Activation energy determinations suggest that thiols reverse autooxidation of tetrahydrobiopterin by a different mechanism than ascorbate

In neutral aqueous solutions tetrahydrobiopterin is oxidized by dioxygen in a reaction that is succinctly described as autooxidation. Ascorbate and thiols moderate this reaction by reversing the oxidative process. In the present study the effect of various thiols on the apparent Arrhenius activation energy of tetrahydrobiopterin autooxidation was characterized and compared to that of ascorbate determined previously. We observed that - in sharp contrast to ascorbate - the efficiency of thiols to protect tetrahydrobiopterin decreased with the elevation of temperature from 22 to 37 °C. Accordingly, the apparent Arrhenius activation energies (in kJ/mol) measured in the presence of thiols were consistently greater than the value determined with tetrahydrobiopterin alone (59.6 ± 1.4) or in the presence of ascorbate (59.9 ± 2.8). Thus, the energy values were 88.8 ± 1.1 with glutathione, 87.6 ± 2.1 with N-acetylcysteine, 79.2 ± 1.6 with cysteine, 75.1 ± 2.4 with dithiotreitol and 70.3 ± 0.9 with homocysteine. Since thiols are as potent reducing agents as ascorbate, these findings suggest that thiols and ascorbate protect tetrahydrobiopterin from oxidation acting at different steps of the oxidation process. It is likely that thiols reduce quinoidal dihydrobiopterin, whereas ascorbate scavenges the trihydrobiopterin radical to tetrahydrobiopterin. Furthermore, the results indicate that thiols are excellent tools to protect tetrahydrobiopterin from autooxidative decomposition in laboratory experiments conducted at relatively low temperatures, whereas the protective effect diminishes at 37 °C, i.e. under physiological conditions.