Association between the NAT1 1095C > A polymorphism and homocysteine concentration.
Explore the source record for details and available documents.
Adult↗
Biomedical subjects
Helene McNulty
Publications and source records attributed to Helene McNulty.
The rate of intestinal absorption of natural food folates is not related to the extent of folate conjugation.
BACKGROUND: Evidence is conflicting as to whether the bioavailability of food folates is influenced by the extent of their conjugation. OBJECTIVE: The objective was to compare the bioavailability of 3 representative food folate sources with various degrees of glutamylation-ie, egg yolk, spinach, and yeast, whose polyglutamyl folate content measured 0%, 50%, and 100%, respectively. DESIGN: In a randomized crossover trial, 13 male subjects, after a prestudy folate saturation procedure, received in random order either placebo or 500 mug total folate, which was provided as concentrated freeze-dried extract removed from the normal food matrix of egg yolk, spinach, or yeast. Blood samples (n = 10) were collected before and up to 10 h after treatments, which were administered at weekly intervals. RESULTS: A significant increase from baseline plasma folate concentrations was observed by 0.5 h after treatment with egg yolk folate or spinach folate and by 1 h after treatment with yeast folate, and the concentrations remained significantly elevated for 3-5 h; no plasma folate response was observed after placebo treatment. The overall responses, calculated as plasma folate area under the curve (AUC) for egg yolk, spinach, and yeast folate, were 122.6 +/- 23.6, 136.2 +/- 21.4, and 102.5 +/- 21.1 nmol . h/L, respectively. No significant differences in AUC were seen between monoglutamyl (egg yolk) folate and either of the polyglutamate-containing folates examined. CONCLUSION: These results suggest that the ratio of monoglutamate to polyglutamate in natural folates is not a factor that limits the extent of intestinal absorption of food folate.
Administration, Oral↗
Global DNA and p53 region-specific hypomethylation in human colonic cells is induced by folate depletion and reversed by folate supplementation.
There is increasing evidence to suggest that reduced folate status may be a causative factor in carcinogenesis, particularly colorectal carcinogenesis. Folate is essential for the synthesis of S-adenosylmethionine, the methyl donor required for all methylation reactions in the cell, including the methylation of DNA. Global DNA hypomethylation appears to be an early, and consistent, molecular event in carcinogenesis. We have examined the effects of folate depletion on human-derived cultured colon carcinoma cells using 2 novel modifications to the Comet (single cell gel electrophoresis) assay to detect global DNA hypomethylation and gene region-specific DNA hypomethylation. Colon cells cultured in folate-free medium for 14 d showed a significant increase in global DNA hypomethylation compared with cells grown in medium containing 3 micromol/L folic acid. This was also true at a gene level, with folate-deprived cells showing significantly more DNA hypomethylation in the region of the p53 gene. In both cases, the effects of folate depletion were completely reversed by the reintroduction of folic acid to the cells. These results confirm that decreased folate levels are capable of inducing DNA hypomethylation in colon cells and particularly in the region of the p53 gene, suggesting that a more optimal folate status in vivo may normalize any DNA hypomethylation, offering potential protective effects against carcinogenesis. This study also introduces 2 novel functional biomarkers of DNA hypomethylation and demonstrates their suitability to detect folate depletion-induced molecular changes.
Azacitidine↗
Riboflavin lowers homocysteine in individuals homozygous for the MTHFR 677C->T polymorphism.
