Exploration of Structure-Activity Relationship of Aromatic Aldehydes Bearing Pyridinylmethoxy-Methyl Esters as Novel Antisickling Agents.

Aromatic aldehydes elicit their antisickling effects primarily by increasing the affinity of hemoglobin (Hb) for oxygen (O2). However, challenges related to weak potency and poor pharmacokinetic properties have hampered their development to treat sickle cell disease (SCD). Herein, we report our efforts to enhance the pharmacological profile of our previously reported compounds. These compounds showed enhanced effects on Hb modification, Hb-O2 affinity, and sickling inhibition, with sustained pharmacological effects in vitro. Importantly, some compounds exhibited unusually high antisickling activity despite moderate effects on the Hb-O2 affinity, which we attribute to an O2-independent antisickling activity, in addition to the O2-dependent activity. Structural studies are consistent with our hypothesis, which revealed the compounds interacting strongly with the polymer-stabilizing αF-helix could potentially weaken the polymer. In vivo studies with wild-type mice demonstrated significant pharmacologic effects. Our structure-based efforts have identified promising leads to be developed as novel therapeutic agents for SCD.

Hydroxyurea in the management of sickle cell disease: pharmacogenomics and enzymatic metabolism.

Hydroxyurea (HU) was approved to be used in the treatment of sickle cell disease (SCD) because of its anti-sickling potential. However, there is variability in HU response among SCD patients and this can be due to physiological, socioeconomic, environmental, metabolic and/or genetic factors. The present review focuses on the latter two. Three quantitative trait loci, HBG2, BCL11A and HMIP, have been suggested as important markers for HU response. Other genes (ASS1, KLF10, HAO2, MAP3K5, PDE7B, TOX, NOS1, NOS2A, FLT1, ARG1, ARG2, UGT1A1, OR51B5/6, SIN3A, SALL2, SAR1A, UTB, OCTN1, CYP2C9, AQP9, MPO, CYP2E1, and GSTT1) have also been considered. Studies implicate catalase, urease, horseradish peroxidase and enzymes of CYP450 family in HU metabolism. However, little is known about these enzymes. Therefore, further studies are needed to elucidate the metabolic pathway of HU, which will facilitate pharmacogenomic studies and help in identification of candidate genes for predicting HU response.

Rational design of pyridyl derivatives of vanillin for the treatment of sickle cell disease.

Hypoxia-induced polymerization of sickle hemoglobin (Hb S) is the principal phenomenon that underlays the pathophysiology and morbidity associated with sickle cell disease (SCD). Opportunely, as an allosteric protein, hemoglobin (Hb) serves as a convenient and potentially critical druggable target. Consequently, molecules that prevent Hb S polymerization (Hb modifiers), and the associated erythrocyte sickling have been investigated-and retain significant interest-as a viable therapeutic strategy for SCD. This group of molecules, including aromatic aldehydes, form high oxygen affinity Schiff-base adducts with Hb S, which are resistant to polymerization. Here, we report the design and synthesis of novel potent antisickling agents (SAJ-009, SAJ-310 and SAJ-270) based on the pharmacophore of vanillin and INN-312, a previously reported pyridyl derivative of vanillin. These novel derivatives exhibited superior in vitro binding and pharmacokinetic properties compared to vanillin, which translated into significantly enhanced allosteric and antisickling properties. Crystal structure studies of liganded Hb in the R2 quaternary state in complex with SAJ-310 provided important insights into the allosteric and antisickling properties of this group of compounds. While these derivatives generally show similar in vitro biological potency, significant structure-dependent differences in their biochemical profiles would help predict the most promising candidates for successful in vivo pre-clinical translational studies and inform further structural modifications to improve on their pharmacologic properties.

