic compounds at an early stage in the carcinogenesis process.

13-cis-retinoic acid and hepatic steatosis in rats

The effect of administration of 13-cis-retinoic acid (100 mg/kg diet) on lipid metabolism was examined in male rats fed either a 20% casein + 0.3% methionine diet, a 20% casein diet, a 10% casein + 0.3% methionine diet, or a 10% casein + 0.6% methionine diet for 10 days. Hepatic triglyceride concentrations of rats fed either 10% casein diet were 3-fold greater than animals receiving diets containing 20% casein. The addition of 13-cis-retinoic acid to the diet further increased the total hepatic lipid (43-56%) and triglyceride (approximately 2-fold) concentrations in rats fed the 10% casein diets. 13-cis-Retinoic acid supplementation did not alter the total liver lipid or triglyceride concentrations in rats fed either of the 20% casein diets. Thus, under specific dietary conditions, the administration of 13-cis-retinoic acid resulted in a marked accumulation of hepatic lipids which did not appear to be related to the total methionine content of the diet nor to the hepatic concentrations of S-adenosylmethionine and glutathione. In addition, all four groups of 13-cis-retinoic acid-fed rats exhibited elevations in the concentration of serum triglycerides, and 10-20% reductions in serum cholesterol concentrations.

Increased levels of S-adenosylmethionine in the livers of rats fed various forms of selenium

The levels of S-adenosylmethionine (SAM) were determined in the livers of male weanling rats fed six different forms of selenium for five weeks. The following forms of selenium were administrated in the diet in logarithmic doses: sodium selenite, sodium selenate, sodium selenide, selenomethionine, selenocystine, and selenium sulfide. An overall increase in hepatic SAM was observed, and all compounds resulted in at least one observation where SAM was significantly elevated (p < 0.01). No dose-response relationship was found to exist, however. A comparison of the relative toxicity of each of the selenicals was based on a dose of 10 ppm dietary selenium for each chemical form. The elevation of SAM resulting from the subchronic administration of selenium may be one mechanism involved in the well-known chemopreventive effects in experimental models.

The relationship between the activity of methionine synthase and the ratio of S-adenosylmethionine to S-adenosylhomocysteine in the brain and other tissues of the pig

Using nitrous oxide to inactivate methionine synthase in vivo, the relationship of the activity of methionine synthase to the S-adenosylmethionine (AdoMet)/S-adenosylhomocysteine (AdoHcy) ratio was examined in neural and other tissues of the pig. Pigs were exposed to 15% nitrous oxide for varying intervals of up to 7 days or studied at varying intervals of recovery in air after 7 days nitrous oxide inhalation, and the rate of inactivation or resynthesis of methionine synthase was related to the corresponding AdoMet/AdoHcy ratios. The rate of inactivation of enzyme during nitrous oxide exposure was considerably faster in the liver and kidney than in the brain and spinal cord with activity levelling off between 10% and 20% of control values. The AdoMet/AdoHcy ratio fell in all tissues during nitrous oxide treatment, the fall being most marked in the brain and spinal cord where a 10-fold change occurred. This change was attributed mainly to a rise in AdoHcy levels. The recovery pattern of methionine synthase was broadly linear but was slower in the spinal cord (0.10 +/- 0.03% per hr; mean +/- SEM) than in any other tissue examined including brain (0.35 +/- 0.04% per hr). Correspondingly, the recovery of the AdoMet/AdoHcy ratio was also significantly slower in the spinal cord. When values for exposure and recovery were combined there was a significant correlation between the activity of methionine synthase and the AdoMet/AdoHcy ratio in both the brain (r = 0.90; P < 0.001) and the spinal cord (r = 0.92; P < 0.001). These results support the concept that the AdoMet/AdoHcy ratio is closely related to the pathogenic process which produces the neurologic lesions associated with a reduction in methionine synthase activity.

