Yoda1 and phosphatidylserine exposure in red cells from patients with sickle cell anaemia

Phosphatidylserine (PS) exposure is increased in red cells from sickle cell anaemia (SCA) patients. Externalised PS is prothrombotic and attractive to phagocytes and activated endothelial cells and thus contributes to the anaemic and ischaemic complications of SCA. The mechanism of PS exposure remains uncertain but it can follow increased intracellular Ca²⁺ concentration ([Ca²⁺]_i). Normally, [Ca²⁺]_i is maintained at very low levels but in sickle cells, Ca²⁺ permeability is increased, especially following deoxygenation and sickling, mediated by a pathway sometimes called P_sickle. The molecular identity of P_sickle is also unclear but recent work has implicated the mechanosensitive channel, PIEZO1. We used Yoda1, an PIEZO1 agonist, to investigate its role in sickle cells. Yoda1 caused an increase in [Ca²⁺]_i and PS exposure, which was inhibited by its antagonist Dooku1 and the PIEZO1 inhibitor GsMTx4, consistent with functional PIEZO1. However, PS exposure did not necessitate an increase in [Ca²⁺]_i. Two PKC inhibitors were also tested, chelerytherine chloride and calphostin C. Both reduced PS exposure whilst chelerytherine chloride also reduced Yoda1-induced increases in [Ca²⁺]_i. Findings are therefore consistent with the presence of PIEZO1 in sickle cells, able to mediate Ca²⁺ entry but that PKC was also involved in both Ca²⁺ entry and PS exposure.

Nanoscale adhesion profiling and membrane characterisation in sickle cell disease using hybrid atomic force microscopy-IR spectroscopy

Sickle cell disease (SCD) presents a significant global health problem. At present there is no effective treatment, with most being supportive for its associated complications such as the vaso-occlusive crises that result from increased cell adhesion. Hypoxic sickle cells have previously shown greater phosphatidylserine (PS) exposure and oxidative damage, as well as being notably "stickier" suggesting that increased cell cohesion and adhesion to the blood vessel endothelium is a possible mechanism for vaso-occlusion. The present work uses the hybrid technique of atomic force microscopy nano-infrared spectroscopy (AFM-IR) to probe changes to the coefficient of friction and C-O IR intensity in SCD on a nanoscale for dried red blood cells (RBCs) fixed under conditions of hypoxia and correlates these observations with adhesive interactions at the membrane. Using functionalised AFM tips, it has been possible to probe adhesive interactions between hydrophilic and hydrophobic moieties exposed at the surface of the dried RBCs fixed under different oxygenation states and for different cell genotypes. The results are consistent with greater PS-exposure and oxidative damage in hypoxic sickle cells, as previously proposed, and also show strong correlation between localised oxidative damage and increased adhesion. A mechanistic explanation involving significant lipid tail disruption as a result of oxidative action, in combination with differing concentrations of externalised PS lipids, is proposed to explain the observed adhesion behaviour of each type of cell.

Factor V Leiden and Thermolabile Methylenetetrahydrofolate Reductase in Extreme Old Age

Both factor V Leiden and the C677T methylenetetrahydrofolate reductase (MTHFR) gene mutation are associated with premature vascular disease, and yet are found at surprisingly high allele frequencies in European populations, 2.7% and 35% respectively. We have investigated the prevalence of these mutations in 87 UK residents over the age of ninety, to look for any evidence of their association with premature death.

Five factor V Leiden heterozygotes were found, giving an allele frequency of 2.9%, similar to that in the general UK population. The frequency of the thermolabile C677T MTHFR mutation was 36% with 11% homozygotes, again similar to that in the UK population; these genotypes are in Hardy-Weinberg equilibrium, suggesting that there is not strong selection against the homozygous state. One person was both heterozygous for factor V Leiden and homozygous for the C677T mutation. This study suggests that neither factor V Leiden nor thermolabile MTHFR are risk factors for premature death.

Effects of o-vanillin on K⁺ transport of red blood cells from patients with sickle cell disease

Aromatic aldehydes like o-vanillin were designed to reduce the complications of sickle cell disease (SCD) by interaction with HbS, to reduce polymerisation and RBC sickling. Present results show that o-vanillin also directly affects RBC membrane permeability. Both the K(+)-Cl(-) cotransporter (KCC) and the Ca(2+)-activated K(+) channel (or Gardos channel) were inhibited with IC50 of about 0.3 and 1 mM, respectively, with activities almost completely abolished by 5 mM. Similar effects were observed in RBCs treated with the thiol reacting reagent N-ethylmaleimide or with the Ca(2+) ionophore A23187, to circumvent any action via HbS polymerisation. The deoxygenation-induced cation conductance (sometimes termed P(sickle)) was partially inhibited, whilst deoxygenation-induced exposure of phosphatidylserine was completely abrogated. Na(+)/K(+) pump activity was also reduced. Notwithstanding, o-vanillin stimulated K(+) efflux through an unidentified pathway and resulted in reduction in cell volume (as measured by wet weight-dry weight). These actions are relevant to understanding how aromatic aldehydes may affect RBC membrane permeability per se as well as HbS polymerisation and thereby inform design of compounds most efficacious in ameliorating the complications of SCD.

