Abstract

Background: Reduced availability of tetrahydrobiopterin (BH₄), an essential cofactor of nitric oxide (NO) synthase (NOS), decreases NO production and increases reactive oxygen species. Both mechanisms contribute to atherosclerotic vascular disease. Although acute supplementation of BH₄ improves endothelial dysfunction, the effect of chronic BH₄ in humans is unknown.

Objective: To investigate the effect of chronic BH₄ supplementation on endothelial function and oxidative stress in hypercholesterolaemia.

Design: Randomised double-blind, placebo-controlled trial.

Setting: University Hospital.

Patients: 22 hypercholesterolaemic patients (low-density lipoprotein (LDL) >4.5 mmol/l) were randomised to 4 weeks of oral BH₄ (400 mg twice daily) or placebo. Age-matched healthy volunteers served as controls.

Main outcome measures: Endothelium-dependent and -independent vasodilatation was assessed by venous occlusion plethysmography. To elucidate the mechanisms of BH₄ effect, NO release and superoxide anion (O₂⁻) production were measured in human aortic endothelial cells exposed to native LDL (2.6 mmol cholesterol/l).

Results: BH₄ plasma levels were significantly increased by oral supplementation. NO-mediated vasodilatation to acetylcholine was reduced in patients compared with controls and restored by BH₄. No effect of BH₄ on endothelium-independent vasodilatation was seen. Furthermore, 8-F₂ isoprostane plasma levels, a marker of vascular oxidative stress, were reduced by BH₄. In LDL-treated endothelial cells, BH₄ levels and NO release were reduced and O₂⁻ production increased compared with control cells. Exogenous BH₄ normalised NO and O₂⁻ production.

Conclusions: In hypercholesterolaemia, endothelial dysfunction and oxidative stress can be reversed by chronic oral treatment with BH₄. Thus, BH₄ availability is essential for maintaining NO synthesis and low O₂⁻ production by endothelial NOS in vivo, and may provide a rational therapeutic approach to prevent cardiovascular disease.