Mechanisms underlying erythrocyte and endothelial nitrite reduction to nitric oxide in hypoxia: role for xanthine oxidoreductase and endothelial nitric oxide synthase.
Reduction of nitrite (NO(2)(-)) provides a major source of nitric oxide (NO) in the circulation, especially in hypoxemic conditions. Our previous studies suggest that xanthine oxidoreductase (XOR) is an important nitrite reductase in the heart and kidney. Herein, we have demonstrated that conversion of nitrite to NO by blood vessels and RBCs was enhanced in the presence of the XOR substrate xanthine (10 micromol/L) and attenuated by the XOR inhibitor allopurinol (100 micromol/L) in acidic and hypoxic conditions only. Whereas endothelial nitric oxide synthase (eNOS) inhibition had no effect on vascular nitrite reductase activity, in RBCs L-NAME, L-NMMA, and L-arginine inhibited nitrite-derived NO production by >50% (P<0.01) at pH 7.4 and 6.8 under hypoxic conditions. Western blot and immunohistochemical analysis of RBC membranes confirmed the presence of eNOS and abundant XOR on whole RBCs. Thus, XOR and eNOS are ideally situated on the membranes of RBCs and blood vessels to generate intravascular vasodilator NO from nitrite during ischemic episodes. In addition to the proposed role of deoxyhemoglobin, our findings suggest that the nitrite reductase activity within the circulation, under hypoxic conditions (at physiological pH), is mediated by eNOS; however, as acidosis develops, a substantial role for XOR becomes evident.
Webb
Andrew J
AJ
William Harvey Research Institute, Centre for Clinical Pharmacology, Barts and the London, Charterhouse Square, London, UK.
Milsom
Alexandra B
AB
Rathod
Krishnaraj S
KS
Chu
Wai Lum
WL
Qureshi
Shehla
S
Lovell
Matthew J
MJ
Lecomte
Florence M J
FM
Perrett
David
D
Raimondo
Carmelo
C
Khoshbin
Espeed
E
Ahmed
Zubair
Z
Uppal
Rakesh
R
Benjamin
Nigel
N
Hobbs
Adrian J
AJ
Ahluwalia
Amrita
A
D000493:Allopurinol / Q000494:pharmacology; D000818:Animals; D001011:Aorta / Q000201:enzymology; D001120:Arginine / Q000378:metabolism; D015687:Cell Hypoxia; D002478:Cells, Cultured; D042783:Endothelial Cells / Q000187:drug effects / Q000201:enzymology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004912:Erythrocytes / Q000187:drug effects / Q000201:enzymology*; D005470:Fluorometry; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D009569:Nitric Oxide / Q000097:blood / Q000378:metabolism*; D052247:Nitric Oxide Synthase Type II / Q000378:metabolism; D052250:Nitric Oxide Synthase Type III / Q000037:antagonists & inhibitors / Q000097:blood / Q000378:metabolism*; D009572:Nitrite Reductases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009573:Nitrites / Q000097:blood / Q000378:metabolism*; D010084:Oxidation-Reduction; D051381:Rats; D017208:Rats, Wistar; D019820:Xanthine / Q000378:metabolism; D014968:Xanthine Dehydrogenase / Q000037:antagonists & inhibitors / Q000097:blood / Q000378:metabolism*; D019323:omega-N-Methylarginine / Q000494:pharmacology
D004791:Enzyme Inhibitors; D009573:Nitrites; D019820:Xanthine; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D000493:Allopurinol; D001120:Arginine; C496314:NOS3 protein, human; D052247:Nitric Oxide Synthase Type II; D052250:Nitric Oxide Synthase Type III; C496316:Nos3 protein, rat; D014968:Xanthine Dehydrogenase; D009572:Nitrite Reductases; D019331:NG-Nitroarginine Methyl Ester