Oxidant-antioxidant status in Egyptian children with sickle cell anemia: a single center based study / Estado oxidante-antioxidante em crianças egípcias com anemia falciforme: estudo baseado em um único centro

OBJECTIVE: the present study was conducted to investigate the oxidant-antioxidant status in Egyptian children with sickle cell anemia. METHODS: the serum levels of total antioxidant capacity (TAO), paraoxonase (PON), vitamin E, nitrite, and malondialdehyde (MDA) were measured in 40 steady state children with homozygous sickle cell anemia (24 males and 16 females) and 20 apparently healthy age- and gender-matched controls. RESULTS: mean serum TAO, PON, vitamin E, and nitrite levels were significantly lower in the group with sickle cell anemia, whereas mean serum MDA was significantly higher in these children compared to controls. No significant differences in mean levels of TAO, PON, nitrite, vitamin E, and MDA were found in sickle cell anemia patients receiving hydroxyurea when compared with those not receiving hydroxyurea. A significant negative correlation between serum nitrite and the occurrence of vaso-occlusive crises (VOC) was observed (r = -0.3, p = 0.04). PON level was found to be positively correlated with patients' weight and BMI (r = -0.4, p = 0.01; r = -0.7, p < 0.001, respectively), but not with frequency of VOC. The area under the curve of serum nitrite in predicting occurrence of VOC was 0.782, versus 0.701 for PON, and 0.650 for TAO (p = 0.006). Serum MDA was not correlated with nitrite, PON, TAO, or vitamin E levels. No significant correlations were detected between serum nitrite and hemoglobin or antioxidant enzymes. CONCLUSION: children with sickle cell anemia have chronic oxidative stress that may result in increased VOC, and decreased serum nitrite may be associated with increases in VOC frequency. A novel finding in this study is the decrease in PON level in these patients, which is an interesting subject for further research. .

OBJETIVO: o presente estudo foi realizado com o objetivo de investigar o estado oxidante-antioxidante em crianc¸as egípcias com anemia falciforme. MÉTODOS: dosamos os níveis séricos da capacidade antioxidante total (CAT), paraoxonase (PON), vitamina E, nitrito e malondialdeído (MDA) em 40 crianças estáveis com anemia falciforme homozigótica (24 meninos e 16 meninas), e 20 controles pareados por idade/sexo aparente-mente saudáveis. RESULTADOS: os níveis séricos médios da CAT, PON, vitamina E e nitrito foram significativamente menores, ao passo que o nível sérico médio de MDA foi significativamente maior em crianças com anemia falciforme (AF), em comparação aos controles. Não foram encontradasdiferenças significativas nos níveis médios de CAT, PON, nitrito, vitamina E e MDA em pacientescom AF em tratamento com hidroxiureia, em comparação aos que receberam hidroxiureia. Encontramos uma correlação negativa significativa entre o nitrito sérico e a ocorrência decrises vaso-oclusivas agudas (CVO) (r = -0,3, p = 0,04). Descobrimos que o nível de PON está correlacionado positivamente com o peso e o IMC dos pacientes (r = -0,4; p = 0,01; r = -0,7; p < 0,001, respectivamente), porém não com a frequência de CVO. A área sob a curva (ASC) donitrito sérico na previsão da ocorrência de CVO foi 0,782, em comparação a 0,701 para PON e 0,650 para CAT (p = 0,006). O MDA não está correlacionado a nitrito, PON, CAT ou vitamina E. Não foram detectadas correlações significativas entre nitrito sérico e hemoglobina ou enzimas antioxidantes. CONCLUSÃO: crianças com AF apresentam estresse oxidativo crônico que pode resultar emaumento das CVO. Em crianças com AF, a redução nos níveis de ...

Oxidant-antioxidant status in Egyptian children with sickle cell anemia: a single center based study.

