Methionine metabolism in mammals

The effect of elevated homocysteine levels on adrenergic vasoconstriction of human resistance arteries: the role of the endothelium and reactive oxygen species

PURPOSE

This study investigated the effect of elevated homocysteine levels on adrenergic contraction of human resistance arteries and tested the hypothesis that homocysteine-induced generation of reactive oxygen species contributes to vascular reactivity changes.

METHODS

Small (<200 microm) subcutaneous arteries were cannulated and pressurized in an arteriograph chamber that allowed the measurement of lumen diameter. Two arteries from the same patient were obtained; one was perfused and superfused (intraluminal pressure = 50 mm Hg) with physiologic saline solution (control, n = 6), and the other was perfused and superfused with physiologic saline solution plus 200 micromol/L homocysteine (HC, n = 6); the reactivity to adrenergic stimulation was assessed. Another group of arteries was incubated in 200 micromol/L homocysteine plus 1200 U/mL superoxide dismutase and 120 U/mL catalase (HC + SC, n = 6), and the reactivity to norepinephrine was determined. The vasoreactivity of homocysteine was further assessed in intact (n = 6) and denuded (n = 6) arteries that were precontracted with an intermediate concentration of norepinephrine and homocysteine (20-200 micromol/L) added to the bath while the lumen diameter was continuously recorded.

RESULTS

Sensitivity to norepinephrine was diminished in HC arteries, which increased the median effective concentration (EC(50)) from 0.24 +/- 0.06 micromol/L in control arteries to 0.65 +/- 0.10 micromol/L in HC arteries (P <.01). Homocysteine also caused concentration-dependent vasodilation of arteries contracted with an intermediate concentration of norepinephrine that was greater in intact than denuded arteries, with the half-maximum responses occurring at 61 +/- 6 micromol/L (intact) and 90 +/- 11 micromol/L (denuded; P <.05). There was no significant difference in sodium nitroprusside sensitivity between control and homocysteine arteries (EC(50) = 61 +/- 3 nmol/L vs 50 +/- 19 nmol/L; P >.05) or in sensitivity to acetylcholine (EC(50) = 19 +/- 7 nmol/L vs 12 +/- 3 nmol/L; P >.05). Arteries in the presence of superoxide dismutase and catalase had similarly impaired reactivity to norepinephrine as did homocysteine arteries (EC(50), 0.58 +/- 0.15 micromol/L; P >.05 vs HC, P <.01 vs control).

CONCLUSION

An elevated homocysteine level in vitro diminishes adrenergic contraction, with a differential endothelial versus smooth muscle influence that appears unrelated to the generation of reactive oxygen species.

Homocysteine

Homocysteine does not occur in the diet but it is an essential intermediate in normal mammalian metabolism of methionine. Each compound, methionine or homocysteine, is the precursor of the other. Similarly, the synthesis of one is the mechanism for the detoxification of the other. The ubiquitous methionine cycle is the metabolic basis for this relationship. In some tissues the transsulfuration pathway diverts homocysteine from the cycle and provides a means for the synthesis of cysteine and its derivatives. Methionine, (or homocysteine) metabolism is regulated by the disposition of homocysteine between these competing sequences. Both pathways require vitamin-derived cofactors, pyridoxine for transsulfuration and both folate and cobalamin in the methionine cycle. The clinical consequences of disruption of these pathways was apparent first in rare inborn errors of metabolism that cause homocystinuria, but recent studies focus on "hyperhomocysteinemia"--a lesser metabolic impairment that may result from genetic variations, acquired pathology, toxicity and nutritional inadequacy. Hyperhomocysteinemia is an independent risk factor for thrombovascular diseases however it is not clear whether the minimally increased concentration of the amino acid is the causative agent or merely a marker for the pathology. Until we resolve that question we cannot predict the potential efficacy of therapies based on folate administration with or without additional cobalamin and pyridoxine.

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.

Correlation between Plasma Total Homocysteine and Copper in Patients with Peripheral Vascular Disease

Background: Increased concentrations of both plasma total homocysteine and copper are separately associated with cardiovascular disease. Correlations between plasma total homocysteine, trace elements, and vitamins in patients with peripheral vascular disease have not been investigated.

Methods: The concentrations of trace elements in plasma were determined by the multielement analytical technique of total-reflection x-ray fluorescence spectrometry. Plasma total homocysteine was determined by HPLC.

Results: In the univariate and multivariate regression analyses, copper was positively correlated with plasma total homocysteine in all subjects (coefficient ± SE, 0.347 ± 0.113; P = 0.0026 and coefficient ± SE, 0.422 ± 0.108; P = 0.0002, respectively), and in patients with peripheral vascular disease (coefficient ± SE, 0.370 ± 0.150; P = 0.016; and coefficient ± SE, 0.490 ± 0.151; P = 0.0025, respectively). Correlation between copper and plasma total homocysteine was not detected in healthy control subjects. The concentration of calcium in plasma (67.5 vs 80.8 μg/g) was significantly lower in the patients than in the control subjects (P = 0.02). When the patients were divided into groups, the patients with suprainguinal lesions had significantly higher copper concentrations (P = 0.04) and significantly lower selenium and calcium concentrations (P = 0.01 and 0.008, respectively) than the healthy subjects. Patients had higher concentrations of autoantibodies against oxidized LDL and concentrations of thiobarbituric acid-reactive substance than the healthy subjects (P &lt;0.0001 and P = 0.001, respectively). The concentrations of plasma total homocysteine and α-tocopherol were significantly higher, and the concentrations of vitamin B6 and β-carotene were lower in the patients than the healthy subjects.

Conclusion: Our findings suggest that the atherogenicity of homocysteine may be related to copper-dependent interactions.

Effects of dietary zinc deficiency on homocysteine and folate metabolism in rats

In rats, zinc deficiency has been reported to result in elevated hepatic methionine synthase activity and alterations in folate metabolism. We investigated the effect of zinc deficiency on plasma homocysteine concentrations and the distribution of hepatic folates. Weanling male rats were fed ad libitum a zinc-sufficient control diet (382.0 nmol zinc/g diet), a low-zinc diet (7.5 nmol zinc/g diet), or a control diet pair-fed to the intake of the zinc-deficient rats. After 6 weeks, the body weights of the zinc-deficient and pair-fed control groups were lower than those of controls, and plasma zinc concentrations were lowest in the zinc-deficient group. Plasma homocysteine concentrations in the zinc-deficient group (2.3 +/- 0.2 micromol/L) were significantly lower than those in the ad libitum-fed and pair-fed control groups (6.7 +/- 0.5 and 3.2 +/- 0.4 micromol/L, respectively). Hepatic methionine synthase activity in the zinc-deficient group was higher than in the other two groups. Low mean percentage of 5-methyltetrahydrofolate in total hepatic folates and low plasma folate concentration were observed in the zinc-deficient group compared with the ad libitum-fed and pair-fed control groups. The reduced plasma homocysteine and folate concentrations and reduced percentage of hepatic 5-methyltetrahydrofolate are probably secondary to the increased activity of hepatic methionine synthase in zinc deficiency.

Measurement of Plasma and Intracellular S-Adenosylmethionine and S-Adenosylhomocysteine Utilizing Coulometric Electrochemical Detection: Alterations with Plasma Homocysteine and Pyridoxal 5′-Phosphate Concentrations

Background: The relative changes in plasma and intracellular concentrations of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) may be important predictors of cellular methylation potential and metabolic alterations associated with specific genetic polymorphisms and/or nutritional deficiencies. Because these metabolites are present in nanomolar concentrations in plasma, methods of detection generally require time-consuming precolumn processing or metabolite derivatization.

