Background: The role of circulating homocysteine as an atherosclerosis risk factor has recently been questioned. However, 5-methyl-tetrahydrofolate (5-MTHF), the circulating metabolite of folic acid participating in homocysteine metabolism, has direct effects on vascular function. We sought to distinguish the effects of plasma versus vascular tissue 5-MTHF and homocysteine on vascular redox and endothelial nitric oxide bioavailability in human vessels.
Methods and results: We used the methyl tetrahydrofolate reductase (MTHFR) gene polymorphism 677C>T as a model of chronic exposure of the vascular wall to varying 5-MTHF levels in 218 patients undergoing coronary artery bypass graft surgery. Vascular superoxide, vascular 5-MTHF, and total homocysteine were determined in saphenous veins and internal mammary arteries obtained during surgery. Nitric oxide bioavailability was evaluated by organ bath studies on saphenous vein rings. MTHFR genotype was a determinant of vascular 5-MTHF (not vascular homocysteine). Both MTHFR genotype and vascular 5-MTHF were associated with vascular nitric oxide bioavailability and superoxide generated by uncoupled endothelial nitric oxide synthase. In contrast, vascular homocysteine was associated only with NADPH-stimulated superoxide.
Conclusions: Genetic polymorphism 677 C>T on MTHFR affects vascular 5-MTHF (but not homocysteine) and can be used as a model to distinguish the chronic effects of vascular 5-MTHF from homocysteine on vascular wall. Vascular 5-MTHF, rather than plasma or vascular homocysteine, is a key regulator of endothelial nitric oxide synthase coupling and nitric oxide bioavailability in human vessels, suggesting that plasma homocysteine is an indirect marker of 5-MTHF rather than a primary regulator of endothelial function.