OBJECTIVE: the present study was conducted to investigate the oxidant-antioxidant status in Egyptian children with sickle cell anemia. METHODS: the serum levels of total antioxidant capacity (TAO), paraoxonase (PON), vitamin E, nitrite, and malondialdehyde (MDA) were measured in 40 steady state children with homozygous sickle cell anemia (24 males and 16 females) and 20 apparently healthy age- and gender-matched controls. RESULTS: mean serum TAO, PON, vitamin E, and nitrite levels were significantly lower in the group with sickle cell anemia, whereas mean serum MDA was significantly higher in these children compared to controls. No significant differences in mean levels of TAO, PON, nitrite, vitamin E, and MDA were found in sickle cell anemia patients receiving hydroxyurea when compared with those not receiving hydroxyurea. A significant negative correlation between serum nitrite and the occurrence of vaso-occlusive crises (VOC) was observed (r=-0.3, p=0.04). PON level was found to be positively correlated with patients' weight and BMI (r=-0.4, p=0.01; r=-0.7, p<0.001, respectively), but not with frequency of VOC. The area under the curve of serum nitrite in predicting occurrence of VOC was 0.782, versus 0.701 for PON, and 0.650 for TAO (p=0.006). Serum MDA was not correlated with nitrite, PON, TAO, or vitamin E levels. No significant correlations were detected between serum nitrite and hemoglobin or antioxidant enzymes. CONCLUSION: children with sickle cell anemia have chronic oxidative stress that may result in increased VOC, and decreased serum nitrite may be associated with increases in VOC frequency. A novel finding in this study is the decrease in PON level in these patients, which is an interesting subject for further research.

No effect of CYP450 and P-glycoprotein on hydroxyurea in vitro metabolism.

Our objectives were (1) to study the HU metabolism via human cytochromes and (2) to test if HU is a substrate of P-gp. HU metabolism was investigated by determining the appearance of urea and HU decreasing upon incubation with human liver microsomes. Quantification was determined using HPLC coupled with UV-detection at 449 nm. Our method was linear between 5 and 1000 microm, precise (coefficients of variation ranging from 1.7 to 9.9%), accurate (97.7-103.9%). The limit of quantification was 7 microm. The ATPase activity of human P-gp membranes was determined by measuring inorganic phosphate liberation. HU and urea measurements in microsomes were not different between 0 and 60 min whatever HU concentration used from 30 to 300 microm. The presence of NADPH in the medium has no effect on HU and urea measurements. In the absence of verapamil, the ATPase activity was unaffected by HU at concentrations of 10, 30, 100 and 300 microm. HU is unlikely to cause clinically relevant drug interactions with the substrates of these enzymes/transporters. However, it will be necessary to validate these in vitro data in patients with sickle cell anemia to evaluate the impact of genetic polymorphisms of these enzymes in a black population.

Fetal hemoglobin in sickle cell anemia: genetic determinants of response to hydroxyurea.

The increase in fetal hemoglobin (HbF) in response to hydroxyurea (HU) varies among patients with sickle cell anemia. Twenty-nine candidate genes within loci previously reported to be linked to HbF level (6q22.3-q23.2, 8q11-q12 and Xp22.2-p22.3), involved in metabolism of HU and related to erythroid progenitor proliferation were studied in 137 sickle cell anemia patients treated with HU. Three-hundred and twenty tagging single nucleotide polymorphisms (SNPs) for genotyping were selected based on HapMap data. Multiple linear regression and the nonlinear regression Random Forest method were used to investigate the association between SNPs and the change in HbF level after 2 years of treatment with HU. Both methods revealed that SNPs in genes within the 6q22.3-23.2 and 8q11-q12 linkage peaks, and also the ARG2, FLT1, HAO2 and NOS1 genes were associated with the HbF response to HU. Polymorphisms in genes regulating HbF expression, HU metabolism and erythroid progenitor proliferation might modulate the patient response to HU.

Crystal structures of HbA2 and HbE and modeling of hemoglobin delta 4: interpretation of the thermal stability and the antisickling effect of HbA2 and identification of the ferrocyanide binding site in Hb.