Methods: We used HPLC with coulometric electrochemical detection for the simultaneous measurement of SAM and SAH in 200 μL of plasma, 106 lymphocytes, or 10 mg of tissue. Filtered trichloroacetic acid extracts were injected directly into the HPLC system without additional processing and were eluted isocratically.

Results: The limits of detection were 200 fmol/L for SAM and 40 fmol/L SAH. In plasma extracts, the interassay CV was 3.4–5.5% and the intraassay CV was 2.8–5.6%. The analytical recoveries were 96.8% and 97.3% for SAM and SAH, respectively. In a cohort of healthy adult women with mean total homocysteine concentrations of 7.3 μmol/L, the mean plasma value was 156 nmol/L for SAM and 20 nmol/L for SAH. In women with increased homocysteine concentrations (mean, 12.1 μmol/L), plasma SAH, but not SAM, was increased (P &lt;0.001), and plasma pyridoxal 5′-phosphate concentrations were reduced (P &lt;0.001). Plasma SAM/SAH ratios were inversely correlated with homocysteine concentrations (r = 0.73; P &lt;0.01), and the SAM/SAH ratio in plasma was directly correlated with the intracellular SAM/SAH ratio in lymphocytes (r = 0.70; P &lt;0.01).

Conclusions: Increased homocysteine in serum is associated with an increase in SAH and a decrease in the SAM/SAH ratio that could negatively affect cellular methylation potential. Accurate and sensitive detection of these essential metabolites in plasma and in specific tissues should provide new insights into the regulation of one-carbon metabolism under different nutritional and pathologic conditions.

Cysteine kinetics and oxidation at different intakes of methionine and cystine in young adults

BACKGROUND

We previously studied methionine kinetics and oxidation with the tracer L-[1-(13)C, methyl-(2)H(3)]methionine.

OBJECTIVES

We sought to explore methionine-cysteine interrelations in adults by using L-[1-(13)C]cysteine under different dietary conditions.

DESIGN

In experiment 1, 12 adults consumed a protein-free diet for 6 d. On day 7, methionine (n = 6) or cysteine (n = 6) oxidation rates were measured during an 8-h continuous infusion of L-[1-(13)C, methyl-(2)H(3)]methionine or L-[1-(13)C]cysteine, respectively. In experiment 2, 6 young men consumed 3 diets for 6 d each before a tracer study on day 7 with L-[1-(13)C]cysteine. The amounts (in mg*kg(-)(1)*d(-)(1)) of methionine and cysteine, respectively, were: high-methionine (HM) diet, 13 and 0; low-methionine (LM) diet, 6.5 and 0; and methionine-plus-cystine (MC) diet, 6.5 and 5.6. Cysteine flux and oxidation rates were determined and sulfur amino acid (SAA, methionine plus cysteine) balances were estimated.

RESULTS

In experiment 1, rates of methionine and cysteine oxidation were similar to losses predicted from obligatory nitrogen losses. In experiment 2, SAA balance was less negative when subjects consumed the HM diet than the LM and MC diets (interaction, P = 0.034), largely because of a difference in fed-state balance (HM compared with LM, P < 0.01; HM compared with MC, P < 0.05). There was no evidence of a sparing effect of dietary cystine on the methionine requirement.

CONCLUSION

These studies support use of [1-(13)C]cysteine for studying whole-body SAA oxidation and conclusions that maintenance of SAA balance is best achieved by supplying methionine at approximately the FAO/WHO/UNU recommendations for total SAA intake (13 mg*kg(-)(1)*d(-)(1)).

Unfiltered coffee increases plasma homocysteine concentrations in healthy volunteers: a randomized trial

BACKGROUND

An elevated plasma homocysteine concentration is a putative risk factor for cardiovascular disease. Observational studies have reported an association between coffee consumption and plasma homocysteine concentrations.

OBJECTIVE

We studied the effect of coffee consumption on plasma homocysteine in a crossover trial. We used unfiltered coffee so as to include the possible effects of coffee diterpenes, which are removed by filtering.

DESIGN

Sixty-four healthy volunteers (31 men and 33 women) with a mean (+/-SD) age of 43 +/- 11 y were randomly assigned to 2 groups. One group (n = 30) drank 1 L unfiltered cafetière (French press) coffee daily for 2 wk. Such coffee is rich in the cholesterol-raising diterpenes kahweol and cafestol. The other group (n = 34) received water, milk, broth, tea, and chocolate drinks instead of coffee. After a washout period of 8 wk, both groups received the alternate intervention for another 2 wk.

RESULTS

Consumption of 1 L unfiltered coffee/d for 2 wk significantly raised fasting plasma homocysteine concentrations by 10%, from 12.8 to 14.0 micromol/L.

CONCLUSIONS

Unfiltered coffee increases plasma homocysteine concentrations in volunteers with normal initial concentrations. It is unclear whether the effect is caused by the cholesterol-raising diterpenes present exclusively in unfiltered coffee or by factors that are also present in filtered coffee.

Augmentative effects of L-cysteine and methylmethionine sulfonium chloride on mucin secretion in rabbit gastric mucous cells

BACKGROUND

Our previous study showed that L-cysteine (Cys) and methylmethionine sulfonium chloride (MMSC) inhibited ethanol-induced gastric mucosal damage and increased the amount of surface mucin in rats. This study examined whether Cys and MMSC augmented mucin secretion and changed distribution of mucin vesicles ultrastructurally in mucous cells by using primary cultured mucous cells from rabbit glandular stomach. Changes in intracellular cyclic adenosine 3',5'-monophosphate (cAMP) and in levels of cytosolic free Ca2+ were investigated by treatment with Cys and MMSC.

METHODS

Mucin content was measured by an enzyme-linked lectin assay. Transmission electron micrography was used to examine ultrastructural distribution of mucin granules. The amount of cAMP or levels of free Ca2+ were measured by enzyme immunoassay or by fura-2. 16,16-Dimethyl prostaglandin E2 (dmPGE2) or ATP was used as the positive control.

RESULTS

L-Cysteine and MMSC increased mucin secretion and decreased cellular mucin content. The same was noted for dmPGE2. Accelerated mucin granule movements toward the plasma membrane were shown by these agents. Intracellular cAMP increased with exposure to dmPGE2 for 20 min, while neither Cys nor MMSC increased cAMP. No increase in cytosolic free Ca2+ levels occurred after treatment with Cys or MMSC, but an increase was induced 10 s after the addition of ATP.

CONCLUSIONS

The present findings indicate that the increase in mucin secretion by Cys and MMSC was not mediated through the cAMP or Ca2+ signal transduction pathway, but might occur through non-receptor-mediated mechanisms.