Identification of the Ca²⁺ entry pathway involved in deoxygenation-induced phosphatidylserine exposure in red blood cells from patients with sickle cell disease

Phosphatidylserine (PS) exposure in red blood cells (RBCs) from sickle cell disease (SCD) patients is increased compared to levels in normal individuals and may participate in the anaemic and ischaemic complications of SCD. Exposure is increased by deoxygenation and occurs with elevation of intracellular Ca²⁺ to low micromolar levels. The Ca²⁺ entry step has not been defined but a role for the deoxygenation-induced pathway, P_sickle, is postulated. Partial P_sickle inhibitors 4-acetamido-4′-isothiocyanostilbene-2,2′-disulphonic acid (SITS), 4,4′-dithiocyano-2,2′-stilbene-disulphonic acid (DIDS) and dipyridamole inhibited deoxygenation-induced PS exposure (DIDS IC₅₀, 118 nM). Inhibitors and activators of other pathways (including these stimulated by depolarisation, benzodiazepines, glutamate and stretch) were without effect. Zn²⁺ and Gd³⁺ stimulated PS exposure to high levels. In the case of Zn²⁺, this effect was independent of oxygen (and hence HbS polymerisation and RBC sickling) but required extracellular Ca²⁺. The effect was completely abolished when Zn²⁺ (100 μM) was added to RBCs suspended in autologous plasma, implying a requirement of high levels of free Zn²⁺.

The clinical effectiveness and cost-effectiveness of primary stroke prevention in children with sickle cell disease: a systematic review and economic evaluation

BACKGROUND

Sickle cell disease (SCD) is a recessive genetic blood disorder, caused by a mutation in the β-globin gene. For children with SCD, the risk of stroke is estimated to be up to 250 times higher than in the general childhood population. Transcranial Doppler (TCD) ultrasonography is a non-invasive technique which measures local blood velocity in the proximal portions of large intracranial arteries. Screening with TCD ultrasonography identifies individuals with high cerebral blood velocity; these children are at the highest risk of stroke. A number of primary stroke prevention strategies are currently used in clinical practice in the UK including blood transfusion, treatment with hydroxycarbamide and bone marrow transplantation (BMT). No reviews have yet assessed the clinical effectiveness and cost effectiveness of primary stroke prevention strategies in children with SCD identified to be at high risk of stroke using TCD ultrasonography.

OBJECTIVE

To assess the clinical effectiveness and cost-effectiveness of primary stroke prevention treatments for children with SCD who are identified (using TCD ultrasonography) to be at high risk of stroke.

DATA SOURCES

Electronic databases were searched from inception up to May 2011, including the Cochrane Database of Systematic Reviews (CDSR), the Cochrane Central Register of Controlled Trials (CENTRAL), the Database of Abstracts of Reviews of Effects (DARE), EMBASE, the Health Technology Assessment (HTA) database, ISI Web of Science Proceedings, ISI Web of Science Citation Index, the NHS Economic Evaluation Database (NHS EED) and MEDLINE.

REVIEW METHODS

The assessment was conducted according to accepted procedures for conducting and reporting systematic reviews and economic evaluations. A de novo Markov model was developed to determine the cost-effectiveness of TCD ultrasonography and blood transfusion, where clinically appropriate, in patients with SCD.

RESULTS

Two randomised controlled trials met the inclusion criteria involving a study population of 209 participants. One compared blood transfusion with standard care for children who are identified as being at high risk of stroke using TCD ultrasonography. In this trial, one patient in the transfusion group had a stroke (1/63) compared with 11 children in the standard care group (11/67). The other trial assessed the impact of halting chronic transfusion in patients with SCD. Sixteen patients in the transfusion-halted group had an event (16/41) (two patients experienced stroke and 14 reverted to abnormal TCD velocity); there were no events in the continued-transfusion group (0/38). No meta-analyses of these trials were undertaken. No relevant economic evaluations were identified for inclusion in the review. The de novo modelling suggests that blood transfusions plus TCD scans (compared with just TCD scans) for patients with SCD at high risk of stroke, aged ≥ 2 years, may be good value for money. The intervention has an incremental cost-effectiveness ratio of £24,075 per quality-adjusted life-year gained, and helps avoid 68 strokes over the lifetime of a population of 1000 patients. The intervention costs an additional £13,751 per patient and generates 0.6 extra years of life in full health per patient. The data available for the economic analysis are limited. Sensitivity analyses and validation against existing data and expert opinion provide some reassurance that the conclusion of the model is reliable but further research is required to validate these findings.

LIMITATIONS

The main limitations relate to the availability of published clinical data; no completed randomised controlled trials were identified which evaluated the efficacy of either BMT or hydroxycarbamide for primary stroke prevention. Both the clinical and cost data available for use in the economic analysis are limited. Sensitivity analyses and validation against existing data and expert opinion provide some reassurance that the conclusions of the model are reliable, but further research is required to validate these findings.

CONCLUSIONS

The use of TCD ultrasonography to identify children at high risk of stroke, and treating these children with prophylactic blood transfusions, appears to be both clinically effective and cost-effective compared with TCD ultrasonography only. However, given the limitations in the data available, further research is required to verify this conclusion. Several research recommendations can be proposed from this review. Clinically, more research is needed to assess the effects and optimal duration of long-term blood transfusion and the potential role of hydroxycarbamide in primary stroke prevention. From an economics perspective, further research is required to generate more robust data on which to base estimates of cost-effectiveness or against which model outputs can be calibrated. More data are required to explain how utility weights vary with age, transfusions and strokes. Research is also needed around the cost of paediatric stroke in the UK.

STUDY REGISTRATION

PROSPERO CRD42011001496.

FUNDING

The National Institute for Health Research Health Technology Assessment programme.