Hemoglobin A(2) (alpha(2)delta(2)) is an important hemoglobin variant which is a minor component (2-3%) in the circulating red blood cells, and its elevated concentration in beta-thalassemia is a useful clinical diagnostic. In beta-thalassemia major, where there is beta-chain production failure, HbA(2) acts as the predominant oxygen deliverer. HbA(2) has two more important features. (1) It is more resistant to thermal denaturation than HbA, and (2) it inhibits the polymerization of deoxy sickle hemoglobin (HbS). Hemoglobin E (E26K(beta)), formed as a result of the splice site mutation on exon 1 of the beta-globin gene, is another important hemoglobin variant which is known to be unstable at high temperatures. Both heterozygous HbE (HbAE) and homozygous HbE (HbEE) are benign disorders, but when HbE combines with beta-thalassemia, it causes E/beta-thalassemia which has severe clinical consequences. In this paper, we present the crystal structures of HbA(2) and HbE at 2.20 and 1.74 A resolution, respectively, in their R2 states, which have been used here to provide the probable explanations of the thermal stability and instability of HbA(2) and HbE. Using the coordinates of R2 state HbA(2), we modeled the structure of T state HbA(2) which allowed us to address the structural basis of the antisickling property of HbA(2). Using the coordinates of the delta-chain of HbA(2) (R2 state), we also modeled the structure of hemoglobin homotetramer delta(4) that occurs in the case of rare HbH disease. From the differences in intersubunit contacts among beta(4), gamma(4), and delta(4), we formed a hypothesis regarding the possible tetramerization pathway of delta(4). The crystal structure of a ferrocyanide-bound HbA(2) at 1.88 A resolution is also presented here, which throws light on the location and the mode of binding of ferrocyanide anion with hemoglobin, predominantly using the residues involved in DPG binding. The pH dependence of ferrocyanide binding with hemoglobin has also been investigated.

Nitric oxide and cyclic GMP levels in sickle cell patients receiving hydroxyurea.

Recent studies suggest that nitric oxide (NO) may partly be responsible for the beneficial effect of hydroxyurea (HU) in sickle cell disease (SCD) patients. NO stimulates cyclic guanosine monophosphate (cGMP) production, which mediates vasodilatation. We investigated the association between NO, cGMP and fetal haemoglobin (HbF) levels after HU administration. Our data showed that chronic HU significantly increased NO, cGMP, and HbF levels in SCD. Recently it was shown that HbF production was stimulated by cGMP-dependent protein kinase. Our results suggest that NO stimulates cGMP production, which then activates a protein kinase and increases the production of HbF.

N-methylcarbamoylated valine of hemoglobin in humans after exposure to N,N-dimethylformamide: evidence for the formation of methyl isocyanate?

N,N-Dimethylformamide (DMF) is reported to cause testicular germ-cell tumors in exposed workers. The reports, however, are not in line with results obtained in animal and in vitro experiments, where DMF was shown not to be mutagenic and also not to be carcinogenic. Considerable interest raised on the formation of a reactive intermediate, presumably methyl isocyanate (MIC), during metabolism of DMF in humans over the last years. We report the formation of N-methylcarbamoylated valine of hemoglobin (Hb) in blood samples from workers exposed to DMF in the polyacrylic fiber industry. N-Methylcarbamoylated Hb was formed by the reaction of MIC with Hb. For this purpose, Hb adducts were monitored by means of a modified Edman degradation involving the release of the N-terminal valine adduct in form of 3-methyl-5-isopropylhydantoin (MIH). For internal standardization of the method, 3-ethyl-5-isopropylhydantoin (EIH) was used. Separation and analysis of MIH and EIH were carried out by gas chromatography and mass spectrometry with electron impact ionization (GC/EI-MS). Hb adducts in form of MIH were quantified in blood samples from exposed persons in concentrations between 26.1 and 412.0 nmol of MIH/g of globin. The observed adducts were proven to be identical to those derived from the in situ reaction between Hb and MIC. Taken together with the fact that only N-methylcarbamoylated Hb can undergo ring closure to the corresponding hydantoin, the reaction is indirect evidence for the occurrence of MIC in vivo. The formation of MIC directly in the cell and its possible distribution through the human body may lead to critical effects after exposure to DMF. Adducts were determined not to be totally specific for exposure to DMF since an identical adduct was also found in blood samples from the general population. However, concentrations were lower by a factor of about 100. The sources for background adducts are currently unknown.