BACKGROUND: Meta-analyses predict that a 25% lowering of plasma homocysteine would reduce the risk of coronary heart disease by 11% to 16% and stroke by 19% to 24%. Individuals homozygous for the methylenetetrahydrofolate reductase (MTHFR) 677C-->T polymorphism have reduced MTHFR enzyme activity resulting from the inappropriate loss of the riboflavin cofactor, but it is unknown whether their typically high homocysteine levels are responsive to improved riboflavin status. METHODS AND RESULTS: From a register of 680 healthy adults 18 to 65 years of age of known MTHFR 677C-->T genotype, we identified 35 with the homozygous (TT) genotype and age-matched individuals with heterozygous (CT, n=26) or wild-type (CC, n=28) genotypes to participate in an intervention in which participants were randomized by genotype group to receive 1.6 mg/d riboflavin or placebo for a 12-week period. Supplementation increased riboflavin status to the same extent in all genotype groups (8% to 12% response in erythrocyte glutathione reductase activation coefficient; P<0.01 in each case). However, homocysteine responded only in the TT group, with levels decreasing by as much as 22% overall (from 16.1+/-1.5 to 12.5+/-0.8 micromol/L; P=0.003; n=32) and markedly so (by 40%) in those with lower riboflavin status at baseline (from 22.0+/-2.9 and 13.2+/-1.0 micromol/L; P=0.010; n=16). No homocysteine response was observed in the CC or CT groups despite being preselected for suboptimal riboflavin status. CONCLUSIONS: Although previously overlooked, homocysteine is highly responsive to riboflavin, specifically in individuals with the MTHFR 677 TT genotype. Our findings might explain why this common polymorphism carries an increased risk of coronary heart disease in Europe but not in North America, where riboflavin fortification has existed for >50 years.
Adolescent↗
A common insertion/deletion polymorphism of the thymidylate synthase (TYMS) gene is a determinant of red blood cell folate and homocysteine concentrations.
Substantial evidence suggests that a low folate/high homocysteine phenotype is pathogenic. We analyzed the impact of the thymidylate synthase (TYMS) 3'UTR ins/del polymorphism on folate and homocysteine levels and assessed the relationship between the TYMS 3'UTR ins/del polymorphism and key genetic and lifestyle variables. Among non-smokers only, the TYMS 3'UTR ins/del polymorphism was significantly associated with red blood cell folate (RBC folate; P=0.002) and homocysteine (P=0.03) concentrations. Median RBC folate concentration was much higher for TYMS 3'UTR del/del subjects (434 microg/l) compared with either ins/ins (282 microg/l) or ins/del (298 microg/l) subjects. The median homocysteine concentration for del/del homozygotes was considerably lower compared with either ins/ins homozygotes or ins/del heterozygotes. A possible additive effect for the impact of the TYMS 3'UTR del/del and MTHFR 677CC genotypes on RBC folate concentration was also observed. Our findings suggest that the TYMS 3'UTR del/del genotype is a significant determinant of elevated RBC folate concentration in a non-smoking population of northwestern European adults and that this genotype confers protection against diseases for which a low folate/high homocysteine phenotype appears to be an etiologic component.
3' Untranslated Regions↗
Effect of folic acid supplementation on mood and serotonin response in healthy males.
Evidence suggests that low folate status may be detrimental to mood and associated with depleted cerebrospinal fluid levels of the neurotransmitter serotonin (5-hydroxytryptamine; 5-HT). A placebo-controlled trial was carried out to determine the effect of folic acid supplementation (100 microg for 6 weeks followed by 200 microg for a further 6 weeks) upon subjective mood (Positive and Negative Affect Schedule) and biochemical markers of mood (5-HT) in healthy males (n 23). Blood samples were obtained at baseline (week 0) and during the intervention at week 6 and week 12. Subjective mood assessments were obtained at week 0 and week 12. The results showed an increase in serum and erythrocyte folate concentrations (P=0.02 and P=0.003, respectively) and a corresponding decrease in plasma homocysteine (P=0.015) in response to the folic acid intervention. Neither subjective mood nor 5-HT levels, however, were significantly altered in response to the change in folate status. Folic acid given at physiological doses did not appear to improve the mood of healthy folate-replete individuals over a 12-week period. Further research is needed to address the effect of folic acid supplementation or of longer duration or increased dose, particularly in the face of sub-optimal folate status.
Adult↗
Homocysteine is lower in the third trimester of pregnancy in women with enhanced folate status from continued folic acid supplementation.
