Restoring Oxygen Carrying Capacity of Sickle Erythrocytes with Nitric Oxide Donors.

Abstract

Sickle cell anemia (SCA) is an inherited blood disorder of hemoglobin function. A genetic mutation results in the substitution of a valine for glutamic acid residue at position 6 of the beta-globin chain yielding the mutant hemoglobin S (HbS). HbS polymerizes within erythrocytes during deoxygenation resulting in altered affinity of oxygen binding. The slightly different P50 (PO2 at which Hb is half-saturated with oxygen) values of sickle erythrocytes obtained during either oxygenation or deoxygenation (hysteresis) demonstrate HbS polymerization induced inhibition of oxygen affinity. Nitric oxide (NO) has been found to be an important signaling molecule in the circulatory system. NO derivatives of Hb provide insights into the physiological role of Hb. NO can bind to Hb at either the heme moiety forming nitrosylhemoglobin (HbNO) or to the conserved beta-93 cysteine yielding S-nitrosohemoglobin (SNO-Hb). In deoxygenated venous blood NO preferentially binds to the hemes of Hb forming HbNO while in oxygenated arterial blood NO binds to the beta-93 cysteine residues forming SNO-Hb. Increased oxygen affinity is seen in both SNO-Hb (Bonaventura C, et al, 1999) and also with HbNO. Decreasing the HbS P50 inhibits intra-erythrocyte HbS polymerization that may be an effective strategy to treat SCA. Clinical trials of NO breathing effects on oxygen affinity are conflicting. One study found an increased oxygen affinity of blood from SCA patients breathing 80 ppm NO with no effect seen in normal controls (Head A, et al, 1997). Another study found that levels of NO bound to Hb are too low to affect overall oxygen affinity (Gladwin M, et al, 1999). The purpose of this in vitro study was to determine the oxygen affinity of deoxygenated sickle erythrocytes pre-treated with exogenous NO donors. Blood from SCA (HbSS) and normal controls (HbAA) were collected and suspended in PBS buffer and deoxygenated with argon gas. The Hb concentration of each sample was calculated and then was either left untreated (control) or treated with varying concentrations of NO donors. The NO donors included: 2-(N, N-diethylamino)-diazenolate-2-oxide (DEANO), S-nitroso-N-acetylpenicillamine (SNAP), sodium nitroprusside (SNP), an aqueous solution of NO, and sodium trioxodinitrate (Angeli's salt, AS). Methemoglobin and protein degradation were negligible. Samples were then transferred via airtight syringes into a stirred and temperature controlled (37°C) chamber of PBS solution at ambient oxygen pressure fitted with a very sensitive oxygen electrode. Oxygen levels were measured in real time. The amount of oxygen extracted from the PBS medium followed first order kinetics. Studies with HbSS red cell suspensions showed that the largest increment in oxygen extraction from the medium was obtained with DEANO pre-treatment. Calculations indicated that low levels of NO treatment, at approximately a 1:1000 ratio of [NO]/[heme], yielded the largest oxygen consumption. The effects of pre-treatment with the other NO donors on sickle erythrocytes (HbSS) were not as pronounced. DEANO is an NO donor yielding a “pure” NO radical as opposed to other redox forms. Similar studies with HbAA and HbSC did not show increases in oxygen extraction. Taken together the data suggest that low levels of NO perturb the quaternary structure of intraerythrocyte HbS polymer allowing depolymerization and increased oxygen affinity. The hope is that these in vitro studies will better characterize the role of NO in its interactions with Hb and the red cell and to use this knowledge for potential therapies in diseases such as SCA.