Quantitation of carbon flow through the hepatic folate-dependent one-carbon pool in rats

Schalinske, Kevin L.; Steele, Robert D.

doi:10.1016/0003-9861(89)90254-3

Cited by 16 publications

(9 citation statements)

References 17 publications

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“…By use of the upper end of this range as a conservative estimate of intracellular dilution of the [3-13 C]serine tracer, the serine 3-C was found to provide all of the 1-C units used for total body homocysteine remethylation. This is greater than the ϳ50% of homocysteine remethylation from the serine 3-C measured in rat liver (23). Although these results might indicate that we underestimated intracellular serine enrichment and therefore overestimated the rate of remethylation using the 3-C of serine, these data strongly support the assumption that serine is the primary 1-C donor.…”

Section: Discussionsupporting

confidence: 39%

Tracer-derived total and folate-dependent homocysteine remethylation and synthesis rates in humans indicate that serine is the main one-carbon donor

Davis

Stacpoole²,

Williamson³

et al. 2004

American Journal of Physiology-Endocrinology and Metabolism

139

107

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, respectively). However, the rate of overall homocysteine remethylation (ϳ8 mol⅐kg Ϫ1 ⅐h Ϫ1) was twice that of previous reports, which suggests a larger role for homocysteine remethylation in methionine metabolism than previously thought. By use of estimates of intracellular [3-13 C]serine enrichment based on a conservative correction of plasma [3-13 C]serine enrichment, serine was calculated to contribute ϳ100% of the methyl groups used for total body homocysteine remethylation under the conditions of this protocol. This contribution represented only a small fraction (ϳ2.8%) of total serine flux. Our dual-tracer procedure is well suited to measure the effects of nutrient deficiencies, genetic polymorphisms, and other metabolic perturbations on homocysteine synthesis and total and folate-dependent homocysteine remethylation. methionine; methylation cycle; cystathionine ELEVATED PLASMA HOMOCYSTEINE CONCENTRATION is considered an independent risk factor for the development of cardiovascular disease (5,30,32). Accordingly, many investigators have sought to define the genetic and environmental factors that affect plasma homocysteine concentration. Associations exist between plasma homocysteine concentrations and gene polymorphisms, as well as lifestyle and other environmental factors (5). Strong evidence implicates nutritional deficiencies of folate, vitamins B 6 and B 12 , and the methylenetetrahydrofolate reductase (MTHFR) 677C 3 T polymorphism as causes of elevated plasma homocysteine concentration (12, 25). Folate and vitamin B 6 deficiencies and the MTHFR 677C 3 T polymorphism are thought to increase circulating homocysteine concentrations by decreasing the availability of 5-methyltetrahydrofolate (5-CH 3 THF) and thereby inhibiting homocysteine remethylation (24). However, a causal relationship between reduced homocysteine remethylation and hyperhomocysteinemia under these conditions has not been confirmed in humans in vivo.Steady-state plasma homocysteine concentration is not solely a function of the rate of its removal by remethylation but is also affected by the rates of homocysteine production, catabolism through transsulfuration, and loss in renal excretion (24). Specific measurements of homocysteine metabolism through these individual pathways are needed to clarify why homocysteine concentration is elevated by particular genetic variations as well as by nutritional and other environmental conditions. To test the hypothesis that homocysteine remethylation is compromised when 5-CH 3 THF availability is reduced, homocysteine remethylation rates must be measured in individuals affected by these homocysteineelevating factors.Remethylation rates have been measured in humans in vivo by use of methionine tracers labeled with stable isotopes at both the carboxyl and methyl groups or else by measurements of separate methyl-and carboxyl-labeled methionine tracers in primed constant infusion experiments (18,26). By use of these methodologies, the effects of dietary sulfur amino acid intake, sex, age, prandial statu...

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Section: Discussionsupporting

confidence: 39%

Tracer-derived total and folate-dependent homocysteine remethylation and synthesis rates in humans indicate that serine is the main one-carbon donor

Davis

Stacpoole²,

Williamson³

et al. 2004

American Journal of Physiology-Endocrinology and Metabolism

139

107

View full text Add to dashboard Cite

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“…Although these compounds have been implicated as significant contributors of one-carbon groups for remethylation (26,27,37,39), the three carbon of serine provided nearly all of the one-carbon units for homocysteine remethylation in folate-adequate Caco-2 cells (Figs. 2 and 5C).…”

