Fig. 8.
Mechanisms of eNOS uncoupling during PAH. (A) Each monomer of eNOS has a reductase and an oxygenase domain, linked together by a calmodulin-binding region. Electrons flow through the reductase domain to the oxygenase domain of an adjacent eNOS monomer. BH4 stabilizes the eNOS dimers and the interaction of eNOS with l-arginine. The electrons then flow to oxygen, which reacts with l-arginine to form citrulline and •NO. When BH4 is oxidized into BH2, l-arginine no longer associates with eNOS. Therefore, oxygen receives the electron from the heme/iron group and forms O2•- instead of •NO, uncoupling the electron transfer from l-arginine hydroxylation. (B) eNOS predominantly localizes to the caveolae, interacting with caveolin-1, inhibiting its activity. Recruitment of Hsp90 and calmodulin displaces caveolin-1 and activates eNOS that produces nitric oxide from arginine. Various conditions, such as low arginine levels, can uncouple eNOS activity, where eNOS generates superoxide instead of nitric oxide. Superoxide and nitric oxide can then react and form peroxynitrite, leading to protein tyrosine nitration and altered protein function, further promoting eNOS uncoupling and effectively decreasing nitric oxide levels.
Abbreviations: eNOS; endothelial nitric oxide synthase, Cav-1; caveolin-1, HSP90; heat shock protein 90, BH4; tetrahydrobiopterin; BH2; dihydrobiopterin; O2•−; superoxide; CaM: calmodulin, FMN: flavin mononucleotide, FAD: flavin adenine dinucleotide, NADPH: nicotinamide adenine dinucleotide phosphate, ADMA: asymmetric dimethyl-l-arginine.