Modeling of biopterin-dependent pathways of eNOS for nitric oxide and superoxide production

Endothelial dysfunction is associated with increase in oxidative stress and low NO bioavailability. The endothelial NO synthase (eNOS) uncoupling is considered an important factor in endothelial cell oxidative stress. Under increased oxidative stress, the eNOS cofactor tetrahydrobiopterin (BH₄) is oxidized to dihydrobiopterin, which competes with BH₄ for binding to eNOS, resulting in eNOS uncoupling and reduction in NO production. The importance of the ratio of BH₄ to oxidized biopterins versus absolute levels of total biopterin in determining the extent of eNOS uncoupling remains to be determined. We have developed a computational model to simulate the kinetics of the biochemical pathways of eNOS for both NO and O₂^•− production to understand the roles of BH₄ availability and total biopterin (TBP) concentration in eNOS uncoupling. The downstream reactions of NO, O₂^•−, ONOO⁻, O₂, CO₂, and BH₄ were also modeled. The model predicted that a lower BH₄]/TBP] ratio decreased NO production but increased O₂^•− production from eNOS. The NO and O₂^•− production rates were independent above 1.5 μM TBP]. The results indicate that eNOS uncoupling is a result of a decrease in BH₄]/TBP] ratio, and a supplementation of BH₄ might be effective only when the BH₄]/TBP] ratio increases. The results from this study will help us understand the mechanism of endothelial dysfunction.