Section: Discussionmentioning

confidence: 99%

“…The percentage of remethylation (%RM from serine) using onecarbon units derived from the three carbon of serine is calculated from RM total and RM from serine %RM from serine ϭ (RM from serine/RM total) ϫ 100 This calculation is equivalent to that reported by Schalinske and Steele (26) for radioisotope studies of serine's role in homocysteine remethylation in rat liver.…”

Section: Methodsmentioning

confidence: 99%

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Folate deprivation reduces homocysteine remethylation in a human intestinal epithelial cell culture model: role of serine in one-carbon donation

Townsend

Davis

Mackey

et al. 2004

American Journal of Physiology-Gastrointestinal and Liver Physi

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III. Folate deprivation reduces homocysteine remethylation in a human intestinal epithelial cell culture model: role of serine in one-carbon donation. Am J Physiol Gastrointest Liver Physiol 286: G588-G595, 2004. First published November 13, 2003 10.1152/ajpgi.00454.2003Little is known about homocysteine metabolism in intestine. To address this question, we investigated homocysteine metabolism under conditions of folate adequacy and folate deprivation in the Caco-2 cell line, a model of human intestinal mucosal cells. Caco-2 cells were cultured in media enriched with [3-13 C]serine and [U-13 C5]methionine tracers, and enrichments of intracellular free amino acid pools of these amino acids as well as homocysteine, cystathionine, and cysteine were measured by using gas chromatography/mass spectrometry. Homocysteine transsulfuration plus folate-dependent and total remethylation were quantified from these amino acid enrichments. Homocysteine remethylation accounted for 19% of the intracellular free methionine pool in cells cultured with supplemental folate, and nearly all one-carbon units used for remethylation originated from the three carbon of serine via folate-dependent remethylation. Labeling of cystathionine and cysteine indicated the presence of a complete transsulfuration pathway in Caco-2 cells, and this pathway produced 13% of the intracellular free cysteine pool. Appearance of labeled homocysteine and cystathionine in culture medium suggests export of these metabolites from intestinal cells. Remethylation was reduced by one-third in folate-restricted cell cultures (P Ͻ 0.001), and only ϳ50% of the one-carbon units used for remethylation originated from the three carbon of serine under these conditions. In conclusion, the three carbon of serine is the primary source of one-carbon units used for homocysteine remethylation in folate-supplemented Caco-2 cell cultures. Remethylation is reduced as a result of folate restriction in this mucosal cell model, and one-carbon sources other than the three carbon of serine contribute to remethylation under this condition. methionine; methylation; Caco-2; transsulfuration; serine FOLATE DEFICIENCY AND GENETIC polymorphisms of several enzymes involved in one-carbon metabolism are linked to cardiovascular disease and certain cancers and particularly colorectal cancer (5, 6, 16, 18a). These associations are likely due to the effects of altered folate metabolism on homocysteine remethylation, DNA synthesis, and DNA methylation (3,11,29). Reduced homocysteine remethylation can result in elevated plasma homocysteine concentration, which is an independent risk factor for cardiovascular disease (19). Altered DNA metabolism is implicated in the etiology of cancers (20).Folate serves as a metabolic carrier of one-carbon units (8). In homocysteine remethylation, folate transports methyl groups as 5-methyltetrahydrofolate (5-CH 3 THF) to methionine synthase, the ubiquitous homocysteine methyltransferase that converts homocysteine to methionine in all cells (Fig. 1, reactions 6-8)...

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“…It is also known that both remethylation pathways seemed to contribute equally by in vitro assay using rat liver [11]. Schalinske and Steele [12] examined the folatedependent remethylation in the rat liver using a radioisotope tracer. However, the regulation of remethylation by both pathways in vivo is not fully understood.…”

Section: Introductionmentioning

confidence: 99%

Distinct effects of folate and choline deficiency on plasma kinetics of methionine and homocysteine in rats

et al. 2006

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Quantitation of carbon flow through the hepatic folate-dependent one-carbon pool in rats

Cited by 16 publications

References 17 publications

Tracer-derived total and folate-dependent homocysteine remethylation and synthesis rates in humans indicate that serine is the main one-carbon donor

Tracer-derived total and folate-dependent homocysteine remethylation and synthesis rates in humans indicate that serine is the main one-carbon donor

Folate deprivation reduces homocysteine remethylation in a human intestinal epithelial cell culture model: role of serine in one-carbon donation

Distinct effects of folate and choline deficiency on plasma kinetics of methionine and homocysteine in rats

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