A non-electrolyte haemolysis assay for diagnosis and prognosis of sickle cell disease

Red blood cells (RBCs) from patients with sickle cell disease (SCD) lyse in deoxygenated isosmotic non-electrolyte solutions. Haemolysis has features which suggest that it is linked to activation of the pathway termed P_sickle. This pathway is usually described as a non-specific cationic conductance activated by deoxygenation, HbS polymerisation and RBC sickling. The current work addresses the hypothesis that this haemolysis will provide a novel diagnostic and prognostic test for SCD, dependent on the altered properties of the RBC membrane resulting from HbS polymerisation. A simple test represented by this haemolysis assay would be useful especially in less affluent deprived areas of the world where SCD is most prevalent. RBCs from HbSS and most HbSC individuals showed progressive lysis in deoxygenated isosmotic sucrose solution at pH 7.4 to a level greater than that observed with RBCs from HbAS or HbAA individuals. Cytochalasin B prevented haemolysis. Haemolysis was temperature- and pH-dependent. It required near physiological temperatures to occur in deoxygenated sucrose solutions at pH 7.4. At pH 6, haemolysis occurred even in oxygenated samples. Haemolysis was reduced in patients on long-term (>5 months) hydroxyurea treatment. Several manoeuvres which stabilise soluble HbS (aromatic aldehydes o-vanillin or 5-hydroxymethyl, and urea) reduced haemolysis, an effect not due to increased oxygen affinity. Conditions designed to elicit HbS polymerisation in cells from sickle trait patients (deoxygenated hyperosmotic sucrose solutions at pH 6) supported their haemolysis. These findings are consistent with haemolysis requiring HbS polymerisation and support the hypothesis that this may be used as a test for SCD.

Acute intermittent porphyria: fatal complications of treatment

Acute neurovisceral attacks of porphyria can be life threatening. They are rare and notoriously difficult to diagnose clinically, but should be considered, particularly in female patients with unexplained abdominal pain, and associated neurological or psychiatric features or hyponatraemia. The diagnosis might be suggested by altered urine colour and can be confirmed by finding an elevated porphobilinogen concentration in fresh urine protected from light. Severe attacks require treatment with intravenous haem arginate and supportive management with safe drugs, including adequate analgesia. Intravenous glucose in water solutions are contraindicated as they aggravate hyponatraemia, which can prove fatal.

The conductance of red blood cells from sickle cell patients: ion selectivity and inhibitors

The abnormally high cation permeability in red blood cells (RBCs) from patients with sickle cell disease (SCD) occupies a central role in pathogenesis. Sickle RBC properties are notably heterogeneous, however, thus limiting conventional flux techniques that necessarily average out the behaviour of millions of cells. Here we use the whole-cell patch configuration to characterise the permeability of single RBCs from patients with SCD in more detail. A non-specific cation conductance was reversibly induced upon deoxygenation and was permeable to both univalent (Na+, K+, Rb+) and also divalent (Ca2+, Mg2+) cations. It was sensitive to the tarantula spider toxin GsMTx-4. Mn2+ caused partial, reversible inhibition. The aromatic aldehyde o-vanillin also irreversibly inhibited the deoxygenation-induced conductance, partially at 1mM and almost completely at 5mM. Nifedipine, amiloride and ethylisopropylamiloride were ineffective. In oxygenated RBCs, the current was pH sensitive showing a marked increase as pH fell from 7.4 to 6, with no change apparent when pH was raised from 7.4 to 8. The effects of acidification and deoxygenation together were not additive. Many features of this deoxygenation-induced conductance (non-specificity for cations, permeability toCa2+ andMg2+, pH sensitivity, reversibility, partial inhibition by DIDS and Mn2+) are shared with the flux pathway sometimes referred to as Psickle. Sensitivity to GsMTx-4 indicates its possible identity as a stretch-activated channel. Sensitivity to o-vanillin implies that activation requires HbS polymerisation but since the conductance was observed in whole-cell patches, results suggest that bulk intracellular Hb is not involved; rather a membrane-bound subfraction is responsible for channel activation. The ability to record P(sickle)-like activity in single RBCs will facilitate further studies and eventual molecular identification of the pathway involved.

Novel permeability characteristics of red blood cells from sickle cell patients heterozygous for HbS and HbC (HbSC genotype)

Individuals heterozygous for HbS and HbC (HbSC) represent about 1/3(rd) of sickle cell disease (SCD) patients. Whilst HbSC disease is generally milder, there is considerable overlap in symptoms with HbSS disease. HbSC patients, as well as HbSS ones, present with the chronic anaemia and panoply of acute vaso-occlusive complications that characterize SCD. However, there are important clinical and haematological differences. Certain complications occur with greater frequency in HbSC patients (like proliferative retinopathy and osteonecrosis) whilst intravascular haemolysis is reduced. Patients with HbSC disease can be considered as a discrete subset of SCD cases. Although much work has been carried out on understanding the pathogenesis of SCD in HbSS homozygotes, including the contribution of altered red blood cell permeability, relatively little pertains directly to HbSC individuals. Results reported in the literature suggest that HbSC cells, and particularly certain subpopulations, present with similar permeability to HbSS cells but there are also important differences - these have not been well characterized. We hypothesise that their unique cell transport properties accounts for the different pattern of disease in HbSC patients and represents a potential chemotherapeutic target not shared in red blood cells from HbSS patients. The distinct pattern of clinical haematology in HbSC disease is emphasised here. We analyse some of the electrophysiological properties of single red blood cells from HbSC patients, comparing them with those from HbSS patients and normal HbAA individuals. We also use the isosmotic haemolysis technique to investigate the behaviour of total red blood cell populations. Whilst both HbSS and HbSC cells show increased monovalent and divalent (Ca(2+)) cation conductance further elevated upon deoxygenation, the distribution of current magnitudes differs, and outward rectification is greatest for HbSC cells. In addition, although Gd(3+) largely abolishes the cation conductance of both HbSS and HbSC cells, only in HbSS ones are currents inhibited by the aminoglycosides like streptomycin. This distinction is retained in isosmotic lysis experiments where both HbSS and HbSC cells undergo haemolysis in sucrose solutions but streptomycin significantly inhibits lysis only in HbSS cells. These findings emphasise similarities but also differences in the permeability properties of HbSS and HbSC cells, which may be important in pathogenesis.