A new high performance liquid chromatographic method for the simultaneous measurement of S-adenosylmethionine and S-adenosylhomocysteine. Concentrations in pig tissues after inactivation of methionine synthase by nitrous oxide

A rapid gradient method has been established to measure simultaneously the tissue levels of S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy). The method involves application of a 15%-25% linear gradient of methanol over a period of 10 min to an initial mobile phase of 15% methanol with 25 mM sodium phosphate, 10 mM 1-heptanesulfonic acid solution at pH 3.2 and a flow rate of 1 mL/min. AdoHcy elutes at 9.5 min and AdoMet at 12.6 min. The assay has a detection limit of 10 pmol and is linear in the concentration range 30-800 pmol. The method was used to assess changes in AdoMet and AdoHcy concentrations of pig tissues after seven days exposure to the anaesthetic gas nitrous oxide which irreversibly inactivates methionine synthase and induces an inability to recycle homocysteine, particularly in neural tissues. The treatment caused significant alterations in cellular AdoMet:AdoHcy ratios which were principally due to a dramatic rise in AdoHcy concentrations.

Regulation of S-adenosyl methionine synthesis in the mouse embryo

In early embryos, methylation is involved in "gamete imprinting" and inactivation of artificially introduced foreign genes. We studied the biosynthesis of the universal methylation cofactor: S-Adenosyl methionine (SAM). In the mouse, SAM conversion from methionine is limited by saturation of the methionine endogenous pool. SAM is present at a practically unchanged level from the unfertilized oocyte to early morula. SAM synthesis is increased at the time of compaction. In blastocysts, although methionine uptake is increased, the conversion rate from methionine is lowered. We observed no differences between C57 Black and Swiss albino random bred strains. In few experiments with human unfertilized oocytes and spared embryos, we observed higher methionine incorporation, and higher conversion to SAM. Next, the effect of two methylation inhibitors was tested, on early mouse embryonic development, at the one-cell and the two-cell stage. We found that ethionine is very toxic, even at the lowest tested concentration of 25 microM. Homocysteine is more potent at the one-cell stage than at the 2-cell stage, and it only partially blocks blastocyst formation from the 2-cell stage even at a concentration of 500 microM. It clearly acts as a methylation inhibitor; it lowers the SAM pool and the methylation index, SAH/SAM ratio (SAH: S-Adenosyl Homocysteine). We also found that homocysteine is an unexpected competitor for methionine influx and efflux.

Choline deficiency and chemical carcinogenesis

We have reviewed the current status of our knowledge concerning the biologic effects of dietary choline (lipotrope) deficiency in modifying chemical carcinogenesis in experimental animals and discussed its possible mechanisms. Choline deficiency produces various pathologic lesions, involving virtually every organ of the body, as a result of a decrease in phospholipid and acetylcholine synthesis and in the supply of labile methyl groups. The liver is the only organ in which a relationship has been consistently demonstrated between choline deficiency and chemically induced tumors. The deficient diet enhances the initiating potency of several carcinogens and acts as a strong cocarcinogen. Diet also exerts a strong promoting effect, though the possibility that it is a complete carcinogen cannot be ruled out. Phase I enzymes of the carcinogen metabolizing system are uniformly depressed by choline deficiency, but very little information is available regarding the effects of diet on Phase II enzymes that detoxify carcinogen metabolites. Possible modifications of carcinogen-induced DNA damage and their repair processes have not been adequately scrutinized. Solid evidence suggests that feeding a choline-deficient diet leads to enhanced liver cell proliferation, an inadequate supply of methyl groups for transmethylation reactions, and membrane lipid peroxidation. Induced cell proliferation and hypomethylation of DNA may alter the state of gene expression, including that of specific cellular oncogenes. Lipid peroxidation may alter the structure and function of membrane receptors related to liver cell growth or may directly damage cellular DNA. Thus these alterations, individually or in combination, could play a critical role in the diet-induced modification of chemical carcinogenesis.

