Submitted on October 21, 2003
Accepted on November 7, 2003
Folate Deprivation Reduces Homocysteine Remethylation in a Human Intestinal Epithelial Cell Culture Model: Role of Serine in One-Carbon Donation
Justin H. Townsend1, Steven R. Davis1, Amy D. Mackey1, and Jesse F. Gregory, III1*
1 Food Science & Human Nutrition Department, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL, USA
* To whom correspondence should be addressed. E-mail: jfgy{at}mail.ifas.ufl.edu.
Little is known about homocysteine metabolism in the 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 [3-13C]serine and [U-13C5]methionine tracers, and the enrichments of the intracellular free amino acid pools of these amino acids, as well as homocysteine, cystathionine, and cysteine were measured using gas chromatography-mass spectrometry. Homocysteine transsulfuration, and 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 3-C 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. The appearance of labeled homocysteine and cystathionine in the culture medium suggests export of these metabolites from intestinal cells. Remethylation was reduced by 1/3rd in folate-restricted cell cultures (P<0.001), and only ~50% of the one-carbon units used for remethylation originated from the 3-C of serine under these conditions. In conclusion, the 3-C 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 3-C of serine contribute to remethylation under this condition.