Age-related changes in adaptation to severe anemia in childhood in developing countries

Severe forms of anemia in children in the developing countries may be characterized by different clinical manifestations at particular stages of development. Whether this reflects developmental changes in adaptation to anemia or other mechanisms is not clear. The pattern of adaptation to anemia has been assessed in 110 individuals with hemoglobin (Hb) E beta-thalassemia, one of the commonest forms of inherited anemia in Asia. It has been found that age and Hb levels are independent variables with respect to erythropoietin response and that there is a decline in the latter at a similar degree of anemia during development. To determine whether this finding is applicable to anemia due to other causes, a similar study has been carried out on 279 children with severe anemia due to Plasmodium falciparum malaria; the results were similar to those in the patients with thalassemia. These observations may have important implications both for the better understanding of the pathophysiology of profound anemia in early life and for its more logical and cost-effective management.

An inward-facing conformation of a putative metal-chelate-type ABC transporter

The crystal structure of a putative metal-chelate-type adenosine triphosphate (ATP)-binding cassette (ABC) transporter encoded by genes HI1470 and HI1471 of Haemophilus influenzae has been solved at 2.4 angstrom resolution. The permeation pathway exhibits an inward-facing conformation, in contrast to the outward-facing state previously observed for the homologous vitamin B12 importer BtuCD. Although the structures of both HI1470/1 and BtuCD have been solved in nucleotide-free states, the pairs of ABC subunits in these two structures differ by a translational shift in the plane of the membrane that coincides with a repositioning of the membrane-spanning subunits. The differences observed between these ABC transporters involve relatively modest rearrangements and may serve as structural models for inward- and outward-facing conformations relevant to the alternating access mechanism of substrate translocation.

The genetic basis of the interaction between pyrimidine 5' nucleotidase I deficiency and hemoglobin E

We have previously described a family in which the interaction between pyrimidine 5' nucleotidase I (P5N-I) deficiency and hemoglobin E resulted in severe haemolytic anaemia. In this study we explored the genetic basis of the severe clinical phenotype and look for evidence of the interaction between these conditions. A P5N-I gene mutation (IVS8 + 1-2delGT) was found in the family, confirming that the severe phenotype results from the interaction between two genetic diseases.

Diagnosis of plasma cell leukaemia: findings of the UK NEQAS for Leucocyte Immunophenotyping scheme

The diagnosis of plasma cell leukaemia, a rare disorder with an aggressive clinical course and poor prognosis, is not always straightforward and may be dependent on the results of immunophenotyping. Samples from two cases of plasma cell leukaemia have been issued by the UK NEQAS for Leucocyte Immunophenotyping Scheme during the last 5 years and on each occasion a significant number of laboratories failed to make the correct diagnosis. The details of the two samples issued and the results of both surveys are presented. The data highlights the need to adhere to guidelines for immunophenotyping, with respect to using the correct antibody panels, the importance of data interpretation in conjunction with morphological appearance as well as the need to participate in external quality assurance schemes.

Bone transplantation

Bony deficiency, particularly loss of bone stock associated with failed joint replacements or tumours, is a challenging problem in orthopaedic surgery. Bone transplantation techniques provide solutions that can be tailored to the clinical problem. However, the risks of bone transplantation are well documented and the biology of allograft incorporation remains unpredictable and poorly understood.

Factor V Leiden mutation screened by PCR and detected with lanthanide-labeled probes

The Factor V Leiden mutation is an important human polymorphism, responsible for increased risk of venous thrombosis in heterozygotes as well as homozygotes. Therefore, screening is a useful possibility, and many detection systems have been described for PCR products. We have developed a simplified and robust assay using oligonucleotide probes for normal and mutant sequences, labeled with europium and samarium, respectively, and measured by time-resolved fluorescence. Populations consisting of 233 Welsh and 148 Irish subjects were examined by both restriction fragment length polymorphism (RFLP) analysis and our assay. The allele frequency was 14/466 in the Welsh and 5/296 in the Irish population, in line with other surveys of European populations. Results were not obtained in 2/381 samples by RFLP, compared with 1/381 with our method. We conclude that our method represents an improved system capable of considerable throughput at reasonable cost.

High-resolution structures of the oxidized and reduced states of cytochrome c554 from Nitrosomonas europaea

Cytochrome c554 (cyt c554) is a tetra-heme cytochrome involved in the oxidation of NH3 by Nitrosomonas europaea. The X-ray crystal structures of both the oxidized and dithionite-reduced states of cyt c554 in a new, rhombohedral crystal form have been solved by molecular replacement, at 1.6 A and 1.8 A resolution, respectively. Upon reduction, the conformation of the polypeptide chain changes between residues 175 and 179, which are adjacent to hemes III and IV. Cyt c554 displays conserved heme-packing motifs that are present in other heme-containing proteins. Comparisons to hydroxylamine oxidoreductase, the electron donor to cyt c554, and cytochrome c nitrite reductase, an enzyme involved in nitrite ammonification, reveal substantial structural similarity in the polypeptide chain surrounding the heme core environment. The structural determinants of these heme-packing motifs extend to the buried water molecules that hydrogen bond to the histidine ligands to the heme iron. In the original structure determination of a tetragonal crystal form, a cis peptide bond between His129 and Phe130 was identified that appeared to be stabilized by crystal contacts. In the rhombohedral crystal form used in the present high-resolution structure determination, this peptide bond adopts the trans conformation, but with disallowed angles of phi and psi.

Some thermodynamic implications for the thermostability of proteins

An analysis of the thermodynamics of protein stability reveals a general tendency for proteins that denature at higher temperatures to have greater free energies of maximal stability. To a reasonable approximation, the temperature of maximal stability for the set of globular, water-soluble proteins surveyed by Robertson and Murphy occurs at T* approximately 283K, independent of the heat denaturation temperature, T(m). This observation indicates, at least for these proteins, that thermostability tends to be achieved through elevation of the stability curve rather than by broadening or through a horizontal shift to higher temperatures. The relationship between the free energy of maximal stability and the temperature of heat denaturation is such that an increase in maximal stability of approximately 0.008 kJ/mole/residue is, on average, associated with a 1 degrees C increase in T(m). An estimate of the energetic consequences of thermal expansion suggests that these effects may contribute significantly to the destabilization of the native state of proteins with increasing temperature.

