The endothelial nitric oxide synthase (eNOS) co-factor tetrahydrobiopterin (BH4) has been shown to play a pivotal role in maintaining endothelial function in experimental vascular disease models. In BH4-deficient states, eNOS becomes enzymatically ‘uncoupled’, generating reactive oxygen species instead of nitric oxide, thus promoting endothelial dysfunction. In humans with coronary artery disease (CAD), higher vascular BH4 levels have been shown to be associated with improved endothelial function, and genetic variation in endogenous BH4 synthesis has implicated a causal role. Accordingly, BH4 has been proposed as a potential therapeutic target in vascular disease states. The work in this thesis aims to further elucidate the roles of exogenous and endogenous BH4 in humans. In a randomised, placebo-controlled clinical trial of oral BH4 therapy in patients with CAD, exogenous BH4 had no effect on endothelial function or vascular oxidative stress. Subsequent pharmacokinetic and pharmacodynamic analysis revealed that oral BH4 significantly augmented BH4 levels in plasma and in venous tissue (but not in arterial tissue), but also increased levels of the oxidation product dihydrobiopterin (BH2), which lacks eNOS cofactor activity. Thus, there was a null effect on overall biopterin redox status. To further understand the mechanics of exogenous BH4 oxidation, ex vivo studies of human blood and vascular tissue demonstrated that exogenous BH4 is very rapidly oxidised to BH2; co-administration with an antioxidant had only a modest effect on preventing BH4 oxidation in blood, with no beneficial effect on biopterin redox state in the vasculature. Finally, using a “Mendelian randomisation” approach, I studied the effects of a haplotype of GCH1 (the gene encoding the rate limiting enzyme in BH4 synthesis) on endogenous BH4 bioavailability and vascular function in healthy individuals. In patients with CAD, this haplotype has been associated with decreased BH4 bioavailability and eNOS uncoupling, however in healthy individuals the haplotype exerted no significant effect, likely due to reduced inflammatory stimulation of GCH1.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:558551 |
| Date | January 2011 |
| Creators | Cunnington, Colin |
| Contributors | Channon, Keith |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:33df2f8a-f16b-4271-988a-4500ed89d35e |
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