BACKGROUND: In many countries, current recommendations are that women take a daily 400-mug folic acid supplement from before conception until the end of the 12th week of gestation for the prevention of neural tube defects. Low folate status is associated with an increased concentration of plasma total homocysteine (tHcy), a risk factor associated with pregnancy complications such as preeclampsia. METHODS: In a longitudinal study, we determined tHcy and corresponding folate status in 101 pregnant women at 12, 20, and 35 weeks of gestation, in 35 nonpregnant controls sampled concurrently, and in a subgroup (n = 21 pregnant women and 19 nonpregnant controls) at 3 days postpartum. RESULTS: Plasma tHcy was significantly lower throughout pregnancy compared with nonpregnant controls, with values lowest in the second trimester before increasing toward nonpregnant values in the third trimester. Importantly, mean tHcy concentrations were lower in pregnant women taking folic acid supplements than in those not, an effect that reached significance in the third trimester (5.45 vs 7.40 micromol/L; P <0.05). During the third trimester, tHcy concentrations were significantly higher in pregnant women with a history of miscarriage than in women with no previous history (8.15 vs 6.38 micromol/L; P <0.01). CONCLUSIONS: This is the first longitudinal study to show that homocysteine concentrations increase in late pregnancy toward nonpregnant values; an increase that can be limited by enhancing folate status through continued folic acid supplementation. These results indicate a potential role for continued folic acid supplementation in reducing pregnancy complications associated with hyperhomocysteinemia.
Adolescent↗
The 5,10-methylenetetrahydrofolate reductase C677T polymorphism interacts with smoking to increase homocysteine.
Elevated homocysteine is a risk marker for several human pathologies. Risk factors for elevated homocysteine include low folate and homozygosity for the T allele of the 5,10-methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism. Because nitric oxide may inhibit folate catabolism and endothelial nitric oxide synthase activity is reduced in smokers, we postulated that smoking status might modify the impact of the MTHFR C677T polymorphism on homocysteine (tHcy) concentrations. We tested this hypothesis in a healthy young adult population for which MTHFR C677T genotypes and tHcy concentrations were previously reported. The MTHFR 677TT genotype was significantly associated with elevated tHcy concentrations in smokers (P = 0.001) but not in non-smokers (P = 0.36). Among smokers, the MTHFR 677TT genotype was significantly associated with high tHcy in heavy smokers (P = 0.003) but not light smokers (P = 0.09), in men (P = 0.003) but not women (P = 0.11), and in subjects from the lowest serum folate quartile (P = 0.49) but not from folate quartiles 2-4 (P = 0.49). After adjustment for nutritional variables, interactions between MTHFR C677T genotype and NOS3 G894T genotype, and between MTHFR genotype, smoking status and gender were statistically significant. We propose that hyperhomocysteinemia in MTHFR 677TT homozygote smokers is the consequence of mild intracellular folate deficiency caused by a smoking-related reduction of NOS3 activity that is exacerbated when serum folate is low.
Adult↗
Folate bioavailability.
The achievement of optimal folate status to prevent neural-tube defects, and possibly other diseases, is hindered by the well-recognised incomplete bioavailability of the natural folates found in foods compared with the synthetic vitamin, folic acid. Folate bioavailability from different foods is considered to be dependent on a number of factors, including the food matrix, the intestinal deconjugation of polyglutamyl folates, the instability of certain labile folates during digestion and the presence of certain dietary constituents that may enhance folate stability during digestion. There is conflicting evidence as to whether the extent of conjugation of polyglutamyl folate (in the absence of specific inhibitors of deconjugation in certain foods) is a limiting factor in folate bioavailability. Estimates of the extent of lower bioavailability of food folates compared with folic acid (relative bioavailability) show great variation, ranging anywhere between 10 and 98%, depending on the methodological approach used. The lack of accurate data on folate bioavailability from natural food sources is of particular concern in those countries in which there is no mandatory folic acid fortification, and therefore a greater reliance on natural food folates as a means to optimise status. Apart from the incomplete bioavailability of food folates, the poor stability of folates in foods (particularly green vegetables) under typical conditions of cooking can substantially reduce the amount of vitamin ingested and thereby be an additional factor limiting the ability of food folates to enhance folate status. A recent workshop convened by the Food Standards Agency concluded that gaining a better understanding of folate bioavailability in representative human diets is a high priority for future research.
