NOS3 and pulmonary arterial hypertension: Peroxynitrite (ONOO−), which forms from the rapid reaction of NO with O2−, has been shown to nitrate susceptible tyrosine residues in a number of proteins [128] and increased protein nitration is thought to reflect the enhanced consumption of NO by O2−.Elevated tyrosine nitration has been observed in human and experimental PAH [53,97,100] which helps to reconcile the data showing impaired endothelial function but no change in eNOS expression.