Singlet molecular oxygen regulates vascular tone and blood pressure in inflammation

Abstract

Singlet molecular oxygen ( 1 O 2 ) has well-established roles in photosynthetic plants, bacteria and fungi 1-3 , but not in mammals. Chemically generated 1 O 2 oxidizes the amino acid tryptophan to precursors of a key metabolite called N-formylkynurenine 4 , while enzymatic oxidation of tryptophan to N-formylkynurenine is catalyzed by a family of dioxygenases, including indoleamine 2,3-dioxygenase 1 5 . Under inflammatory conditions, this hemecontaining enzyme becomes expressed in arterial endothelial cells, where it contributes to the regulation of blood pressure 6 . However, whether indoleamine 2,3-dioxygenase 1 forms 1 O 2 and whether this contributes to blood pressure control is unknown. Here we show that arterial indoleamine 2,3-dioxygenase 1 regulates blood pressure via formation of 1 O 2 . We observed that in the presence of hydrogen peroxide, the enzyme generates 1 O 2 and that this is associated with the stereoselective oxidation of L-tryptophan to a tricyclic hydroperoxide via a previously unrecognized oxidative activation of the dioxygenase activity. The tryptophanderived hydroperoxide acts as a hitherto undiscovered signaling molecule in vivo, which induces arterial relaxation and decreases blood pressure dependent on cysteine residue 42 of protein kinase G1α. Our findings demonstrate a pathophysiological role for 1 O 2 in mammals through formation of an amino acid-derived hydroperoxide that regulates vascular tone and blood pressure under inflammatory conditions. Several small molecules, such as nitric oxide and hydrogen peroxide (H 2 O 2 ) regulate cellular signaling via interaction with proteins containing redox active metals and/or cysteine residues. Of these molecules, nitric oxide, formed from L-arginine by endothelial nitric oxide synthase, is an important regulator of vascular tone 7 . Sustained increases in nitric oxide synthesis by inducible nitric oxide synthase, as observed in pathological settings such as sepsis, are associated with profound hypotension 8 . Paradoxically, inhibitors of the nitric oxide pathway have generally failed to ameliorate severe septic shock 9,10 , suggesting involvement of additional mediators of hypotension.Based on functional similarity with the metabolism of L-arginine by nitric oxide synthase, we 3 reported previously that metabolism of L-tryptophan (Trp) to N-formylkynurenine (NFK) and kynurenine by endothelial indoleamine 2,3-dioxygenase 1 (IDO1) ( Fig. 1a) contributes to the regulation of vascular tone and blood pressure in inflammation 6 . We also showed that commercial kynurenine relaxed pre-constricted arteries, which suggested that kynurenine is an endotheliumderived relaxant factor 6 . Although others have since confirmed these findings 11,12 , we noticed that recently purchased kynurenine no longer caused arterial relaxation, and that HPLC-purified kynurenine and NFK also failed to relax naïve mouse arteries ( Fig. 1b). However, purified Trp relaxed pre-constricted mouse abdominal aortas that expressed IDO1, irrespective of whether IDO1 expression was...

Oxidative Stress: A Unifying Paradigm in Hypertension

“…These molecular events induce protein oxidation and dysregulated cell signalling, leading to inflammation, proliferation, apoptosis, migration, and fibrosis, which are important processes contributing to impaired vascular function, cardiovascular remodelling, renal dysfunction, immune cell activation, and sympathetic nervous system excitation in hypertension. [1][2][3][4] A major source of cardiovascular ROS is a family of nonphagocytic NADPH oxidases (Nox1, Nox2, and Nox4 in rodents and Nox1, Nox2, Nox4, and Nox5 in humans). 5,6 Expression and activation of Nox isoforms are increased in hypertension and are a likely cause of oxidative stress in cardiovascular, renal, and immune cells in hypertension-associated target organ damage.…”

“…ROS also influence many other systems involved in hypertension, and the reader is referred to recent papers for further details. [2][3][4][5][6][7][8] Reactive Oxygen Species, Oxidative Stress, and Redox Signalling in Hypertension…”

Generation of Singlet Molecular Oxygen by Lipid Hydroperoxides and Nitronium Ion†

“…In biological systems, another interesting aspect of lipid peroxidation is the ultra‐weak chemiluminescence emitted during the production of excited molecules, such as singlet molecular oxygen [O 2 ( 1 Δ g )] and triplet excited carbonyls (15–17). Indeed, O 2 ( 1 Δ g ) is a powerful excited and reactive species generated by plants (18,19) and mammals through the action of biological hydroperoxides (20–26), peroxidase‐catalyzed enzymatic processes (27–30) or oxygenase (31). Recently, Stanley et al demonstrated O 2 ( 1 Δ g ) generation by arterial indoleamine 2,3‐dioxygenase 1 (IDO1) in the presence of H 2 O 2 (31).…”

“…Indeed, O 2 ( 1 Δ g ) is a powerful excited and reactive species generated by plants (18,19) and mammals through the action of biological hydroperoxides (20–26), peroxidase‐catalyzed enzymatic processes (27–30) or oxygenase (31). Recently, Stanley et al demonstrated O 2 ( 1 Δ g ) generation by arterial indoleamine 2,3‐dioxygenase 1 (IDO1) in the presence of H 2 O 2 (31). Under these conditions, IDO1 generated O 2 ( 1 Δ g ), which induced stereoselective oxidation of L‐tryptophan to a tricyclic hydroperoxide derivative, consequently contributing to blood pressure regulation under inflammatory conditions (31–33).…”

“…Recently, Stanley et al demonstrated O 2 ( 1 Δ g ) generation by arterial indoleamine 2,3‐dioxygenase 1 (IDO1) in the presence of H 2 O 2 (31). Under these conditions, IDO1 generated O 2 ( 1 Δ g ), which induced stereoselective oxidation of L‐tryptophan to a tricyclic hydroperoxide derivative, consequently contributing to blood pressure regulation under inflammatory conditions (31–33). It is proposed that lipid peroxidation can also yield O 2 ( 1 Δ g ) from peroxyl radicals (ROO · ) through the Russell mechanism (34,35).…”