Biological Availability↗
Determining bioavailability of food folates in a controlled intervention study.
BACKGROUND: The concept of dietary folate equivalents (DFEs) in the United States recognizes the differences in bioavailability between natural food folates and the synthetic vitamin, folic acid. However, many published reports on folate bioavailability are problematic because of several confounding factors. OBJECTIVE: We compared the bioavailability of food folates with that of folic acid under controlled conditions. To broadly represent the extent to which natural folates are conjugated in foods, we used 2 natural sources of folate, spinach (50% polyglutamyl folate) and yeast (100% polyglutamyl folate). DESIGN: Ninety-six men were randomly assigned according to their screening plasma homocysteine (tHcy) concentration to 1 of 4 treatment groups for an intervention period of 30 d. Each subject received (daily under supervision) either a folate-depleted "carrier" meal or a drink plus 1) placebo tablet, 2) 200 microg folic acid in a tablet, 3) 200 microg natural folate provided as spinach, or 4) 200 microg natural folate provided as yeast. RESULTS: Among the subjects who completed the intervention, responses (increase in serum folate, lowering of tHcy) relative to those in the placebo group (n = 18) were significant in the folic acid group (n = 18) but not in the yeast folate (n = 19) or the spinach folate (n = 18) groups. Both natural sources of folate were significantly less bioavailable than was folic acid. Overall estimations of folate bioavailability relative to that of folic acid were found to be between 30% (spinach) and 59% (yeast). CONCLUSION: Relative bioavailability estimates were consistent with the estimates from the metabolic study that were used as a basis to derive the US DFE value.
Administration, Oral↗
The short-term bioavailabilities of [6S]-5-methyltetrahydrofolate and folic acid are equivalent in men.
The natural folate derivative, 5-methyltetrahydrofolate ([6S]-5-MTHF), could be an option for supplementation and fortification but its bioavailability remains unclear. This study compared the bioavailability of [6S]-5-MTHF with that of folic acid (FA) by measuring plasma folate responses after a single ingestion of equivalent doses of the two folate forms. In a double-blind, crossover study, 13 men (presaturated with FA) received in random order each of the following treatments administered orally at 1-wk intervals: 1) placebo capsule; 2) 500 micro g FA capsule; and 3) 500 micro g [6S]-5-MTHF capsule. Plasma total folate concentrations were measured before and up to 10 h after each treatment (n = 10 samples per treatment). Plasma folate concentrations increased significantly (compared with baseline) from 0.5 to 5 h after both folate treatments. The maximum plasma folate response did not differ between the two treatments (mean +/- SEM, 33.4 +/- 3.9 vs. 31.8 +/- 3.9 nmol/L, P = 0.7, for FA and [6S]-5-MTHF, respectively) and typically occurred in individuals between 0.5 and 3 h postprandially. The area under the plasma folate response curve was significantly greater after both folate treatments compared with placebo, and the response did not differ between the treatments. These results indicate that the short-term bioavailabilities of [6S]-5-MTHF and FA are equivalent. Supplementation with the natural folate derivative could have all the beneficial effects associated with FA, but without the potential disadvantage of masking the anemia of vitamin B-12 deficiency.
Adult↗
The thymidylate synthase tandem repeat polymorphism is not associated with homocysteine concentrations in healthy young subjects.
