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

https://doi.org/10.1016/j.freeradbiomed.2011.06.009Get rights and content

Abstract

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.

Section snippets

Model description

Using known biochemical pathways for eNOS, NO production was modeled for a single endothelial cell [41], [42]. For eNOS uncoupling and O2•− production, we used the biochemical pathway described by Berka and co-workers [43], [44]. In brief, the electron transport from the reductase to the oxygenase domain in eNOS occurs after biopterin binds to the enzyme [1]. After the binding of biopterin, l-arginine, and CaM to the NOS dimer, the heme in the oxygenase domain undergoes a series of redox

NO production decreases and O2•− production increases nonlinearly with a reduction in tetrahydrobiopterin availability

Under normal physiological conditions, a small amount (5–10%) of total biopterin is in oxidized biopterin (BH2 and BH3) form [23], [25]. However, the amount of oxidized biopterin can increase as much as 90% in endothelial cell dysfunction [23], [25]. To understand the impact of BH4 availability on the NO and O2•− production from eNOS, six cases were simulated with a [BH4]/[TBP] ratio from 0.99 to 0.05. A high value for the ratio indicates that the majority of TBP is in the reduced form BH4

Discussion

In this study, we developed a computational model for eNOS biochemical pathways for the production of NO and O2•− to understand the eNOS uncoupling and related endothelial dysfunction mechanism. We analyzed the effects of [BH4]/[TBP] ratio, total biopterin, eNOS concentration, and feedback inhibition of NO consumption on NO and O2•− production.

Acknowledgment

This study was funded by National Institutes of Health Grant NIH R01 HL084337.

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