High-resolution crystal structure of deoxy hemoglobin complexed with a potent allosteric effector.

The crystal structure of human deoxy hemoglobin (Hb) complexed with a potent allosteric effector (2-[4-[[(3,5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid) = RSR-13) is reported at 1.85 A resolution. Analysis of the hemoglobin:effector complex indicates that two of these molecules bind to the central water cavity of deoxy Hb in a symmetrical fashion, and that each constrains the protein by engaging in hydrogen bonding and hydrophobic interactions with three of its four subunits. Interestingly, we also find that water-mediated interactions between the bound effectors and the protein make significant contributions to the overall binding. Physiologically, the interaction of RSR-13 with Hb results in increased oxygen delivery to peripheral tissues. Thus, this compound has potential therapeutic application in the treatment of hypoxia, ischemia, and trauma-related blood loss. Currently, RSR-13 is in phase III clinical trials as a radiosensitizing agent in the treatment of brain tumors. A detailed structural analysis of this compound complexed with deoxy Hb has important implications for the rational design of future analogs.

Phase I trial to determine the safety, pharmacodynamics, and pharmacokinetics of RSR13, a novel radioenhancer, in newly diagnosed glioblastoma multiforme.

PURPOSE: To determine the safety, pharmacokinetics, and pharmacodynamic effect of 2-[4-(3, 5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylproprionic++ + acid (RSR13) 100 mg/kg/d with radiation therapy (RT) for glioblastoma multiforme (GBM). RSR13, a synthetic allosteric modifier of hemoglobin (HgB), is a novel radioenhancing agent that noncovalently binds to HgB, thereby reducing oxygen binding affinity and increasing tissue oxygen release to hypoxic tissues. PATIENTS AND METHODS: In this multi-institutional, dose frequency-seeking trial, 19 adult patients with newly diagnosed GBM received RSR13 100 mg/kg every other day or daily along with cranial RT (60 Gy/30 fractions). RSR13 was given over 1 hour by central venous access with 4 L/min of O(2 )by nasal cannula, followed by RT within 30 minutes. Pharmacokinetic (PK) and pharmacodynamic (PD) determinations were performed. The PD end point was shift in P50, the oxygen half-saturation pressure of HgB. RESULTS: Grade 3 dose-limiting toxicity occurred in none of the patients with every-other-day dosing and in two of the 10 patients with daily dosing. Grade 2 or greater toxicity occurred in three out of nine and six out of 10, respectively. PK and PD data demonstrate that a substantial PD effect was reliably achieved, that PD effect was related to RBC RSR13 concentration, and that there was no significant drug accumulation even with daily dosing. The mean shift in P50 was 9.24 +/- 2.6 mmHg (a 34% increase from baseline), which indicates a substantial increase in tendency toward oxygen unloading. CONCLUSION: Daily RSR13 (100 mg/kg) during cranial RT is well tolerated and achieves the desired PD end point. A phase II trial of daily RSR13 for newly diagnosed malignant glioma is currently accruing patients within the New Approaches to Brain Tumor Therapy Central Nervous System Consortium to determine survival outcome.

Acetylation of human hemoglobin by methyl acetylphosphate. Evidence of broad regio-selectivity revealed by NMR studies.

