Tetrahydrobiopterin, superoxide, and vascular dysfunction

Abstract

(6R)-5,6,7,8-Tetrahydrobiopterin (BH₄) is an endogenously produced pterin that is found widely distributed in mammalian tissues. BH₄ works as a cofactor of aromatic amino acid hydroxylases and nitric oxide synthases. In the vasculature a deficit of BH₄ is implicated in the mechanisms of several diseases including atherosclerosis, hypertension, diabetic vascular disease, and vascular complications from cigarette smoking and environmental pollution. These ill-effects are connected to the ability of BH₄ to regulate reactive oxygen species levels in the endothelium. The possibility of using BH₄ as a therapeutical agent in cardiovascular medicine is becoming more compelling and many biochemical and physiological aspects involved in this application are currently under investigation. This review summarizes our current understanding of BH₄ reactivity and some aspects of cellular production and regulation.

Introduction

It is clear that tetrahydrobiopterin is vital to human health in many ways and that research to elucidate its exact role is even more essential with new therapies arising. This year the first BH₄ formulation (Sapropterin, Kuvan) received FDA approval for use in patients afflicted with phenylketonuria (PKU) [1], a genetic condition causing accumulation of phenylalanine in the blood [2]. While this treatment will not be positive for all PKU patients, certainly it represents a tremendous benefit for those that until now only relied on a strict diet, of questionable taste, to control their disease. In a broader sense, this application opens the door to the use of BH₄ in the treatment of other human conditions where BH₄ supply is limiting. This step certainly requires a more clear understanding of the mechanisms regulating BH₄ in different organs and tissues, its pharmacology, and its physiological effects.

On the demonstration that nitric oxide (NO) production from nitric oxide synthases is dependent on optimal BH₄ levels, we have witnessed a renewed interest in understanding BH₄ sources and reactivity. It is now recognized that tissue availability and distribution in different redox forms may explain its biological activity. Although these variables have been shown to affect iron-dependent enzymes such as phenylalanine and tyrosine hydroxylase, they only recently have been connected with NOS and NO synthesis. Some confounding characteristics that have delayed a full understanding of the role of BH₄ in the regulation of eNOS activity include a high turnover rate, low redox potential, and reactivity with antioxidant molecules such as thiols and ascorbate [4], [5], [6], [7]. Also, in the endothelium the differential activity of the enzymes assisting in the recycling of oxidized products of BH₄ (i.e., dihydrofolate reductase, dihydropteridine reductase) has limited our understanding as to what extent BH₄ may be considered a limiting pathophysiological factor in vascular disease.

There is now a large body of literature implicating BH₄ and its oxidized metabolites in the regulation of superoxide anion radical (O₂^·−) release in the endothelium. There is also strong evidence showing that this activity is linked to loss of NO. The shift in endothelial O₂^·− and NO levels is thought to significantly change endothelial redox state and create a chronic state of BH₄ deficiency. This way BH₄ could cause a sizeable state of oxidant stress in the endothelium conducive to cellular changes and loss of function. Within this context, this review focuses on the BH₄ biochemistry, and its role in reactive oxygen species formation and current evidence on the potential implications of this agent in vascular medicine.