Genetic basis of hemolytic anemia caused by pyrimidine 5' nucleotidase deficiency

Pyrimidine 5' nucleotidase (P5'N-1) deficiency is an autosomal recessive condition causing hemolytic anemia characterized by marked basophilic stippling and the accumulation of high concentrations of pyrimidine nucleotides within the erythrocyte. It is implicated in the anemia of lead poisoning and is possibly associated with learning difficulties. Recently, a protein with P5'N-1 activity was analyzed and a provisional complementary DNA (cDNA) sequence published. This sequence was used to study 3 families with P5'N-1 deficiency. This approach generated a genomic DNA sequence that was used to search GenBank and identify the gene for P5'N-1. It is found on chromosome 7, consists of 10 exons with alternative splicing of exon 2, and produces proteins 286 and 297 amino acids long. Three homozygous mutations were identified in this gene in 4 subjects with P5'N-1 deficiency: codon 98 GAT-->GTT, Asp-->Val (linked to a silent polymorphism codon 92, TAC-->TAT), codon 177, CAA-->TAA, Gln-->termination, and IVS9-1, G-->T. The latter mutation results in the loss of exon 9 (201 bp) from the cDNA. None of these mutations was found in 100 normal controls. The DNA analysis was complicated by P5'N-1 pseudogenes found on chromosomes 4 and 7. This study is the first description of the structure and location of the P5'N-1 gene, and 3 mutations have been identified in affected patients from separate kindreds. (Blood. 2001;97:3327-3332)

Crystal structures of a novel ferric reductase from the hyperthermophilic archaeon Archaeoglobus fulgidus and its complex with NADP+

BACKGROUND

Studies performed within the last decade have indicated that microbial reduction of Fe(III) to Fe(II) is a biologically significant process. The ferric reductase (FeR) from Archaeoglobus fulgidus is the first reported archaeal ferric reductase and it catalyzes the flavin-mediated reduction of ferric iron complexes using NAD(P)H as the electron donor. Based on its catalytic activity, the A. fulgidus FeR resembles the bacterial and eukaryotic assimilatory type of ferric reductases. However, the high cellular abundance of the A. fulgidus FeR (approximately 0.75% of the total soluble protein) suggests a catabolic role for this enzyme as the terminal electron acceptor in a ferric iron-based respiratory pathway [1].

RESULTS

The crystal structure of recombinant A. fulgidus FeR containing a bound FMN has been solved at 1.5 A resolution by multiple isomorphous replacement/ anomalous diffraction (MIRAS) phasing methods, and the NADP+- bound complex of FeR was subsequently determined at 1.65 A resolution. FeR consists of a dimer of two identical subunits, although only one subunit has been observed to bind the redox cofactors. Each subunit is organized around a six-stranded antiparallel beta barrel that is homologous to the FMN binding protein from Desulfovibrio vulgaris. This fold has been shown to be related to a circularly permuted version of the flavin binding domain of the ferredoxin reductase superfamily. The A. fulgidus ferric reductase is further distinguished from the ferredoxin reductase superfamily by the absence of a Rossmann fold domain that is used to bind the NAD(P)H. Instead, FeR uses its single domain to provide both the flavin and the NAD(P)H binding sites. Potential binding sites for ferric iron complexes are identified near the cofactor binding sites.

CONCLUSIONS

The work described here details the structures of the enzyme-FMN, enzyme-FMN-NADP+, and possibly the enzyme-FMN-iron intermediates that are present during the reaction mechanism. This structural information helps identify roles for specific residues during the reduction of ferric iron complexes by the A. fulgidus FeR.

Crystal structure of the "cab"-type beta class carbonic anhydrase from the archaeon Methanobacterium thermoautotrophicum

The structure of the "cab"-type beta class carbonic anhydrase from the archaeon Methanobacterium thermoautotrophicum (Cab) has been determined to 2.1-A resolution using the multiwavelength anomalous diffraction phasing technique. Cab exists as a dimer with a subunit fold similar to that observed in "plant"-type beta class carbonic anhydrases. The active site zinc is coordinated by protein ligands Cys(32), His(87), and Cys(90), with the tetrahedral coordination completed by a water molecule. The major difference between plant- and cab-type beta class carbonic anhydrases is in the organization of the hydrophobic pocket. The structure reveals a Hepes buffer molecule bound 8 A away from the active site zinc, which suggests a possible proton transfer pathway from the active site to the solvent.

Crystal structure of the Acidaminococcus fermentans 2-hydroxyglutaryl-CoA dehydratase component A

Acidaminococcus fermentans degrades glutamate via the hydroxyglutarate pathway, which involves the syn-elimination of water from (R)-2-hydroxyglutaryl-CoA in a key reaction of the pathway. This anaerobic process is catalyzed by 2-hydroxyglutaryl-CoA dehydratase, an enzyme with two components (A and D) that reversibly associate during reaction cycles. Component A (CompA), a homodimeric protein of 2x27 kDa, contains a single, bridging [4Fe-4S] cluster and uses the hydrolysis of ATP to deliver an electron to the dehydratase component (CompD), where the electron is used catalytically. The structure of the extremely oxygen-sensitive CompA protein was solved by X-ray crystallography to 3 A resolution. The protein was found to be a member of the actin fold family, revealing a similar architecture and nucleotide-binding site. The key differences between CompA and other members of the actin fold family are: (i) the presence of a cluster binding segment, the "cluster helix"; (ii) the [4Fe-4S] cluster; and (iii) the location of the homodimer interface, which involves the bridging cluster. Possible reaction mechanisms are discussed in light of the close structural similarity to members of the actin-fold family and the functional similarity to the nitrogenase Fe- protein.