Liver-specific methionine adenosyltransferase MAT1A gene expression is associated with a specific pattern of promoter methylation and histone acetylation: implications for MAT1A silencing during transformation

Methionine adenosyltransferase (MAT) is the enzyme that catalyzes the synthesis of S-adenosylmethionine (AdoMet), the main donor of methyl groups in the cell. In mammals MAT is the product of two genes, MAT1A and MAT2A. MAT1A is expressed only in the mature liver whereas fetal hepatocytes, extrahepatic tissues and liver cancer cells express MAT2A. The mechanisms behind the tissue and differentiation state specific MAT1A expression are not known. In the present work we examined MAT1A promoter methylation status by means of methylation sensitive restriction enzyme analysis. Our data indicate that MAT1A promoter is hypomethylated in liver and hypermethylated in kidney and fetal rat hepatocytes, indicating that this modification is tissue specific and developmentally regulated. Immunoprecipitation of mononucleosomes from liver and kidney tissues with antibodies mainly specific to acetylated histone H4 and subsequent Southern blot analysis with a MAT1A promoter probe demonstrated that MAT1A expression is linked to elevated levels of chromatin acetylation. Early changes in MAT1A methylation are already observed in the precancerous cirrhotic livers from rats, which show reduced MAT1A expression. Human hepatoma cell lines in which MAT1A is not expressed were also hypermethylated at this locus. Finally we demonstrate that MAT1A expression is reactivated in the human hepatoma cell line HepG2 treated with 5-aza-2'-deoxycytidine or the histone deacetylase inhibitor trichostatin, suggesting a role for DNA hypermethylation and histone deacetylation in MAT1A silencing.

Lifestyle and cardiovascular disease risk factors as determinants of total cysteine in plasma: the Hordaland Homocysteine Study

BACKGROUND

Plasma total homocysteine (tHcy) is a cardiovascular disease risk factor and is related to several components of the established cardiovascular disease risk profile. Cysteine is structurally and metabolically related to homocysteine, but data on its association with cardiovascular disease and cardiovascular disease risk factors are sparse.

OBJECTIVE

Our objective was to search for the determinants of plasma total cysteine (tCys) and compare them with those of tHcy.

DESIGN

In this cross-sectional study, we studied 7591 healthy men and 8585 healthy women aged 40-67 y with no history of hypertension, diabetes mellitus, coronary heart disease, or cerebrovascular disease.

RESULTS

In the group aged 40-42 y, tCys was significantly higher in men (&mean;: 273 micromol/L; 2.5-97.5 percentile: 219-338 micromol/L) than in women (253 micromol/L; 202-317 micromol/L) (P < 0.001). In the group aged 65-67 y, there was no significant sex difference in tCys: men (296 micromol/L; 233-362 micromol/L) and women (296 micromol/L; 234-361 micromol/L). As with tHcy, tCys was positively associated with age, total cholesterol concentration, diastolic blood pressure, and coffee consumption. Body mass index was a strong determinant of tCys but was not related to tHcy. Several factors known to influence tHcy, including smoking status, folate and vitamin intake, heart rate, and physical activity, were not associated or were only weakly associated with tCys.

CONCLUSION

Plasma tCys is strongly related to several factors that constitute the cardiovascular disease risk profile. This should be an incentive to determine the role of tCys in cardiovascular disease.

Changes of plasma total homocysteine levels during the menstrual cycle

BACKGROUND

It is known that plasma total homocysteine (tHcy) levels are lower in premenopausal and pregnant women compared with postmenopausal women. To confirm the suggestion that sex steroid hormones are nongenetic factors affecting homocysteine metabolism, we investigated the effect of natural steroid hormone levels on the fasting plasma tHcy in healthy women during the menstrual cycle.

MATERIALS AND METHODS

Fifteen premenopausal women were enrolled in this study. Plasma tHcy, estradiol, progesterone and cortisol concentrations were measured in the luteal and follicular phase. The plasma tHcy concentration was determined by high performance liquid chromatography with fluorescence detection, and the steroid hormones by RIA methods.

RESULTS

Mean homocysteine values increased from 7.8 micromol L-1 in the luteal phase to 8.9 micromol L-1 in the follicular phase (P < 0.000005, Student's paired t-test). We also found slight negative but insignificant correlations of homocysteine levels with estradiol in both phases of the menstrual cycle. In the case of cortisol and progesterone, no significant correlations with plasma homocysteine were found.

CONCLUSION

The study provides the first evidence of significant differences in plasma homocysteine concentration during the menstrual cycle. From our observed findings it is necessary to account for the phase of the menstrual cycle when determining homocysteine in premenopausal women.

Folic acid inhibits homocysteine-induced proliferation of human arterial smooth muscle cells

PURPOSE

An elevated plasma homocysteine level has been identified as an independent risk factor for atherosclerosis. Whether this represents a marker for vascular disease or a direct effect on the vasculature remains unclear. Because vascular smooth muscle cells (VSMCs) play an integral role in the atherosclerotic process, we studied the effect of homocysteine on human infragenicular VSMC proliferation and the role of folic acid in reversing the homocysteine effect.

METHODS

Human infragenicular VSMCs harvested from amputation specimens were studied. Various cell groups were exposed to physiologic (6.25 micromol/L and 12.5 micromol/L) and pathologic (25 micromol/L to 500 micromol/L) concentrations of homocysteine. Similar groups were simultaneously exposed to 20 nmol/L of folic acid. Cell counts and DNA synthesis, as reflected by [methyl-(3)H]-thymidine incorporation, were performed at 6 days and 24 hours, respectively. Additional groups were exposed to various combinations of folic acid (20 nmol/L), vitamin B(6) (145 nmol/L), and vitamin B(12) (0.45 nmol/L) in the presence of homocysteine (25, 50, and 250 micromol/L).

RESULTS

Homocysteine resulted in a dose-dependent increase in DNA synthesis and cell proliferation. Cell counts increased significantly at homocysteine concentrations ranging from 25 micromol/L to 500 micromol/L (P <.05), with a maximal increase of 98% at 500 micromol/L of homocysteine. The addition of 20 nmol/L folic acid resulted in significant inhibition of cell proliferation at all homocysteine concentrations studied (P <.001). Maximal inhibition of 70% occurred in the cells exposed to 50 micromol/L of homocysteine. The increases in [methyl-(3)H]-thymidine incorporation ranged from 36% at 6 micromol/L homocysteine to a maximum of 247% at 500 micromol/L homocysteine. All increases were statistically significant (P <.05). The addition of 20 nmol/L folic acid resulted in significant inhibition of DNA synthesis (P <.002). Vitamins B(6) and B(12) did not demonstrate significant antiproliferative properties.

CONCLUSION

A possible role of homocysteine in the formation of atherosclerotic lesions is through a direct proliferative effect on VSMCs in a dose-dependent fashion. Folic acid intake at levels available in dietary supplements may prove protective in hyperhomocysteinemia-induced atherosclerosis. Vitamins B(6) and B(12) alone do not appear to exhibit a substantial inhibitory effect in the setting of elevated homocysteine levels.

Total homocysteine and cardiovascular disease

Recent data have shown that an elevated plasma level of the amino acid homocysteine (Hcy) is a common, independent, easily modifiable and possibly causal risk factor for cardiovascular disease (CVD) which may be of equal importance to hypercholesterolemia, hypertension and smoking. This paper reviews the biochemical, clinical, epidemiological and experimental data underlying this conclusion and is critically questioning whether elevated tHcy is a causal factor.

