Diffusion-limited Reaction of Free Nitric Oxide with Erythrocytes

Abstract

Nitric oxide (NO) 1 is one of the 10 smallest, stable molecules of the hundreds of millions in nature (1). According to Stokes' Law, the diffusibility of a molecule in the condensed phase is inversely proportional to its molecular radius, which thus makes NO one of the most rapidly diffusible molecules known. Its diffusion constant (D) is approximately 3300 -3800 m 2 /s, whether measured in aqueous solution (2) or in intact tissue (e.g. brain (3)). Membranes and other hydrophobic structures in tissue are no barrier to diffusion of NO because of its solubility in hydrophobic phases (4).The reaction of free NO with oxyhemoglobin is rapid (bimolecular rate constant k ϭ 3.4 ϫ 10 7 M Ϫ1 s Ϫ1 (5)), and from this rate constant it can be calculated that the half-life of NO in the presence of a concentration of hemoglobin equivalent to that in the bloodstream (15 g/dl) would be very short, approximately 2 ϫ 10 Ϫ6 s. As we have pointed out previously (6, 7), the extremely rapid diffusibility of NO coupled with its rapid reaction with oxyhemoglobin apparently poses a difficulty in the postulate that free NO is the endothelium-derived relaxing factor.Using an electrochemical method, we describe here the results of measurements of the disappearance of NO upon reaction with either oxyhemoglobin in solution or oxyhemoglobin when contained within intact erythrocytes. We find that, as reported in 1927 for the reaction of O 2 with deoxyhemoglobin (8), the NO reaction with intact RBCs is considerably slower than with an equivalent concentration of free oxyhemoglobin. We present a mathematical analysis of this phenomenon, which demonstrates that the rate of the reaction of NO with intraerythrocytic hemoglobin is limited by the rate of diffusion of NO into the cell. From our data, we estimate that in whole blood the half-life of NO will be less than 2 ms, which, although quite rapid, is considerably longer than in the presence of free hemoglobin. EXPERIMENTAL PROCEDURESPreparation of NO Solution-6 ml of phosphate-buffered saline (PBS: 15 mM phosphate (potassium) plus 0.09% NaCl pH 7.4) in a plastic vial was used in preparing saturated NO solution. The solution was bubbled with argon gas (Aldrich) for 30 min and then changed to NO gas (Aldrich) for 20 min. The NO gas was passed first through a gaswashing bottle containing 1 M deaerated KOH solution.RBC and Free Hemoglobin Preparation-Blood was withdrawn from rats and centrifuged at 2300 ϫ g for 10 min. The plasma and buffy coat were discarded, and the RBC pellet was washed 3 times with PBS (pH 7.4). The packed RBCs then were added to PBS and the solution was stirred gently. Cells were counted with a hemocytometer and were stored on ice for use. To prepare free oxyHb, 2 ml of counted RBCs was centrifuged at 2300 g for 10 min (4°C). The packed RBCs were then added to 40 ml of 5 mM phosphate solution (pH 8), stirred and allowed to incubate for 30 min for hemolysis.Electrochemical Measurements-All electrochemical measurements were carried out at 25 Ϯ 2°C by a BAS 100B electrochemical a...

Nitric oxide metabolites induced in Anopheles stephensi control malaria parasite infection

“…All reactions of · NO with Hb in the vasculature are decreased by the RBC membrane [37], as · NO reacts at least 1,000 times faster with free Hb than with Hb contained by the RBC membrane [38]. The midgut conditions resemble that of a cell-free Hb system since the RBCs nearest to the midgut epithelium, the source of · NO, are actively digested and lysed.…”

The potential of Angeli's salt to decrease nitric oxide scavenging by plasma hemoglobin

“…That NO is made in a compartment adjacent to the blood where there is about 10 mM Hb (heme concentration * ), led to questioning how it can function without being scavenged by the Hb [34]. In normal physiology, the reason that endothelial-derived NO is not scavenged to the extent predicted, based purely on kinetic calculations, is that red blood cell (RBC) encapsulated Hb in the blood reacts with NO much more slowly than does cell-free Hb [35][36][37][38][39][40][41][42][43][44][45]. Three mechanisms contribute to reduced NO scavenging by RBCs [46].…”

“…Three mechanisms contribute to reduced NO scavenging by RBCs [46]. : (1) the rate of the reaction is largely limited by external diffusion of NO through the plasma to the surface of the RBC [44], especially due to the presence of a red cell free zone adjacent to the vessel walls where NO is made [37][38][39]; (2) NO diffusion is partially blocked by a physical barrier across the RBC membrane [40,43,47]; and (3) RBC-encapsulated Hb is efficiently compartmentalized in the lumen; it does not extravasate into the endothelium and interstitium [44,[48][49][50][51][52].…”

Nitric oxide red blood cell membrane permeability at high and low oxygen tension

“…Hemoglobin reacts rapidly with NO, making it difficult to understand how endothelialderived NO can reach the smooth muscle cells with so much Hb present in blood [6]. Several mechanisms act to reduce NO scavenging by red blood cells (RBCs) compared to cell-free hemoglobin, thus enabling NO to act as the endothelium-derived relaxation factor [7][8][9][10][11][12][13][14][15][16]. These mechanisms include (1) a RBC free zone created adjacent to the endothelium due to the velocity gradient in laminar flow [8,10,11], (2) an unstirred layer surrounding the RBCs that results in NO uptake being rate-limited by diffusion of NO to the RBC [7,14], and (3) an intrinsic, physical RBC membrane barrier to NO diffusion [12,13,[15][16][17][18].…”