Thymidylate synthase (TYMS) and 5,10-methylenetetrahydrofolate reductase (MTHFR) may compete for their common cofactor 5,10-methylenetetrahyhdrofolate (5,10-meTHF). Limiting 5,10-meTHF results in elevated homocysteine, especially in individuals homozygous for the T allele of the MTHFR C677T polymorphism. The TYMS gene has a tandem repeat polymorphism (two repeats or three repeats, designated 2R or 3R, respectively), which may also affect homocysteine concentrations. The 3R allele is subject to increased translational efficiency in vitro and the 3R3R genotype is associated with both decreased serum folate and elevated plasma homocysteine (tHcy) in a population of Singapore Chinese. We assessed the relationship between TYMS genotype and key biochemical and genetic variables in a random sample of 392 healthy young Northwestern European men and women. The tHcy concentrations for 3R3R homozygotes (median 8.5 micromol/l) did not differ significantly from those for 2R2R homozygotes (median 8.7 micromol/l) or 2R3R heterozygotes (median 9.3 micromol/l) in the population as a whole (P=0.43), or in subsets of subjects with low serum folate (P=0.60) or the MTHFR 677TT genotype (P=0.90). Furthermore, there was no trend toward elevated tHcy in 3R3R homozygotes. Similarly, the TYMS tandem repeat polymorphism was not associated with serum folate concentrations. Our findings indicate that the TYMS 3R3R genotype is not a determinant of homocysteine in this sample of healthy young Caucasian adults from Northern Ireland.
Adult↗
Protocol for the production of concentrated extracts of food folate for use in human bioavailability studies.
To provide a tool to study folate bioavailability under controlled conditions, a methodology was developed to produce extracts representative of natural food folates but removed from their matrix and sufficiently concentrated so as to elicit a response in biomarkers of folate status without distorting usual dietary intake patterns. Egg, spinach, and yeast were selected to represent the wide range in extent of folate conjugation found in foods (0, 60, and 100% polyglutamyl folate, respectively). The protocol, which was based on extracting food folates using only reagents safe for human consumption, was optimized in the laboratory (thermal extraction for 10 min in a 2% ascorbate solution at pH 5) and then adapted for industrial scale production in a food-processing facility. Results showed that the extracts were 2.3-12 times more concentrated in folate compared with their corresponding food sources. Neither the mono- to polyglutamate ratio nor the distribution of the main folate derivatives was altered during processing, making these extracts suitable for use in human bioavailability studies.
Biological Availability↗
Genetic evidence that nitric oxide modulates homocysteine: the NOS3 894TT genotype is a risk factor for hyperhomocystenemia.
OBJECTIVE: Mild hyperhomocystenemia is an independent, graded risk factor for cardiovascular disease. Genetic determinants of hyperhomocystenemia include functional polymorphisms in several folate/homocysteine metabolic enzymes. Nitric oxide may also modulate plasma homocysteine (tHcy) concentrations, either by direct inhibition of methionine synthase or via an indirect effect on folate catabolism. METHODS AND RESULTS: The hypothesis that the endothelial nitric oxide synthase (NOS3) G894T polymorphism is a genetic determinant of tHcy concentrations was tested in 2 independent healthy adult populations. In both populations, NOS3 genotype was significantly associated with tHcy concentrations in nonsmokers with low folate (P=0.03 for each). Models were constructed to adjust for known determinants of tHcy concentrations and test for interactions between NOS3 genotype and these determinants in nonsmokers from each population. NOS3 genotype remained a significant determinant of tHcy concentrations after adjustment. Interactions between NOS3 genotype and serum folate were significant in both populations, and the interaction between NOS3 genotype and MTHFR C677T genotype was significant in the larger population. CONCLUSIONS: These data indicate that the NOS3 894TT genotype is a risk factor for elevated tHcy in healthy nonsmoking adults with low serum folate and supports the hypothesis that nitric oxide modulates homocysteine through an effect on folate catabolism.
Adult↗
Genetic and nutritional factors contributing to hyperhomocysteinemia in young adults.
A modestly elevated total plasma homocysteine concentration (tHcy) is generally accepted as an independent and graded risk factor for various pathologies, including vascular diseases, neural tube defects, Alzheimer disease, and pregnancy complications. We analyzed 5 common functional polymorphisms in enzymes involved in homocysteine metabolism (ie, methylenetetrahydrofolate reductase [MTHFR] 677C>T and 1298A>C, methionine synthase [MTR] 2756A>G, cystathionine beta-synthase [CBS] 844ins68, and methionine synthase reductase [MTRR] 66A>G) in 452 young adults, and quantified their independent and interactive effects on tHcy concentrations. Serum folate, red cell folate, vitamin B(12), and tHcy concentrations were significantly influenced by MTHFR 677C>T genotypes. A particularly strong interaction was observed between the MTHFR 677TT genotype and serum folate, which led to a high tHcy phenotype that was more pronounced in males. The genetic contribution to the variance in tHcy was estimated to be approximately 9%, compared with approximately 35% that could be attributed to low folate and vitamin B(12). Our study indicates that dietary factors are centrally important in the control of tHcy levels in young adults with additional, but somewhat weaker, genetic effects. These data underscore the potential benefits that may be gained by improving the dietary status of young adults, and provide support for the implementation of folate/B-vitamin food fortification programs.