Potential new cardiovascular risk factors: left ventricular hypertrophy, homocysteine, lipoprotein(a), triglycerides, oxidative stress, and fibrinogen

The 1996 Bethesda Conference acknowledged left ventricular hypertrophy, hyperhomocysteinemia, lipoprotein(a) excess, hypertriglyceridemia, oxidative stress, and hyperfibrinogenemia as possible new cardiac risk factors. This review summarizes the current literature that supports these conditions as cardiac risk factors. Left ventricular hypertrophy is an independent risk factor for vascular disease. Improvement or progression of left ventricular hypertrophy influences subsequent cardiovascular complications. Clinical trials are under way to assess the potential benefit of decreasing homocysteine levels. The role of lipoprotein(a) excess in vascular disease is controversial. The atherogenic potential of lipoprotein(a) seems to be neutralized by effective reduction of low-density lipoprotein cholesterol levels. Increasing evidence supports an independent role of hypertriglyceridemia in cardiovascular disease and a possible clinical benefit from decreasing triglyceride levels. Among antioxidant micronutrients, supplementation with vitamin E has been shown to be beneficial in primary and secondary prevention studies. Data supporting the use of other antioxidants are much weaker. Preliminary evidence suggests that reducing fibrinogen levels in patients with high baseline levels and coronary disease may be beneficial. Despite the potential relation between new risk factors and cardiovascular disease, routine clinical application of these conditions as cardiovascular risk factors would be premature. Evidence is needed that these conditions extend prognostic ability beyond conventional risk factors and that modification of these conditions can reduce the risk for cardiovascular events.

Normalization of hyperhomocysteinemia with L-thyroxine in hypothyroidism

BACKGROUND

Hyperhomocysteinemia is an independent risk factor for coronary, peripheral, and cerebrovascular disease. Elevated plasma homocysteine levels were described in a preliminary report on primary hypothyroidism.

OBJECTIVE

To determine whether restoration of euthyroidism by L-thyroxine replacement therapy would reduce or normalize plasma homocysteine levels.

DESIGN

Prospective cohort study.

SETTING

Outpatient endocrinology department of a tertiary center.

PATIENTS

14 patients (10 women and 4 men; 25 to 77 years of age): 4 with newly diagnosed chronic (Hashimoto) hypothyroidism and 10 who had been rendered acutely hypothyroid (thyroid-stimulating hormone level > 25 mU/L) by total thyroidectomy for thyroid carcinoma.

MEASUREMENTS

Total plasma homocysteine levels were measured at baseline and 3 to 9 months later, after euthyroidism had been attained by L-thyroxine replacement therapy.

RESULTS

Median baseline plasma homocysteine levels in both sexes (women, 11.65 micromol/L [range, 7.2 to 26.5 micromol/L]; men, 15.1 micromol/L [range, 14.1 to 16.3 micromol/L]) were higher (P = 0.002) than those in healthy female (n = 35) and male (n = 36) volunteers (women, 7.52 micromol/L [range, 4.3 to 14.0 micromol/L]; men, 8.72 micromol/L [range, 5.94 to 14.98 micromol/L]). Eight patients (57%) had baseline plasma homocysteine levels that exceeded the upper limit of sex-specific reference ranges. Upon attainment of euthyroidism, all patients had a diminution in plasma homocysteine levels. The median overall change of -5.5 micromol/L (range, -15.4 to -1.8 micromol/L) corresponds to a difference of -44% (range, -58% to -13%) (P < 0.001). Homocysteine levels returned to normal in 7 of the 8 patients with elevated pretreatment values.

CONCLUSIONS

Hypothyroidism may be a treatable cause of hyperhomocysteinemia, and elevated plasma homocysteine levels may be an independent risk factor for the accelerated atherosclerosis seen in primary hypothyroidism.

Regulation of homocysteine metabolism

We have used a combination of in vivo and in vitro techniques to measure factors regulating homocysteine metabolism and the plasma concentration of this atherogenic amino acid. The germane findings include: 1. Homocysteine metabolism in rat kidney proceeds predominantly through the transsulfuration pathway, whose enzymes are enriched within the proximal cells of kidney tubules. Furthermore, the rat kidney possesses significant reserve capacity to handle both acute and chronic elevations in plasma homocysteine concentrations. 2. Plasma homocysteine concentrations are lower in diabetic rats. Insulin administration corrects this perturbation. Therefore, insulin and/or one of its counter-regulatory hormones affects homocysteine metabolism, possibly through an increased flux in the hepatic transsulfuration pathway. In support of these data, glucagon administration to rats produced similar results. Further support was provided by studies with isolated rat hepatocytes, from which homocysteine export was reduced when incubated in the presence of glucagon.

Homocysteine metabolism in cardiovascular cells and tissues: implications for hyperhomocysteinemia and cardiovascular disease

We have determined the activity and protein levels of CBS in a number of cardiovascular cells and tissues by direct enzyme assay and Western blot analysis, respectively. We have also determined the activity of BHMT in these same tissues and cells and have come to the conclusion that neither enzyme is expressed. This results suggests that in the human cardiovascular system homocysteine metabolism is limited to the remethylation pathway catalyzed by MS. Thus, hyperhomocysteinemia in conjunction with a limited metabolic capacity for homocysteine in the cardiovascular system could result in cellular dysfunction.

Biochemical analysis of myelin lipids and proteins in a model of methyl donor pathway deficit: effect of S-adenosylmethionine

S-Adenosylmethionine (SAMe) is the methyl donor to numerous acceptor molecules. We used cycloleucine (CL), which prevents the conversion of methionine to SAMe by inhibiting ATP-l-methionine-adenosyltransferase (MAT), to characterize the lipid and protein changes induced in peripheral nerve and brain myelin in rats during development. We also investigated the effect of exogenous SAMe by administering SAMe-1,4-butane disulfonate (SAMe-SD4). CL was given on days 7, 8, 12, and 13 and SAMe-SD4 was given daily from day 7; the animals were killed on day 18. CL accumulates in the brain reaching a concentration within 24 h compatible with its ID(50) in vitro and interacting with methionine metabolism; brain MAT activity and SAMe levels were lower and methionine levels higher than in controls. CL significantly reduced brain and nerve weight gains, brain myelin content, proteins, phospholipids, and galactolipids. Among phospholipids in nerve and brain, only sphingomyelin was significantly increased, by 35-50%. Sciatic nerve protein analyses showed some significant changes: protein zero in sciatic nerve remained unchanged but the 14.0- and 18.5-kDa isoforms of myelin basic protein showed a dramatic increase. Among the main proteins, in purified brain myelin, the proteolipid protein and dimer-20 isoform decreased after CL. SAMe-SD4 highlights some sensitive parameters by counteracting, at least partially, some alterations of PL--particularly galactolipids and sphingomyelins--and proteins induced by CL. The partial beneficial effects might also be explained by the age-related limited bioavailability of exogenous SAMe, a finding, to our knowledge, not yet reported elsewhere. This study demonstrates that availability of methyl donors is closely related to the formation of myelin components.

Homocysteine and methionine metabolism in ESRD: A stable isotope study

BACKGROUND

Hyperhomocysteinemia has a high prevalence in the end-stage renal disease (ESRD) population, which may contribute to the high cardiovascular risk in these patients. The cause of hyperhomocysteinemia in renal failure is unknown, and therapies have not been able to normalize plasma homocysteine levels. Insight into methionine-homocysteine metabolism in ESRD is therefore necessary.

METHODS

Using a primed, continuous infusion of [2H3-methyl-1-13C]methionine, we measured whole body rates of methionine and homocysteine metabolism in the fasting state in four hyperhomocysteinemic hemodialysis patients and six healthy control subjects.