MgATP-Bound and nucleotide-free structures of a nitrogenase protein complex between the Leu 127 Delta-Fe-protein and the MoFe-protein

A mutant form of the nitrogenase iron protein with a deletion of residue Leu 127, located in the switch II region of the nucleotide binding site, forms a tight, inactive complex with the nitrogenase molybdenum iron (MoFe) protein in the absence of nucleotide. The structure of this complex generated with proteins from Azotobacter vinelandii (designated the L127Delta-Av2-Av1 complex) has been crystallographically determined in the absence of nucleotide at 2.2 A resolution and with bound MgATP (introduced by soaking) at 3.0 A resolution. As observed in the structure of the complex between the wild-type A. vinelandii nitrogenase proteins stabilized with ADP.AlF(4-), the most significant conformational changes in the L127Delta complex occur in the Fe-protein component. While the interactions at the interface between the MoFe-protein and Fe-proteins are conserved in the two complexes, significant differences are evident at the subunit-subunit interface of the dimeric Fe-proteins, with the L127Delta-Av2 structure having a more open conformation than the wild-type Av2 in the complex stabilized by ADP.AlF(4-). Addition of MgATP to the L127Delta-Av2-Av1 complex results in a further increase in the separation between Fe-protein subunits so that the structure more closely resembles that of the wild-type, nucleotide-free, uncomplexed Fe-protein, rather than the Fe-protein conformation in the ADP.AlF(4-) complex. The L127Delta mutation precludes key interactions between the Fe-protein and nucleotide, especially, but not exclusively, in the region corresponding to the switch II region of G-proteins, where the deletion constrains Gly 128 and Asp 129 from forming hydrogen bonds to the gamma-phosphate and activating water for attack on this group, respectively. These alterations account for the inability of this mutant to support mechanistically productive ATP hydrolysis. The ability of the L127Delta-Av2-Av1 complex to bind MgATP demonstrates that dissociation of the nitrogenase complex is not required for nucleotide binding.

Crystal structure of the all-ferrous [4Fe-4S]0 form of the nitrogenase iron protein from Azotobacter vinelandii

The structure of the nitrogenase iron protein from Azotobacter vinelandii in the all-ferrous [4Fe-4S](0) form has been determined to 2.25 A resolution by using the multiwavelength anomalous diffraction (MAD) phasing technique. The structure demonstrates that major conformational changes are not necessary either in the iron protein or in the cluster to accommodate cluster reduction to the [4Fe-4S](0) oxidation state. A survey of [4Fe-4S] clusters coordinated by four cysteine ligands in proteins of known structure reveals that the [4Fe-4S] cluster of the iron protein has the largest accessible surface area, suggesting that solvent exposure may be relevant to the ability of the iron protein to exist in three oxidation states.

Non-depolarizing neuromuscular blocking activity of bisquaternary amino di- and tripeptide derivatives

Herein we describe the synthesis of novel di- and tripeptide derivatives with two quaternary nitrogen groups attached and the biological testing of these compounds for neuromuscular blocking (NMB) activity in vitro and in vivo. The short peptide scaffold was selected because it offers potential for desired distance between the two pharmacophoric quaternary nitrogen groups, short duration of action, straightforward synthesis, and compatibility with an injectable formulation. From a small series of compounds 20c,e are identified as effective non-depolarizing NMB agents in vitro and in vivo in anesthetized cats and Rhesus monkeys with potencies similar to those of the clinical reference compounds rocuronium (4) and suxamethonium (2) (monkey ED(90) = 0.68, 0.23, 0.16, 5.04 micromol/kg, respectively). These new peptide derivatives 20c,e have similar potency and onset time but longer duration and slower recovery than the clinically used reference compounds. The structure-activity relationships described for this chemical series lead to the conclusion that the di- or tripeptide fragment can be regarded as an alternative template to the steroid or aliphatic ester of previously reported NMBs and within this tripeptide-derived series clog P correlates well with in vitro NMB activity.

Are aberrant BCR--ABL transcripts more common than previously thought?

We report the use of multiplex polymerase chain reaction (PCR), using 4% polyacrylamide gel electrophoresis (PAGE) for the detection of BCR-ABL transcripts in Philadelphia-positive disease. Three out of 50 cases [two out of 37 chronic myeloid leukaemia (CML), one out of 13 acute lymphoblastic leukaemia (ALL)] possessed rare breakpoints; an e19a2 and e13a3 in CML and an e1a3 in the ALL. We suggest that multiplex PCR using 4% PAGE and optimized for smaller transcript detection may lead to a higher detection rate of rare BCR-ABL breakpoints. Multiplex PCR, however, failed to distinguish e13a2 from e1a3 transcripts. Finally, the presence of e13a3 in CML supports the view that abl exon 2 sequences are unnecessary for the pathogenesis of 'classic' CML.

Hemoglobin F and hemoglobin E/beta-thalassemia

Hemoglobin (Hb) F levels are high and variable in Hb E/beta-thalassemia, ranging from 10% to 80%. The high levels are secondary to expansion of the erythron with ineffective erythropoiesis and selection in favor of cells able to make more gamma-globin. The variability in levels reflects the complex processes involved in Hb F production. Important determinants include age, alpha-thalassemia, and genetic determinants of gamma-chain synthesis. Overall, percentage of Hb F correlates with total hemoglobin levels and decreases steadily with age. alpha-thalassemia is associated with a lower percentage of Hb F levels but a higher total hemoglobin level in Hb E/beta-thalassemia.