5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
Folate bioavailability: UK Food Standards Agency workshop report.
The UK Food Standards Agency convened a group of expert scientists to review current research investigating folate bioavailability. The workshop aimed to overview current research and establish priorities for future research. Discrepancies were observed in the evidence base for folate bioavailability, especially with regard to the relative bioavailability of natural folates compared with folic acid. A substantial body of evidence shows folic acid to have superior bioavailability relative to food folates; however, the exact relative bioavailability still needs to be determined, and in particular with regard to mixed diets. The bioavailability of folate in a mixed diet is probably not a weighted average of that in the various foods consumed; thus the workshop considered that assessment of folate bioavailability of whole diets should be a high priority for future research.
Biological Availability↗
Fasting and post-methionine loading concentrations of homocysteine, vitamin B2, and vitamin B6 in patients on antiepileptic drugs.
Explore the source record for details and available documents.
Adult↗
Demonstration of glycated insulin in human diabetic plasma and decreased biological activity assessed by euglycemic-hyperinsulinemic clamp technique in humans.
The presence and biological significance of circulating glycated insulin has been evaluated by high-pressure liquid chromatography (HPLC), electrospray ionization mass spectrometry (ESI-MS), radioimmunoassay (RIA), receptor binding, and hyperinsulinemic-euglycemic clamp techniques. ESI-MS analysis of an HPLC-purified plasma pool from four male type 2 diabetic subjects (HbA(1c) 8.1 +/- 0.2%, plasma glucose 8.7 +/- 1.3 mmol/l [means +/- SE]) revealed two major insulin-like peaks with retention times of 14-16 min. After spectral averaging, the peak with retention time of 14.32 min exhibited a prominent triply charged (M+3H)(3+) species at 1,991.1 m/z, representing monoglycated insulin with an intact M(r) of 5,970.3 Da. The second peak (retention time 15.70 min) corresponded to native insulin (M(r) 5,807.6 Da), with the difference between the two peptides (162.7 Da) representing a single glucitol adduct (theoretical 164 Da). Measurement of glycated insulin in plasma of type 2 diabetic subjects by specific RIA gave circulating levels of 10.1 +/- 2.3 pmol/l, corresponding to approximately 9% total insulin. Biological activity of pure synthetic monoglycated insulin (insulin B-chain Phe(1)-glucitol adduct) was evaluated in seven overnight-fasted healthy nonobese male volunteers using two-step euglycemic-hyperinsulinemic clamps (2 h at 16.6 micro g x kg(-1) x min(-1), followed by 2 h at 83.0 micro g x kg(-1) x min(-1); corresponding to 0.4 and 2.0 mU x kg(-1) x min(-1)). At the lower dose, the exogenous glucose infusion rates required to maintain euglycemia during steady state were significantly lower with glycated insulin (P < 0.01) and approximately 70% more glycated insulin was required to induce a similar rate of insulin-mediated glucose uptake. Maximal responses at the higher rates of infusion were similar for glycated and control insulin. Inhibitory effects on endogenous glucose production, insulin secretion, and lipolysis, as indicated by measurements of C-peptide, nonesterified free fatty acids, and glycerol, were also similar. Receptor binding to CHO-T cells transfected with human insulin receptor and in vivo metabolic clearance revealed no differences between glycated and native insulin, suggesting that impaired biological activity is due to a postreceptor effect. The present demonstration of glycated insulin in human plasma and related impairment of physiological insulin-mediated glucose uptake suggests a role for glycated insulin in glucose toxicity and impaired insulin action in type 2 diabetes.
Animals↗