Oxidation of tetrahydrobiopterin by biological radicals and scavenging of the trihydrobiopterin radical by ascorbate

One-electron oxidation of (6R)-5,6,7,8-tetrahydrobiopterin (H4B) by the azide radical generates the radical cation (H4B•+) which rapidly deprotonates at physiological pH to give the neutral trihydrobiopterin radical (H3B•); pKa (H4B•+ right arrow over left arrow H3B• + H+) = (5.2 ± 0.1). In the absence of ascorbate both the H4B•+ and H3B• radicals undergo disproportionation to form quinonoid dihydrobiopterin (qH2B) and the parent H4B with rate constants k(H4B•+ + H4B•+) = 6.5 × 103 M−1 s−1 and k(H3B• + H3B•) = 9.3 × 104 M−1 s−1, respectively. The H3B• radical is scavenged by ascorbate (AscH−) with an estimated rate constant of k(H3B• + AscH−) not, vert, similar 1.7 × 105 M−1 s−1. At physiological pH the pterin rapidly scavenges a range of biological oxidants often associated with cellular oxidative stress and nitric oxide synthase (NOS) dysfunction including hydroxyl (√OH), nitrogen dioxide (NO2•), glutathione thiyl (GS•), and carbonate (CO3•−) radicals. Without exception these radicals react appreciably faster with H4B than with AscH− with k(√OH + H4B) = 8.8 × 109 M−1 s−1, k(NO2• + H4B) = 9.4 × 108 M−1 s−1, k(CO3•− + H4B) = 4.6 × 109 M−1 s−1, and k(GS• + H4B) = 1.1 × 109 M−1 s−1, respectively. The glutathione disulfide radical anion (GSSG•−) rapidly reduces the pterin to the tetrahydrobiopterin radical anion (H4B•−) with a rate constant of k(GSSG•− + H4B) not, vert, similar 4.5 × 108 M−1 s−1. The results are discussed in the context of the general antioxidant properties of the pterin and the redox role played by H4B in NOS catalysis.