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Clinical Chemistry and Laboratory Medicine (CCLM)

Published in Association with the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM)

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Folic acid supplementation does not reduce intracellular homocysteine, and may disturb intracellular one-carbon metabolism

Desirée E.C. Smith1, a / Jacqueline M. Hornstra2, a / Robert M. Kok1 / Henk J. Blom1, 3 / 2, 3

1Department Clinical Chemistry, VU University Medical Center, Amsterdam, The Netherlands

2Department of Internal Medicine, VU University Medical Center, Amsterdam, The Netherlands

3Institute for Cardiovascular Research, ICaR-VU, VU University Medical Center, Amsterdam, The Netherlands

aDesirée E.C. Smith and Jacqueline M. Hornstra contributed equally to the article.

Corresponding author: Prof. Dr. Yvo M. Smulders, Department of Internal Medicine, VU University Medical Center, De Boelelaan 1117, 1007 MB Amsterdam, The Netherlands, Phone: +31 20 4444307, Fax: +31 20 4444313

Citation Information: Clinical Chemistry and Laboratory Medicine. Volume 51, Issue 8, Pages 1643–1650, ISSN (Online) 1437-4331, ISSN (Print) 1434-6621, DOI: 10.1515/cclm-2012-0694, June 2013

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Background: In randomized trails, folic acid (FA) lowered plasma homocysteine, but failed to reduce cardiovascular risk. We hypothesize this is due to a discrepancy between plasma and intracellular effects of FA.

Methods: In a double-blind trial, 50 volunteers were randomized to received 500 µg FA daily for 8 weeks, or placebo. Plasma and peripheral blood mononuclear cell (PBMC) concentrations of homocysteine, S-adenosylmethionine (SAM), S-adenosylhomocysteine, methionine, cystathionine and 5-methyltetrahydrofolate (bioactive folate) were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). PBMCs were used as a cellular model since they display the full spectrum of one-carbon (1C) enzymes and reactions.

Results: At baseline, plasma concentrations were a poor reflection of intracellular concentrations for most 1C metabolites, except 5-methyltetrahydrofolate (R=0.33, p=0.02), homocysteine (Hcy) (R=0.35, p=0.01), and cystathionine (R=0.45, p=0.001). FA significantly lowered plasma homocysteine (p=0.00), but failed to lower intracellular homocysteine or change the concentrations of any of the other PBMC 1C metabolites. At baseline, PBMC homocysteine concentrations correlated to PBMC SAM. After FA supplementation, PBMC homocysteine no longer correlated with PBMC SAM, suggesting a loss of SAM’s regulatory function. In vitro experiments in lymphoblasts confirmed that at higher folate substrate concentrations, physiological concentrations of SAM no longer effectively inhibit the key regulatory enzyme methylenetetrahydrofolate reductase (MTHFR).

Conclusions: FA supplementation does not reduce intracellular concentrations of Hcy or any of its closely related substances. Rather, FA may disturb physiological regulation of intracellular 1C metabolism by interfering with SAM’s inhibitory effect on MTHFR activity.

Keywords: cardiovascular disease; folate; homocysteine; intracellular concentrations; S-adenosylmethionine

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