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(Circulation Research. 2008;103:957.)
© 2008 American Heart Association, Inc.

Molecular Medicine

Mechanisms Underlying Erythrocyte and Endothelial Nitrite Reduction to Nitric Oxide in Hypoxia

Role for Xanthine Oxidoreductase and Endothelial Nitric Oxide Synthase

Andrew J. Webb^*, Alexandra B. Milsom^*, Krishnaraj S. Rathod, Wai Lum Chu, Shehla Qureshi, Matthew J. Lovell, Florence M.J. Lecomte, David Perrett, Carmelo Raimondo, Espeed Khoshbin, Zubair Ahmed, Rakesh Uppal, Nigel Benjamin, Adrian J. Hobbs, Amrita Ahluwalia

From the Centre for Clinical Pharmacology (A.J.W., A.B.M., K.S.R., M.J.L.,W.L.C., S.Q., F.M.J.L., D.P., A.A.), William Harvey Research Institute, Barts & The London School of Medicine & Dentistry; Cardiothoracic Surgery (C.R., E.K., R.U.) and Department of Anaesthetics (Z.A.), St Bartholomew’s Hospital, Barts & The London; Peninsula Medical School (N.B.), Plymouth; and Department of Pharmacology (A.J.H.), University College London, United Kingdom.

Correspondence to Amrita Ahluwalia, William Harvey Research Institute, Centre for Clinical Pharmacology, Barts and the London, Charterhouse Square, London EC1M 6BQ, United Kingdom. E-mail a.ahluwalia{at}qmul.ac.uk

Reduction of nitrite (NO₂^–) 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 µmol/L) and attenuated by the XOR inhibitor allopurinol (100 µmol/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.

Key Words: blood vessels • cardiovascular research • hypoxia • nitric oxide

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