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.); pK(a) (H4B. reversible arrow H3B. + H+) = (5.2 +/- 0.1). In the absence of ascorbate both the H4B.+ and H3B. radicals undergo disproportionation to form quinonoid dihydrobiopterin (qH(2)B) and the parent H4B with rate constants k(H4B.+ + H4B.+) = 6.5 X 10(3) M-1 s(-1) and k(H3B. + H3B.) = 9.3 X 10(4) M-1 s(-1), respectively. The H3B. radical is scavenged by ascorbate (AscH(-)) with an estimated rate constant of k(H3B. + AscH(-)) similar to1.7 X 10(5) 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)-O-.), nitrogen dioxide (NO2.), glutathione thiyl (GS(.)), and carbonate (CO3.-) radicals. Without exception these radicals react appreciably faster with H,B than with AscH- with k((OH)-O-. + H4B) = 8.8 X 10(9) M-1 s(-1), k(NO2. + H4B) = 9.4 X 10(8) M-1 s(-1), k(CO3.- + H4B) = 4.6 X 10(9) M-1 s(-1), and k(GS(.) + H4B) = 1.1 X 10(9) 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) similar to4.5 X 10(8) 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. (C) 2002 Elsevier Science Inc.