RESULTS

Remethylation of homocysteine was significantly decreased in the hemodialysis patients: 2.6+/-0.2 (SEM) vs. 3.8+/-0.3 micromol. kg(-1)x hr(-1) in the control subjects (P = 0.03), whereas transsulfuration was not 2.5+/-0.3 vs. 3.0+/-0.1 micromol. kg(-1) x hr(-1) (P = 0.11). The transmethylation rate was proportionally and significantly lower in the ESRD patients as compared with controls: 5.2+/-0.4 vs. 6.8+/-0.3 micromol. kg(-1) x hr(-1) (P = 0.02). Methionine fluxes to and from body protein were similar.

CONCLUSIONS

The conversion of homocysteine to methionine is substantially (approximately 30%) decreased in hemodialysis patients, whereas transsulfuration is not. Decreased remethylation may explain hyperhomocysteinemia in ESRD. This stable isotope technique is applicable for developing new and effective homocysteine-lowering treatment regimens in ESRD based on pathophysiological mechanisms.

Measurement of muscle protein synthesis by positron emission tomography with L-[methyl-11C]methionine: effects of transamination and transmethylation

BACKGROUND

Positron emission tomography with L-[methyl-11C]methionine provides a measure of regional protein synthesis rate (PSR) in skeletal muscle. However, the validity of the method depends on incorporation of methionine into protein with minimal transamination, transmethylation, or both. To test directly these assumptions, uptake of L-[methyl-14C]methionine in skeletal muscle was measured in control and cycloheximide-treated rats.

METHODS

Normal and cycloheximide-treated rats (n = 8/group) were injected with 50 microCi of L-[methyl-14C]methionine and arterial blood sampled over 90 minutes. After killing, thigh muscle was homogenized, centrifuged, and treated with trichloroacetic acid. PSR from circulating methionine was estimated from trichloroacetic acid-precipitable radioactivity, arterial time-activity curves, and plasma methionine concentrations.

RESULTS

In normal rats, approximately 70% of the tissue radioactivity was precipitated with trichloroacetic acid. In normal animals, PSR was 0.22 nmoles x min(-1) x g(-1), in excellent agreement with previous results. In the cycloheximide-treated group, PSR was 0.0032 nmoles x min(-1) x g(-1); approximately 98% reduction compared with controls.

CONCLUSION

These studies support the hypothesis that L-[methyl-11(14C]methionine accumulates in skeletal muscle as 11(14)C-labeled protein.

Methionine adenosyltransferase S-nitrosylation is regulated by the basic and acidic amino acids surrounding the target thiol

S-Adenosylmethionine serves as the methyl donor for many biological methylation reactions and provides the propylamine group for the synthesis of polyamines. S-Adenosylmethionine is synthesized from methionine and ATP by the enzyme methionine adenosyltransferase. The cellular factors regulating S-adenosylmethionine synthesis have not been well defined. Here we show that in rat hepatocytes S-nitrosoglutathione monoethyl ester, a cell-permeable nitric oxide donor, markedly reduces cellular S-adenosylmethionine content via inactivation of methionine adenosyltransferase by S-nitrosylation. Removal of the nitric oxide donor from the incubation medium leads to the denitrosylation and reactivation of methionine adenosyltransferase and to the rapid recovery of cellular S-adenosylmethionine levels. Nitric oxide inactivates methionine adenosyltransferase via S-nitrosylation of cysteine 121. Replacement of the acidic (aspartate 355) or basic (arginine 357 and arginine 363) amino acids located in the vicinity of cysteine 121 by serine leads to a marked reduction in the ability of nitric oxide to S-nitrosylate and inactivate hepatic methionine adenosyltransferase. These results indicate that protein S-nitrosylation is regulated by the basic and acidic amino acids surrounding the target cysteine.

Folate and homocysteine metabolism in copper-deficient rats

To investigate the effect of copper deficiency on folate and homocysteine metabolism, we measured plasma, red-cell and hepatic folate, plasma homocysteine and vitamin B-12 concentrations, and hepatic methionine synthase activities in rats. Two groups of male Sprague-Dawley rats were fed semi-purified diets containing either 0. 1 mg (copper-deficient group) or 9.2 mg (control group) of copper per kg. After 6 weeks of dietary treatment, copper deficiency was established as evidenced by markedly decreased plasma and hepatic copper concentrations in rats fed the low-copper diet. Plasma, red-cell, hepatic folate, and plasma vitamin B-12 concentrations were similar in both groups, whereas plasma homocysteine concentrations in the copper-deficient group were significantly higher than in the control group (P<0.05). Copper deficiency resulted in a 21% reduction in hepatic methionine synthase activity as compared to the control group (P<0.01). This change most likely caused the increased hepatic 5-methyltetrahydrofolate and plasma homocysteine concentrations in the copper-deficient group. Our results indicate that hepatic methionine synthase may be a cuproenzyme, and plasma homocysteine concentrations are influenced by copper nutriture in rats. These data support the concept that copper deficiency can be a risk factor for cardiovascular disease.

Elevated expression of liver gamma-cystathionase is required for the maintenance of lactation in rats

Liver gamma-cystathionase activity increases in rats during lactation; its inhibition due to propargylglycine is followed by a significant decrease in lactation. This is reversible by N-acetylcysteine administration. To study the role of liver gamma-cystathionase and the intertissue flux of glutathione during lactation, we used lactating and virgin rats fed liquid diets. Virgin rats were divided into two groups as follows: one group was fed daily a diet containing the same amount of protein that was consumed the previous day by lactating rats (high protein diet-fed rats); the other virgin group was fed the normal liquid diet (control). The expression and activity of liver gamma-cystathionase were significantly greater in lactating rats and in high protein diet-fed virgin rats compared with control rats. The total glutathione [reduced glutathione (GSH) + oxidized glutathione (GSSG)] released per gram of liver did not differ in lactating rats or in high protein diet-fed rats, but it was significantly higher in these two groups than in control virgin rats. Liver size and the GSH + GSSG released by total liver were significantly higher in lactating rats than in high protein diet-fed virgin rats, and this difference was similar to the amount of glutathione taken up by the mammary gland (454.2 +/- 36.0 nmol/min). The uptake of total glutathione by the lactating mammary gland was much higher than the uptakes of free L-cysteine and L-cystine, which were negligible. These data suggest that the intertissue flux of glutathione is an important mechanism of L-cysteine delivery to the lactating mammary gland, which lacks gamma-cystathionase activity. This emphasizes the physiologic importance of the increased expression and activity of liver gamma-cystathionase during lactation.

Molecular and cellular effects of methotrexate

In 1998, knowledge about the mechanisms of action of methotrexate (MTX) in immunoinflammatory disorders further increased. The most interesting results to date came from studies showing that most of the anti-inflammatory actions of MTX are mediated by adenosine, which may account for the profound effects of MTX on cytokines, cytokine inhibitors, and cell differentiation. Finally, potential novel pharmacologic strategies are discussed based on actual knowledge about the molecular and cellular actions of MTX.

