Serial Review: Biomedical Implications for Hemoglobin Interactions with Nitric Oxide Serial Review Editors: Mark T. Gladwin and Rakesh Patel THE BIOCHEMISTRY OF NITRIC OXIDE, NITRITE, AND HEMOGLOBIN: ROLE IN BLOOD FLOW REGULATION MARK T. GLADWIN,* ,y JACK H. CRAWFORD, z and RAKESH P. P ATEL z * Critical Care Medicine Department, Clinical Center, and y Laboratory of Chemical Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institute of Health, Bethesda, MD, USA; and z Department of Pathology and the Center for Free Radical Biology, University of Alabama at Birmingham, Birmingham, AL, USA (Received 19 November 2003; Accepted 26 November 2003) Abstract—Nitric oxide (NO) plays a fundamental role in maintaining normal vasomotor tone. Recent data implicate a critical function for hemoglobin and the erythrocyte in regulating the activity of NO in the vascular compartment. Intravascular hemolysis releases hemoglobin from the red blood cell into plasma (cell-free plasma hemoglobin), which is then able to scavenge endothelium-derived NO 600-fold faster than erythrocytic hemoglobin, thereby disrupting NO homeostasis. This may lead to vasoconstriction, decreased blood flow, platelet activation, increased endothelin-1 expression (ET-1), and end-organ injury, thus suggesting a novel mechanism of disease for hereditary and acquired hemolytic conditions such as sickle cell disease and cardiopulmonary bypass. Furthermore, therapy with NO gas inhalation or infusion of sodium nitrite during hemolysis may attenuate this disruption in vasomotor balance by oxidizing plasma cell-free hemoglobin, thereby preventing the consumption of endogenous NO and the associated pathophysiological changes. In addition to providing an NO scavenging role in the physiological regulation of NO- dependent vasodilation, hemoglobin and the erythrocyte may deliver NO as the hemoglobin deoxygenates. While this process has previously been ascribed to S-nitrosated hemoglobin, recent data from our laboratories suggest that deoxygenated hemoglobin reduces nitrite to NO and vasodilates the human circulation along the physiological oxygen gradient. This newly described role of hemoglobin as a nitrite reductase is discussed in the context of blood flow regulation, oxygen sensing, and nitrite-based therapeutics. D 2004 Elsevier Inc. All rights reserved. Keywords—Nitrite, hemoglobin, Vasodilation, Blood flow, Oxygen sensing, S-nitroso-hemoglobin, Iron-nitrosyl- hemoglobin, Free radicals INTRODUCTION Nitric oxide (NO) is a critical regulator of vascular homeostasis [1–5]. Constitutive local production of NO by endothelial cell nitric oxide synthase (eNOS) accounts for 25–30% of basal human blood flow [6– 10]. In addition to maintaining basal vasodilator tone, NO tonically inhibits platelet aggregation, leukocyte adhesion, and smooth muscle proliferation, modulates respiration, and exerts antioxidant and anti-inflamma- tory activity. Under physiological conditions, reactions of vascular-derived NO with hemoglobin are thought to be the most important pathway for limiting NO bioac- tivity. As described in more detail elsewhere in this review series, reaction of the iron-containing heme groups of oxy- and deoxyhemoglobin with NO produ- ces methemoglobin and nitrate ions and iron-nitrosyl- hemoglobin, respectively. These reactions of NO and This article is part of a series of reviews on ‘‘Biomedical Implications for Hemoglobin Interactions with Nitric Oxide.’’ The full list of papers may be found on the home page of the journal. Address correspondence to: Dr. Mark T. Gladwin, National Institutes of Health, Building 10, Room 7D-43, 10 Center Drive, Bethesda, MD 20892-1662; Fax: (301) 402-1213; E-mail: mgladwin@nih.gov. OR Dr. Rakesh Patel, Department of Pathology, Center for Free Radical Biology, University of Alabama at Birmingham, 901 19th Street South, BMR-2, rm 307, Birmingham, AL 35294; Fax: (205) 934-7447; E-mail: patel@path.uab.edu. Free Radical Biology & Medicine, Vol. 36, No. 6, pp. 707 –717, 2004 Copyright D 2004 Elsevier Inc. Printed in the USA. All rights reserved 0891-5849/$-see front matter doi:10.1016/j.freeradbiomed.2003.11.032 707