Nitrogenase: standing at the crossroads

Nitrogenase catalyzes the ATP-dependent reduction of dinitrogen to ammonia, which is central to the process of biological nitrogen fixation. Recent progress towards establishing the mechanism of action of this complex metalloenzyme reflects the contributions of a combination of structural, biochemical, spectroscopic, synthetic and theoretical approaches to a challenging problem with implications for a range of biochemical and chemical systems.

FLT3 internal tandem duplication mutations in adult acute myeloid leukaemia define a high-risk group

Genomic DNA from 106 cases of adult de novo acute myeloid leukaemia (AML) was screened by polymerase chain reaction (PCR) and gel electrophoresis for FLT3 internal tandem duplication (ITD) mutations within the juxtamembrane (JM) domain. FLT3 mutations were detected in 14 cases (13.2%) and occurred in FAB types M1 (4 out of 14 cases), M3 (1 out of 10 cases), M4 (5 out of 37 cases) and M5 (4 out of 11 cases). Sequence analysis of four cases with abnormal PCR electrophoretic patterns revealed in frame duplications in the region of exon 11 of between 27 and 111 base pairs. Three are predicted to result in the tandem duplication of adjacent amino acid residues and one to result in a tandem duplication plus insertion of a novel amino acid motif. Statistical analysis showed the FLT3 mutations to be a strong prognostic factor, with patients lacking the mutation surviving significantly longer from diagnosis (mean 29.1 months) than those with an ITD (mean 12.8 months; P = 0.0002). Thirteen of the 14 patients with FLT3 mutations died within 18 months of diagnosis. FLT3 mutations were of prognostic significance in good risk disease (P = 0.04), as well as in patients with standard risk disease (P = 0.0096). This study demonstrates that the FLT3 ITD mutation occurs in a significant percentage of adult AML cases and is an important adverse prognostic factor that appears independent of conventional karyotypic findings.

Solid phase synthesis of tertiary amines on amide REM resins: Grignard and metal hydride compatible resins

Four new amide REM resins (AM REM 2-5) are described, and their use is illustrated for the synthesis of tertiary amines 6-9 and 13-16. Amide REM resins 4 and 5, which have a phenyl ring attached to the amide nitrogen, are found to give superior product yields and purities, and the resins are stable to a wider range of reagents and conditions compared to REM resin 1.

A closer look at the active site of gamma-class carbonic anhydrases: high-resolution crystallographic studies of the carbonic anhydrase from Methanosarcina thermophila

The prototype of the gamma-class of carbonic anhydrase has been characterized from the methanogenic archaeon Methanosarcina thermophila. Previously reported kinetic studies of the gamma-class carbonic anhydrase are consistent with this enzyme having a reaction mechanism similar to that of the mammalian alpha-class carbonic anhydrase. However, the overall folds of these two enzymes are dissimilar, and apart from the zinc-coordinating histidines, the active site residues bear little resemblance to one another. The crystal structures of zinc-containing and cobalt-substituted gamma-class carbonic anhydrases from M. thermophila are reported here between 1.46 and 1.95 A resolution in the unbound form and cocrystallized with either SO(4)(2)(-) or HCO(3)(-). Relative to the tetrahedral coordination geometry seen at the active site in the alpha-class of carbonic anhydrases, the active site of the gamma-class enzyme contains additional metal-bound water ligands, so the overall coordination geometry is trigonal bipyramidal for the zinc-containing enzyme and octahedral for the cobalt-substituted enzyme. Ligands bound to the active site all make contacts with the side chain of Glu 62 in manners that suggest the side chain is likely protonated. In the uncomplexed zinc-containing enzyme, the side chains of Glu 62 and Glu 84 appear to share a proton; additionally, Glu 84 exhibits multiple conformations. This suggests that Glu 84 may act as a proton shuttle, which is an important aspect of the reaction mechanism of alpha-class carbonic anhydrases. A hydrophobic pocket on the surface of the enzyme may participate in the trapping of CO(2) at the active site. On the basis of the coordination geometry at the active site, ligand binding modes, the behavior of the side chains of Glu 62 and Glu 84, and analogies to the well-characterized alpha-class of carbonic anhydrases, a more-defined reaction mechanism is proposed for the gamma-class of carbonic anhydrases.

Analyzing your complexes: structure of the quinol-fumarate reductase respiratory complex

The integral membrane protein complex quinol-fumarate reductase catalyzes the terminal step of a major anaerobic respiratory pathway. The homologous enzyme succinate-quinone oxidoreductase participates in aerobic respiration both as complex II and as a member of the Krebs cycle. Last year, two structures of quinol-fumarate reductases were reported. These structures revealed the cofactor organization linking the fumarate and quinol sites, and showed a cofactor arrangement across the membrane that is suggestive of a possible energy coupling function.

Structure of a thioredoxin-like [2Fe-2S] ferredoxin from Aquifex aeolicus

The 2.3 A resolution crystal structure of a [2Fe-2S] cluster containing ferredoxin from Aquifex aeolicus reveals a thioredoxin-like fold that is novel among iron-sulfur proteins. The [2Fe-2S] cluster is located near the surface of the protein, at a site corresponding to that of the active-site disulfide bridge in thioredoxin. The four cysteine ligands are located near the ends of two surface loops. Two of these ligands can be substituted by non-native cysteine residues introduced throughout a stretch of the polypeptide chain that forms a protruding loop extending away from the cluster. The presence of homologs of this ferredoxin as components of more complex anaerobic and aerobic electron transfer systems indicates that this is a versatile fold for biological redox processes.