Differential effect of thioacetamide on hepatic methionine adenosyltransferase expression in the rat

Liver-specific and non-liver-specific methionine adenosyltransferase (MAT) are products of two genes, MAT1A and MAT2A, respectively, that catalyze the formation of S-adenosylmethionine (SAM), the principal methyl donor. Mature liver expresses mainly MAT1A. We showed a switch from MAT1A to MAT2A gene expression in human liver cancer cells that may offer a growth advantage. To gain a better understanding of the chronology and significance of the change in MAT expression, we examined changes in hepatic MAT expression after acute treatment of rats with a hepatocarcinogen, thioacetamide (TAA). TAA treatment for 3 weeks did not change the MAT1A mRNA level but reduced the liver-specific MAT protein level to below 30% of control. TAA also acutely reduced the activity of liver-specific MAT when added to normal liver homogenates. In contrast, both the mRNA and protein levels of non-liver-specific MAT were induced. Because liver-specific MAT exhibits a much higher Km for methionine (mmol/L) than non-liver-specific MAT ( approximately 10 micromol/L), MAT activity was decreased at 5 mmol/L but increased at 20 micromol/L methionine concentration. The SAM level, SAM-to-S-adenosylhomocysteine (SAH) ratio, and DNA methylation all fell during treatment. In summary, TAA treatment induced differential changes in hepatic MAT expression. The reduction in liver-specific MAT protein level represents a novel mechanism of inactivation of liver-specific MAT. This along with induction in MAT2A contributed to a fall in the SAM-to-SAH ratio. The resulting DNA hypomethylation may be important in the process of hepatocarcinogenesis.

Plasma total homocysteine response to oral doses of folic acid and pyridoxine hydrochloride (vitamin B6) in healthy individuals. Oral doses of vitamin B6 reduce concentrations of serum folate

Plasma total homocysteine response was compared in four groups of healthy individuals given orally divided doses of vitamin supplementations for a duration of 5 weeks. The vitamin supplements; A, 0.3 mg folic acid; B, 120 mg vitamin B6; C, combination of 0.3 mg folic acid and 120 mg vitamin B6 or D, 0.6 mg folic acid reduced the concentrations of plasma total homocysteine 20, 17, 32 and 24%, respectively. However, the intergroup comparisons did not show a significant difference in the effects of vitamin supplements. Multivariate analysis with correction for differences in pre-supplement values indicated a significant effect of vitamin B6 supplementation on plasma total homocysteine and serum folate. Our data show that plasma total homocysteine concentrations are reduced with low to medium divided doses of folic acid alone or in combination with vitamin B6.

Electrophysiological effects of homocysteine in isolated rat right ventricular papillary muscles and left atria

There is clinical and epidemiological evidence that elevated plasma homocysteine (Hcy) levels are associated with increased myocardial infarction mortality; however, very little is known about Hcy's direct cardiac effects. Thus, we aimed to characterize the cellular electrophysiologic effects of Hcy, a sulfur-containing amino acid in isolated rat hearts. A conventional microelectrode technique was used in left atria and right ventricular papillary muscles. At concentrations higher than 10(6) M, Hcy significantly decreased the maximum rate of rise of the depolarization phase (Vmax) in both cardiac preparations in a dose-dependent manner. Hcy at 10(-4)-5 x 10(-4) M concentrations increased the action potential duration (APD) at late stages of repolarization (at 75% and 90% of APD) both in atria and in ventricles. There was a slight decrease in action potential amplitude in ventricular papillary muscles and atria at concentrations higher that 10(-5) M. The resting membrane potential and the early repolarization phase (APD25 and APD50) remained unchanged in every preparation studied at all concentrations of Hcy administered. The present data suggest that homocysteine may decrease the Na+ channel activity in in vitro cardiac preparations. reserved.

Impaired methionine synthesis and hypomethylation in rats exposed to valproate during gestation

BACKGROUND

The antiepileptic drug valproic acid (VPA) may be teratogenic. The mechanism of teratogenicity remains unclear, but it has been hypothesized that VPA interferes with folate metabolism.

OBJECTIVE

To study the effect of VPA on the methionine cycle and transmethylation reactions in pregnant rats.

METHODS

Wistar rats were treated with VPA (300 mg/kg/day) on gestation days 8, 9, and 10, alone or in combination with folinic acid (FOL, 4 mg/kg/day) on gestation days 8, 9, and 10 or S-adenosylmethionine (SAM, 10 mg/kg/day) throughout gestation days 1 to 10.

RESULTS

VPA induced a reduction in maternal methionine serum concentration (p < 0.05) caused by a 24% reduction of methionine synthase activity in liver. This provoked hepatic DNA hypomethylation, although the methylation ratio (S-adenosylmethionine/S-adenosylhomocysteine) was not altered. Homocysteine, folate, and vitamin B12 serum concentrations, as well as methionine adenosyltransferase and betaine homocysteine methyltransferase hepatic activities, did not change. In fetuses exposed to VPA, no effect was observed in hepatic methionine content, but the methylation ratio was reduced (p < 0.01), leading again to hepatic DNA hypomethylation. Coadministration of FOL prevented VPA-induced alterations in methionine synthesis and corrected fetal DNA hypomethylation. By contrast, SAM did not exert a protective effect on fetal DNA methylation.

CONCLUSION

Impaired methionine synthesis and DNA hypomethylation may be involved in VPA-induced teratogenesis.

Inhibition of glutathione synthesis with propargylglycine enhances N-acetylmethionine protection and methylation in bromobenzene-treated Syrian hamsters

The finding that liver necrosis caused by the environmental glutathione (GSH)-depleting chemical, bromobenzene (BB) is associated with marked impairment in O- and S-methylation of BB metabolites in Syrian hamsters raises questions concerning the role of methyl deficiency in BB toxicity. N-Acetylmethionine (NAM) has proven to be an effective antidote against BB toxicity when given after liver GSH has been depleted extensively. The mechanism of protection by NAM may occur via a replacement of methyl donor and/or via an increase of GSH synthesis. If replacement of the methyl donor is an important process, then blocking the resynthesis of GSH in the methyl-repleted hamsters should not decrease NAM protection. This hypothesis was examined in this study. Propargylglycine (PPG), an irreversible inhibitor of cystathionase, was used to inhibit the utilization of NAM for GSH resynthesis. Two groups of hamsters were pretreated with an intraperitoneal (ip) dose of PPG (30 mg/kg) or saline 24 h before BB administration (800 mg/kg, ip). At 5 h after BB treatment, an ip dose of NAM (1200 mg/kg) was given. Light microscopic examinations of liver sections obtained 24 h after BB treatment indicated that NAM provided better protection (P < 0.05) in the PPG + BB + NAM group than in the BB + NAM group. Liver GSH content, however, was lower in the PPG + BB + NAM group than in the BB + NAM group. The Syrian hamster has a limited capability to N-deacetylated NAM. The substitution of NAM with methionine (Met; 450 mg/kg) resulted in a higher level of GSH in the BB + Met group than in the BB + NAM group (P < 0.05). The enhanced protection by PPG in the PPG + BB + NAM group was accompanied by higher (P < 0.05) urinary excretions of specificO- and S-methylated bromothiocatechols than in the BB + NAM group. The results suggest that NAM protection occurs primarily via a replacement of the methyl donor and that methyl deficiency occurring in response to GSH repletion plays a potential role in BB toxicity.

Association of dietary protein intake and coffee consumption with serum homocysteine concentrations in an older population

BACKGROUND

Elevated blood concentrations of total homocysteine (tHcy) have been implicated in the pathogenesis of atherosclerotic cardiovascular disease. Previous studies identified suboptimal nutritional status and dietary intake of folate, vitamin B-6, and vitamin B-12 as determinants of elevated tHcy.

OBJECTIVE

We identified other nutritional factors associated with tHcy in 260 retired schoolteachers in the Baltimore metropolitan area.

DESIGN

We performed observational analyses of baseline and 2-4-mo follow-up data collected in a study designed to test the feasibility of conducting a large-scale clinical trial of vitamin supplements by mail. The study population consisted of 151 women and 109 men with a median age of 64 y. At baseline, each participant completed a food-frequency questionnaire. At follow-up, fasting serum tHcy was measured.

