Figure 4.

Potential mechanisms by which cardiovascular risk factors lead to oxidative stress and endothelial NOS uncoupling. (A) In many types of vascular disease, NADPH oxidases are up-regulated in the vascular wall and generate superoxide (O₂^−•). In experimental diabetes mellitus and angiotensin II-induced hypertension, this has been shown to be mediated by protein kinase C (PKC).^168,169 Expression of endothelial NOS is also increased in vascular disease. H₂O₂, the dismutation product of O₂^−• can increase endothelial NOS expression via transcriptional and post-transcriptional mechanisms (SOD, superoxide dismutase).¹⁷⁰ In addition, also protein kinase C activation can enhance endothelial NOS expression,¹⁷¹ and protein kinase C inhibitors reduce endothelial NOS expression levels in vascular disease.¹⁶⁹ The products of NADPH oxidases and endothelial NOS, O₂^−• and NO·, rapidly recombine to form peroxynitrite (ONOO⁻). This can oxidize the essential cofactor of endothelial NOS (6R-)5,6,7,8-tetrahydrobiopterin (BH₄) to trihydrobiopterin radical (BH₃^•).^172,173 BH₃^• can disproportionate to the quinonoid 6,7-[8H]-H₂-biopterin (BH₂). As a consequence, oxygen reduction and O₂ reduction by endothelial NOS are uncoupled from NO^· formation, and a functional NOS is converted into a dysfunctional O₂^−•-generating enzyme that contributes to vascular oxidative stress. The enhanced endothelial NOS expression (see above) aggravates the situation. (B) Oxidation of BH₄ to biologically inactive products such as the BH₃^• radical or BH₂ also reduces the affinity of the substrate l-arginine (l-Arg) to NO, and NOS catalyzes the uncoupled reduction in O₂, leading to the production of O₂^−• (and possibly also H₂O₂).