Methionine metabolism and cancer

This paper summarizes recent developments linking methionine metabolism and S-adenosylmethionine to DNA methylation and gene expression in relation to cancer. Methionine, obtained in the diet and synthesized by several reactions in the body, is the sole precursor of S-adenosylmethionine, the primary methyl donor in the body. Disruptions in methionine metabolism and methylation reactions may be involved in cancer processes. S-Adenosylmethionine is involved in, inter alia, the methylation of a small percentage of cytosine bases of DNA. Recent evidence suggests that enzymatic DNA methylation is an important component of gene control and may serve as a silencing mechanism for gene function. Some carcinogens interfere with enzymatic DNA methylation, and thus may allow oncogene activation. Demethylation may be a necessary, but not always sufficient, condition for enhanced transcription. DNA hypomethylation has been observed in many cancer cells and tumors. The hypothesis that oncogenic transformation may be prevented or even reversed by a diet containing excess methionine and/or choline needs to be further investigated.

A radioenzymatic method for S-adenosyl-L-methionine determination in biological fluids

Current methods for the radioenzymatic assay of S-adenosyl-L-methionine (AdoMet) in biological fluids have been modified in order to increase sensitivity. The modified procedure has allowed to measure AdoMet content also in plasma and cerebrospinal fluid where the concentrations have been found to range between 17 and 72 ng/ml in the different animal species.

Tissue distribution of captopril, reducible captopril conjugates and S-methylcaptopril in the rat

The tissue distribution of captopril, an antihypertensive drug possessing a free sulfhydryl group, and its sulfur-conjugated metabolites was studied in rats by gas chromatography-mass spectrometry at 15, 30 and 60 min following a single 10 mg/kg oral dose of captopril. It was found that tissue accumulation of captopril was rapid with both free and oxidized-forms already present at 15 min post-dose. A maximum concentration of captopril was achieved at 30 min in tissues studied, being substantially higher in kidney (14.2 micrograms/g), with lesser amounts occurring in liver, lung, heart, blood cells, spleen and plasma in that order. Oxidized disulfide forms of captopril were usually present in the same or slightly higher proportion than free captopril except for liver which only contained detectable disulfides at 15 min after oral dosing. S-methylcaptopril was also present at 30 min in all tissues examined with highest levels occurring in liver and kidney (1.05 micrograms/g) followed by plasma, lung, heart, spleen and blood cells.

Reversed-phase ion-pair liquid chromatographic procedure for the simultaneous analysis of S-adenosylmethionine, its metabolites and the natural polyamines

A method using reversed-phase ion-pair liquid chromatography with dual detection was developed for the simultaneous determination of the S-adenosylmethionine (SAM) analogues and the natural polyamines. The separation is obtained with a gradient elution and by adjusting the concentration of octanesulfonic acid used as ion-pairing agent, the ionic strength of the eluent, the pH and the acetonitrile content of the eluents. The SAM analogues are analyzed by UV detection at 254 nm and the polyamines by fluorescence detection after post-column derivatization with o-phthalaldehyde. The method allows the determination of the SAM analogues and the polyamines in one single run by direct injection of tissue extracts. The procedure is applied to the study in rats and in hepatoma tissue culture cells of the biochemical effects of alpha difluoromethylornithine, a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase.

An inverse correlation between hepatic ornithine decarboxylase and S-adenosylmethionine in rats

The comparative effects of the subchronic administration to rats of ethionine-supplemented and of chemically defined methyl-deficient diets on the hepatic levels of S-adenosylmethionine (SAM) and of ornithine decarboxylase (ODC), an enzyme marker of cell proliferation, were studied. Both treatments led to decreased hepatic levels of SAM and to marked increased activities in ODC. Both systems led to significant inverse correlations between ODC and SAM. In rats fed the methyl-deficient diets, hepatic levels of SAM were generally proportional to the dietary content of methionine and choline. The metabolic increases in S-adenosylmethionine decarboxylase (SAMDC) observed in the livers of methyl-deficient rats were proportional to the changes seen in ODC.