The development of chemical modification agents that reduce the tendency of sickle hemoglobin (HbS) to aggregate represents an important chemotherapeutic goal. Methyl acetylphosphate (MAP) has been reported to bind to the 2,3-diphosphoglycerate (2,3-DPG) binding site of hemoglobin, where it selectively acetylates residues, resulting in increased solubility of HbS. We have prepared [1-(13)C]MAP and evaluated the adduct formation with hemoglobin using (1)H-(13)C HMQC and HSQC NMR studies. These spectra of the acetylated hemoglobin adducts showed 10-11 well resolved adduct peaks, indicating that the acetylation was not highly residue specific. The chemical shift pattern observed is in general similar to that obtained recently using [1'-(13)C]aspirin as the acetylating agent (Xu, A. S. L., Macdonald, J. M., Labotka, R. J., and London, R. E. (1999) Biochim. Biophys. Acta 1432, 333-349). Blocking the 2, 3-DPG binding site with inositol hexaphosphate (IHP) resulted in a selective reduction in intensity of adduct resonances, presumably corresponding to residues located in the 2,3-DPG binding cleft. The pattern of residue protection appeared to be identical to that observed in our previous study using IHP and labeled aspirin. Pre-acetylation of hemoglobin using unlabeled MAP, followed by acetylation with [1'-(13)C]aspirin indicated a general protective effect, with the greatest reduction of intensity for resonances corresponding to acetylated residues in the 2,3-DPG binding site. These studies indicated that both MAP and aspirin exhibit similar, although not identical, acetylation profiles and target primarily the betaLys-82 residue in the 2,3-DPG binding site, as well as sites such as betaLys-59 and alphaLys-90, which are not located in the beta-cleft of hemoglobin.

In vivo production of nitric oxide in rats after administration of hydroxyurea.

The metabolism of nitrovasodilators such as glyceryl trinitrate and nitroprusside provides the active moiety of these drugs (that is, nitric oxide). This process is not limited to the known nitrovasodilators, but also occurs with nitroaromatic antimicrobials. Here we report that the administration of hydroxyurea, an antitumor drug, to rats at pharmacological doses formed detectable nitrosyl hemoglobin, which increased with dose. At higher doses, nitrosyl hemoprotein complexes could also be detected in liver tissue. [15N]hydroxyurea was synthesized and compared with [14N]hydroxyurea. These observations verified that nitric oxide detected as nitrosyl hemoglobin or nitrosyl hemoprotein complexes in rats was the result of the metabolism of hydroxyurea. The time course and dose-dependence of nitric oxide generation were also investigated. Hydroxyurea's antineoplastic activity is caused by its direct action on ribonucleotide reductase, the rate-limiting enzyme in DNA synthesis. Because nitric oxide also inhibits ribonucleotide reductase, this metabolite may supplement this action of hydroxyurea. In addition, the known ability of hydroxyurea to ease the pain of sickle cell anemia patients may be the result of vasodilation by the drug-derived nitric oxide.

Comparative toxicity of methyl isocyanate and its hydrolytic derivatives in rats. I. Pulmonary histopathology in the acute phase.

The present study describes the acute histopathological changes induced by methyl isocyanate (MIC) in the lungs of rats at 24 h after a single exposure to varied concentrations/doses of MIC by inhalation and subcutaneous (s.c.) routes and also delineates the effects due to the hydrolytic derivatives of MIC, viz., methylamine (MA) and N,N'-dimethyl urea (DMU). MIC, either inhaled or administered s.c., resulted in a wide range and extent of histopathological changes in the lungs, proportional to the exposure concentration/dose. The salient, effects of inhaled MIC are acute necrotizing bronchitis of the entire respiratory tract accompanied by varying degrees of confluent congestion, hyperemia and interstitial and intra-alveolar edema, while MIC administered s.c. led to prominent vascular endothelial damage, congestion and severe interstitial pneumonitis with apparently normal bronchial epithelium; and intra-alveolar edema only with the high dose. The only noteworthy lesion produced by MA and DMU (to some extent) was interstitial pneumonitis, suggesting their possible involvement in the subsequent inflammatory response of MIC. Except, for the endothelial changes, the overall spectrum of the histopathological lesions is quite comparable to those observed in the lungs of Bhopal victims during the acute phase.

Comparative toxicity of methyl isocyanate and its hydrolytic derivatives in rats. II. Pulmonary histopathology in the subacute and chronic phases.