RESULTS

In multivariable linear regression and generalized linear models, there was an independent, inverse dose-response relation between dietary protein and In tHcy (P = 0.002) and a positive, significant dose-response relation between coffee consumption and In tHcy (P for trend = 0.01). Other significant predictors of In tHcy were creatinine (positive; P = 0.0001) and prestudy use of supplemental B vitamins (inverse; P = 0.03). In stratified analyses restricted to persons receiving standard multivitamin therapy, the association of 1n tHcy with dietary protein and coffee persisted.

CONCLUSIONS

These results support the hypothesis that increased protein intake and decreased coffee consumption may reduce tHcy and potentially prevent atherosclerotic cardiovascular disease and other disease outcomes.

Metabolic effects of a methylthioadenosine phosphorylase substrate analog on African trypanosomes

The effects of 5'-deoxy-5'-(hydroxyethylthio)adenosine (HETA), a trypanocidal analog of 5'-deoxy-5'-(methylthio)adenosine (MTA), on polyamine synthesis and S-adenosylmethionine (AdoMet) metabolism were examined in bloodstream forms of Trypanosoma brucei brucei. HETA was cleaved by trypanosome MTA phosphorylase at the same rate as the natural substrate, MTA, in a phosphate-dependent reaction. Fluorine substitution at the 2-position of the purine ring increased activity by approximately 50%, whereas substitution with an amino group reduced activity to about one-third of the control. HETA was accumulated by trypanosomes with internal concentrations of 100-250 microM and >800 microM after a 15-min incubation with 1 and 10 microM, respectively. Trypanosomes preincubated with HETA metabolized it at a rate of 21.9 nmol/hr/mg protein. Preincubation of cells with HETA at 1 or 10 microM inhibited spermidine synthesis from [3H]ornithine by 22-37%, and increased the cytosolic levels of AdoMet by 2- to 5-fold and that of MTA by up to 8-fold. S-Adenosylhomocysteine (AdoHcy) levels also increased 1.5- to 7-fold in treated cells, whereas decarboxylated AdoMet decreased 65%. Preincubation of trypanosomes with HETA for 4 hr also reduced the incorporation of [35S]methionine in trichloroacetic acid-precipitable material by 50-60%, and reduced the methyl group incorporation into protein from [U-14C]methionine by 65-70%. Thus, HETA interferes with a series of biochemical events involving the participation of AdoMet and methionine in polyamine synthesis, protein synthesis, and transmethylation reactions.

Association between hyperhomocysteinaemia and hypertension in Sri Lankans

This study examined the relationship between homocysteine and its metabolites, and hypertension in a cohort of Sri Lankan patients with essential hypertension. Serum homocysteine, cysteine, cysteinylglycine and glutathione were measured in 86 patients with a diagnosis of essential hypertension and compared with those of an age- and sex-matched control group. Patients with hypertension had significantly higher mean serum concentrations of homocysteine, cysteine and cysteinylglycine. The odds ratio for hypertension for those with a mean serum homocysteine concentration above 18 mumol/l was 2.8. Hyperhomocysteinaemia is a risk factor for hypertension in Sri Lankans and can lead to a threefold increase in risk.

Amino acid metabolism in liver disease

The impairment of transsulphuration during methionine degradation in hepatic failure can be counteracted by treatment with S-adenosylmethionine. Regarding the pathogenesis of hepatic encephalopathy, no convincing evidence exists for tryptophan, glutamine or glutamate being involved. Portal-systemic shunting-induced hyperammonaemia may reduce plasma branched-chain amino acids. The glucose effect on urea synthesis does not exist in cirrhosis.

Homocysteine, a Risk Factor for Cardiovascular Disease

Fasting hyperhomocysteinemia is an independent risk factor for coronary artery disease, stroke, peripheral vascular atherosclerosis, and for arterial and venous thromboembolism. The risk for cardiovascular disease with homocysteine is similar to conventional risk factors. The interaction of hyperhomocysteinemia with hypertension and smoking is strong and the combined effect is more than multiplicative. The combined effect of homocysteine and cholesterol is additive. Homocysteine produces atherosclerosis, thromboembolism, and vascular endothelial cell injury. Vascular dysfunction produced by homocysteine may be due to endothelial cell damage. Homocysteinemia-induced atherosclerosis is probably due to various factors including endothelial cell injury, inability to sustain S-nitroso-homocysteine formation because of imbalance between production of nitric oxide by dysfunctional endothelium and homocysteine, smooth muscle cell proliferation, and thromboembolism. There is strong evidence that endothelial cell injury is associated with oxidative stress produced by homocysteine. Hyperhomocysteinemia is associated with numerous conditions, including coronary disease, stroke, peripheral vascular disease (carotid artery and cerebrovascular atherosclerosis), venous thrombosis, renal disease, diabetes mellitus, and organ transplant. Folic acid, vitamin B12 and B6 have been shown to be beneficial in reducing plasma homocysteine levels. Folic acid is specifically very effective, safe and inexpensive.

Hyperhomocysteinemia induces elastolysis in minipig arteries: structural consequences, arterial site specificity and effect of captopril-hydrochlorothiazide

Hyperhomocysteinemia is a risk factor for arterial diseases, and the deterioration of the arterial elastic structures is one of the possible mechanisms underlying this epidemiological association. The aim of this paper is to quantitatively characterize such structural alterations and to explore their causes in a previous model of dietary induced mild hyperhomocysteinemia in minipigs. After four months, both a morphodensitometrical analysis of the elastic structure and a biochemical analysis of elastin and elastase activities were performed on the infrarenal abdominal aorta (IRAA) and the proximal left interventricular coronary artery (LIVCA) of control (C), hyperhomocysteinemic (H) and captopril-hydrochlorothiazide (Cp-Htz, 25 + 12.5 mg/d)-treated (H+/-Cp) minipigs (n = 8/group). Hyperhomocysteinemia was found to induce an increase in parietal elastolytic metalloproteinase activities. It resulted in opening and enlargement of fenestrae through the medial elastic laminae and in a decrease in medial elastin content (p < 10(-3)), expressed as well as volume density (%) as weight concentration (microg elastin/mg dry tissue). The thickness of the media and its basic lamellar organization was unchanged. The reduction in volume density was more dramatic in LIVCA (H: 4.7 +/- 0.9 vs C: 8.8 +/- 2.4), where it was evenly distributed within the media, than in IRAA (H: 6.7 +/- 1.1 vs C: 9.3 +/- 1.2), where the deep medial layers were less affected. Cp-Htz partly prevented the hyperhomocysteinemia-induced reduction of the medial elastin content in LIVCA (5.7 +/- 1.2) and IRAA (7.9 +/- 1.4). This effect, occurring in the subintimal layers of the media in both arteries but not in the deeper layers, resulted in a less beneficial effect in LIVCA than in IRAA. This result parallels the moderate beneficial therapeutic effect of ACE inhibitors against coronary atherosclerosis in humans. This paper reports for the first time a quantitative analysis of the arterial site-dependent deterioration of the elastic structure caused by mild hyperhomocysteinemia and the involvement of metalloproteinases in this process. These results confirm that the plaque-independent damage to elastic structure previously described in hyperhomocysteinemic-atherosclerotic minipigs was mainly due to homocysteine. This highlights that the metalloproteinase-related elastolysis and the subsequent structural deterioration is one of the major events underlying the epidemiological association between mild hyperhomocysteinemia and arterial diseases.

