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.