This paper describes the long-term (subacute and chronic) histopathological effects in the lungs of rats subjected to a single exposure to methyl isocyanate (MIC) by both the inhalation and subcutaneous (s.c.) routes as well as the role of methylamine (MA) and N,N'-dimethyl-urea (DMU), the hydrolytic derivatives of MIC in eliciting the observed changes. At the subacute phase, the intraalveolar and interstitial edema were prominent only in the inhalation group as against the more pronounced inflammatory response in the s.c. route. With the progress of time the evolution of lesions appeared to be similar, culminating in the development of significant interstitial pneumonitis and fibrosis. MA, one of the hydrolytic derivatives of MIC, also caused interstitial pneumonitis progressing to fibrosis, albeit to a lesser extent than MIC, indicating its contribution to the long-term pulmonary damage. The diffuse interstitial pulmonary fibrosis observed at 10 weeks after a single exposure to MIC by either route is of greater significance in the context of the occurrence of pulmonary fibrosis in the late autopsies of Bhopal gas victims and also clinical sequelae in some of the survivors.

Do the hydrolysis products, methylamine and N,N'-dimethylurea, play any role in the methyl isocyanate-induced haematological and biochemical changes in rabbits?

The subcutaneous administration of methyl isocyanate (MIC) to female rabbits, resulted in significant increases in haemoglobin concentration, erythrocyte volume fraction and leucocyte number in blood, as well as plasma total proteins, and urea. The present study was designed to investigate whether the hydrolytic products of MIC, methylamine (MA) and N,N'-dimethylurea (DMU) play any role in eliciting these changes. Both MA and DMU administered subcutaneously in an equimolar dose to that of 1.0 LD50 MIC, 2.2 mmol kg-1, had no influence on these parameters, although there was a marginal increase in the plasma urea level shortly after the administration of DMU. This study establishes that the observed haematological and biochemical changes induced by MIC intoxication in rabbits are mostly due to MIC.

Comparison of crystal and solution hemoglobin binding of selected antigelling agents and allosteric modifiers.

This paper details comprehensive binding studies (solution and X-ray) of human hemoglobin A with a group of halogenated carboxylic acids that were investigated as potential antisickling agents. It is, to our knowledge, the first study to compare solution and crystal binding for a series of compounds under similar high-salt conditions used for cocrystallization. The compounds include [(3,4-dichlorobenzyl)oxy]acetic acid, [(p-bromobenzyl)oxy]acetic acid, clofibric acid, and bezafibrate. The location and stereochemistry of binding sites have been established by X-ray crystallography, while the number of binding sites and affinity constants were measured by using equilibrium dialysis. The solution binding studies were conducted with deoxygenated hemoglobin and carbonmonoxyhemoglobin with low (50 mM) and high (2 M) salt concentrations. It was concluded that the observed crystal structures are consistent with the binding observed in solution and that the number of binding sites is independent of salt concentration, while the binding constant increases with increasing salt concentration. The studies also reveal that relatively small changes in the chemical structure of a drug molecule can result in entirely different binding sites on the protein. Moreover, the X-ray studies provide a possible explanation for the multiplicity in function exhibited by these compounds as allosteric modulators and/or antisickling agents. Finally, the studies that these compounds bind differently to the R and T states of hemoglobin, an observation of special significance to the original design of these agents.

Increasing haemoglobin oxygen affinity to prevent sickling: abnormal haemoglobin variants as models.

Two abnormal high oxygen affinity haemoglobins have been studied for their ability to inhibit the polymerization of deoxy-Hb S. They were used as models to predict the effect of chemically modifying haemoglobin to increase oxygen affinity since the oxy(R)-conformation is the most potent inhibitor known of cell sickling. The participation of these variants in deoxy-Hb S polymers has been described quantitatively in terms of an exclusion coefficient, f, that relates their behaviour to that of deoxy-Hb S, and qualitatively in terms of an exponent of hybridization. Hb Bethesda was a potent inhibitor of polymerization, its behaviour being similar to that of Hb F studied previously. Hb Radcliffe demonstrated atypical behaviour, with hybrid molecules of the formula alpha 2 beta S beta Rad participating in the polymerization as effectively as Hb S, as has been shown for Hb C. These data have implications for the development of anti-sickling agents designed to increase oxygen affinity by covalent binding to Hb S.