title: Folate deprivation reduces homocysteine remethylation in a human intestinal epithelial cell culture model: role of serine in one-carbon donation.
abstract: Little 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)C(5)]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 < 0.001), and only approximately 50% 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.
authors: Food Science and Human Nutrition Department, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611-0370, USA. jfgy@mail.ifas.ufl.edu
authors: Justin H
authors: JH
authors: Townsend
authors: Steven R
authors: SR
authors: Davis
authors: Amy D
authors: AD
authors: Mackey
authors: Jesse F
authors: JF
authors: Gregory
mesh_terms: D000596:Amino Acids / Q000302:isolation & purification / Q000378:metabolism; D018938:Caco-2 Cells; D003470:Culture Media; D004847:Epithelial Cells / Q000378:metabolism*; D005494:Folic Acid Deficiency / Q000378:metabolism*; D006710:Homocysteine / Q000378:metabolism*; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D007422:Intestines / Q000166:cytology; D007700:Kinetics; D008715:Methionine / Q000378:metabolism; D008745:Methylation; D012694:Serine / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D003470:Culture Media; D006710:Homocysteine; D012694:Serine; D008715:Methionine