Computational analysis of interactions of oxidative stress and tetrahydrobiopterin reveals instability in eNOS coupling

In cardiovascular and neurovascular diseases, an increase in oxidative stress and endothelial dysfunction has been reported. There is a reduction in tetrahydrobiopterin (BH₄), which is a cofactor for the endothelial nitric oxide synthase (eNOS), resulting in eNOS uncoupling. Studies of the enhancement of BH₄ availability have reported mixed results for improvement in endothelial dysfunction. Our understanding of the complex interactions of eNOS uncoupling, oxidative stress and BH₄ availability is not complete and a quantitative understanding of these interactions is required. In the present study, we developed a computational model for eNOS uncoupling that considers the temporal changes in biopterin ratio in the oxidative stress conditions. Using the model, we studied the effects of cellular oxidative stress (Q_supcell) representing the non-eNOS based oxidative stress sources and BH₄ synthesis (Q_BH4) on eNOS NO production and biopterin ratio (BH₄/total biopterins (TBP)). Model results showed that oxidative stress levels from 0.01 to 1 nM·s⁻¹ did not affect eNOS NO production and eNOS remained in coupled state. When the Q_supcell increased above 1 nM·s⁻¹, the eNOS coupling and NO production transitioned to an oscillatory state. Oxidative stress levels dynamically changed the biopterin ratio. When Q_supcell increased from 1 to 100 nM·s⁻¹, the endothelial cell NO production, TBP levels and biopterin ratio reduced significantly from 26.5 to 2 nM·s⁻¹, 3.75 to 0.002 μM and 0.99 to 0.25, respectively. For an increase in BH₄ synthesis, the improvement in NO production rate and BH₄ levels were dependent on the extent of cellular oxidative stress. However, a 10-fold increase in Q_BH4 at higher oxidative stresses did not restore the NO-production rate and the biopterin ratio. Our mechanistic analysis reveals that a combination of enhancing tetrahydrobiopterin level with a reduction in cellular oxidative stress may result in significant improvement in endothelial dysfunction.