Drug and environmental factors associated with adverse pregnancy outcomes. Part III: Folic acid: pharmacology, therapeutic recommendations, and economics

OBJECTIVE

To review folic acid's mechanism of action, adverse effects, therapeutic recommendations, compliance, and cost.

DATA SOURCES

A MEDLINE search was conducted through December 1997. Additional sources were obtained from Current Contents and citations from the references obtained. Search terms included folate, folic acid, neural tube defect, homocysteine, and methylenetetrahydrofolate reductase.

STUDY SELECTION

Animal and human studies examining the effects of folate were reviewed.

DATA EXTRACTION

Data collected included mechanism of action, safety issues, dosing recommendations, compliance with recommendations, and economics.

DATA SYNTHESIS

Folic acid decreases neural tube defect risk through an effect on methionine-homocysteine metabolism. In addition, increased folate intake may reduce cardiovascular morbidity and mortality. Since toxicity is minimal, everyone can potentially benefit from increased folate consumption. To help achieve this, the Food and Drug Administration has mandated that cereal grain be fortified with 140 micrograms of folic acid per 100 g of grain, which will add approximately 0.1 mg of folate to the average diet. Studies recommend supplementing with 0.2 mg to promote optimal homocysteine concentrations and for preventing neural tube defects.

CONCLUSIONS

Despite fortification, most women will still receive less folate than the 0.4 mg/d recommended by the Public Health Service. All population groups would benefit from increased folate intake. Current studies indicate 200 micrograms/d may be the minimum effective amount of fortification needed for normalizing homocysteine concentrations and preventing a significant number of neural tube defects; thus, a higher level of food fortification may be warranted.

Intestinal mucosal amino acid catabolism

The small intestine is not only responsible for terminal digestion and absorption of nutrients, but it also plays an important role in catabolism of arterial glutamine and dietary amino acids. Most of glutamine and almost all of glutamate and aspartate in the diet are catabolized by small intestinal mucosa, and CO2 accounts for 56-64% of their metabolized carbons. The small intestinal mucosa also plays an important role in degrading arginine, proline and branched-chain amino acids, and perhaps methionine, lysine, phenylalanine, threonine, glycine and serine in the diet, such that 30-50% of these dietary amino acids are not available to extraintestinal tissues. Dietary amino acids are major fuels for the small intestinal mucosa and are essential precursors for intestinal synthesis of glutathione, nitric oxide, polyamines, purine and pyrimidine nucleotides, and amino acids (alanine, citrulline and proline), and are obligatory for maintaining intestinal mucosal mass and integrity. Because intestinal amino acid catabolism plays an important role in modulating dietary amino acid availability to extraintestinal tissues, it has important implications for the utilization efficiency of dietary protein and amino acids in animals and humans.

Homocysteine and vitamins in cardiovascular disease

On the basis of recent retrospective and prospective studies, it is now widely accepted that increased total plasma homocysteine is a risk factor for cardiovascular disease. Impaired enzyme function as a result of genetic mutation or deficiency of the essential B vitamins folic acid, B12, and B6 can lead to hyperhomocysteinemia. Oxidized forms of homocysteine account for 98–99% of total plasma homocysteine. Although there is uncertainty as to whether increased homocysteine is causal or merely a proxy for cardiovascular disease, several lines of evidence suggest that it may play a role in atherothrombotic disease. Homocysteine appears to alter the anticoagulant properties of endothelial cells to a procoagulant phenotype. Mildly increased homocysteine causes dysfunction of the vascular endothelium. Folic acid effectively lowers homocysteine concentration in the plasma. Intervention studies are urgently needed to determine if lowering homocysteine is effective in decreasing the morbidity and mortality of cardiovascular disease.

Oxidative stress in chronic renal failure

Cardiovascular disease is the major cause of morbidity and mortality in chronic renal failure. The aim of this review is to summarise current evidence suggesting that there is increased free radical production, antioxidant depletion and changes in lipoprotein composition in renal failure which will lead to oxidation of LDL and hence to accelerated development of atherosclerosis.

Changes in methionine adenosyltransferase during liver regeneration in the rat

Liver-specific and non-liver-specific methionine adenosyltransferase (MAT) are products of two genes (MAT1A and MAT2A, respectively) that catalyze the formation of S-adenosylmethionine (SAM), the principal methyl donor. We previously showed that MAT2A expression was associated with more rapid cell growth. Here we examined changes in hepatic MAT gene expression and related consequences after two-thirds partial hepatectomy (PH) in rats. The mRNA levels of both MAT forms increased from 3 to 6 h, but the MAT1A level then fell below baseline from 12 to 24 h, whereas the MAT2A level remained elevated up to 4 days after PH. The increase in the MAT2A mRNA level was due to increased gene transcription and mRNA stabilization. The change in the MAT1A mRNA level was posttranscriptional and did not require de novo protein synthesis. Changes in MAT activity were consistent with an increased amount of MAT isozymes. SAM levels, the ratio of SAM to S-adenosylhomocysteine (SAH), and DNA methylation fell from 6 to 24 h, whereas SAH levels increased slightly at 12 and 24 h after PH. Both increased SAM utilization and MAT2A gene expression likely contributed to the fall in SAM.

Effects of intermediates of methionine metabolism and nucleoside analogs on S-adenosylmethionine transport by Trypanosoma brucei brucei and a drug-resistant Trypanosoma brucei rhodesiense

The effects of purine nucleoside analogs, polyamines, and established trypanocidal agents on the uptake of [8-14C]adenosine and S-[methyl-3H]adenosylmethionine (AdoMet) by bloodform trypanosomes of drug-susceptible Trypanosoma brucei brucei and a drug-resistant Trypanosoma brucei rhodesiense clinical isolate were compared. AdoMet uptake was not antagonized by omithine or methionine (500 microM), adenosine (100 microM), or other purine nucleosides, including methylthioadenosine (MTA) at 500 microM. Hydroxyethylthioadenosine (HETA), a trypanocidal analog of methylthioadenosine, and sinefungin, an analog of AdoMet, were competitive with AdoMet transport in both isolates. Dipyridamole, an antagonist of the adenosine P2 transporter, also competed with AdoMet transport in both isolates. The trypanocidal diamidines pentamidine, Berenil, CGP 40215, and the decarboxylated S-adenosylmethionine (dAdoMet) analog MDL 73811 (5'-¿[(Z)-4-amino-2-butenyl]¿methyl-amino¿-5'-deoxyadenosine) competed with P2 adenosine transport but did not inhibit AdoMet transport at 100 microM. Methylglyoxalbis(guanylhydrazone) (MGBG), an analog of dAdoMet, was a strong competitive inhibitor of adenosine transport at 100 microM, but did not inhibit AdoMet transport. The polyamines putrescine, spermine, and spermidine (1 mM) were examined for competition with adenosine and AdoMet transport. Putrescine significantly inhibited P2 adenosine transport in both strains (in the presence of saturating inosine), but AdoMet transport was not affected by these polyamines. P2 adenosine transport in both strains was highly inhibited by melarsen oxide and melamine, its key organic component, whereas AdoMet uptake was not affected by these agents. These findings further characterize distinguishing features of the unique AdoMet transporter in African trypanosomes, and indicate that the P2 adenosine transporter remains functional in melarsen- and diamidine-resistant clinical isolates.