Overexpression, purification, and crystallization of the membrane-bound fumarate reductase from Escherichia coli

Quinol-fumarate reductase (QFR) from Escherichia coli is a membrane-bound four-subunit respiratory protein that shares many physical and catalytic properties with succinate-quinone oxidoreductase (EC 1.3.99.1) commonly referred to as Complex II. The E. coli QFR has been overexpressed using plasmid vectors so that more than 50% of the cytoplasmic membrane fraction is composed of the four-subunit enzyme complex. The growth characteristics required for optimal levels of expression with minimal degradation by host cell proteases and oxidation factors were determined for the strains harboring the recombinant plasmid. The enzyme is extracted from the enriched membrane fraction using the nonionic detergent Thesit (polyoxyethylene(9)dodecyl ether) in a monodisperse form and then purified by a combination of anion-exchange, perfusion, and gel filtration chromatography. The purified enzyme is highly active and contains all types of redox cofactors expected to be associated with the enzyme. Crystallization screening of the purified QFR by vapor diffusion resulted in the formation of crystals within 24 h using a sodium citrate buffer and polyethylene glycol precipitant. The crystals contain the complete four-subunit QFR complex, diffract to 3.3 A resolution, and were found to be in space group P2(1)2(1)2(1) with unit cell dimensions a = 96.6 A, b = 138.1 A, and c = 275.3 A. The purification and crystallization procedures are highly reproducible and the general procedure may prove useful for Complex IIs from other sources.

Structures of the superoxide reductase from Pyrococcus furiosus in the oxidized and reduced states

Superoxide reductase (SOR) is a blue non-heme iron protein that functions in anaerobic microbes as a defense mechanism against reactive oxygen species by catalyzing the reduction of superoxide to hydrogen peroxide [Jenney, F. E., Jr., Verhagen, M. F. J. M., Cui, X. , and Adams, M. W. W. (1999) Science 286, 306-309]. Crystal structures of SOR from the hyperthermophilic archaeon Pyrococcus furiosus have been determined in the oxidized and reduced forms to resolutions of 1.7 and 2.0 A, respectively. SOR forms a homotetramer, with each subunit adopting an immunoglobulin-like beta-barrel fold that coordinates a mononuclear, non-heme iron center. The protein fold and metal center are similar to those observed previously for the homologous protein desulfoferrodoxin from Desulfovibrio desulfuricans [Coelho, A. V., Matias, P., Fülöp, V., Thompson, A., Gonzalez, A., and Carrondo, M. A. (1997) J. Bioinorg. Chem. 2, 680-689]. Each iron is coordinated to imidazole nitrogens of four histidines in a planar arrangement, with a cysteine ligand occupying an axial position normal to this plane. In two of the subunits of the oxidized structure, a glutamate carboxylate serves as the sixth ligand to form an overall six-coordinate, octahedral coordinate environment. In the remaining two subunits, the sixth coordination site is either vacant or occupied by solvent molecules. The iron centers in all four subunits of the reduced structure exhibit pentacoordination. The structures of the oxidized and reduced forms of SOR suggest a mechanism by which superoxide accessibility may be controlled and define a possible binding site for rubredoxin, the likely physiological electron donor to SOR.

Conformational flexibility of B-DNA at 0.74 A resolution: d(CCAGTACTGG)(2)

The affinity and specificity of a ligand for its DNA site is a function of the conformational changes between the isolated and complexed states. Although the structures of a hydroxypyrrole-imidazole-pyrrole polyamide dimer with 5'-CCAGTACTGG-3' and the trp repressor recognizing the sequence 5'-GTACT-3' are known, the baseline conformation of the DNA site would contribute to our understanding of DNA recognition by these ligands. The 0.74 A resolution structure of a B-DNA double helix, 5'-CCAGTACTGG-3', has been determined by X-ray crystallography. Six of the nine phosphates, two of four bound calcium ions and networks of water molecules hydrating the oligonucleotide have alternate conformations. By contrast, nine of the ten bases have a single, unique conformation with hydrogen atoms visible in most cases. The polyamide molecules alter the geometry of the phosphodiester backbone, and the water molecules mediating contacts in the trp repressor/operator complex are conserved in the unliganded DNA. Furthermore, the multiple conformational states, ions and hydration revealed by this ultrahigh resolution structure of a B-form oligonucleotide are potentially general considerations for understanding DNA-binding affinity and specificity by ligands.

Structure of a photoactive rhodium complex intercalated into DNA

Intercalating complexes of rhodium(III) are strong photo-oxidants that promote DNA strand cleavage or electron transfer through the double helix. The 1.2 A resolution crystal structure of a sequence-specific rhodium intercalator bound to a DNA helix provides a rationale for the sequence specificity of rhodium intercalators. It also explains how intercalation in the center of an oligonucleotide modifies DNA conformation. The rhodium complex intercalates via the major groove where specific contacts are formed with the edges of the bases at the target site. The phi ligand is deeply inserted into the DNA base pair stack. The primary conformational change of the DNA is a doubling of the rise per residue, with no change in sugar pucker from B-form DNA. Based upon the five crystallographically independent views of an intercalated DNA helix observed in this structure, the intercalator may be considered as an additional base pair with specific functional groups positioned in the major groove.

Structural effects of DNA sequence on T.A recognition by hydroxypyrrole/pyrrole pairs in the minor groove

Synthetic polyamides composed of three types of aromatic amino acids, N-methylimidazole (Im), N-methylpyrrole (Py) and N-methyl-3-hydroxypyrrole (Hp) bind specific DNA sequences as antiparallel dimers in the minor groove. The side-by-side pairings of aromatic rings in the dimer afford a general recognition code that allows all four base-pairs to be distinguished. To examine the structural consequences of changing the DNA sequence context on T.A recognition by Hp/Py pairs in the minor groove, crystal structures of polyamide dimers (ImPyHpPy)(2) and the pyrrole counterpart (ImPyPyPy)(2) bound to the six base-pair target site 5'-AGATCT-3' in a ten base-pair oligonucleotide have been determined to a resolution of 2.27 and 2.15 A, respectively. The structures demonstrate that the principles of Hp/Py recognition of T.A are consistent between different sequence contexts. However, a general structural explanation for the non-additive reduction in binding affinity due to introduction of the hydroxyl group is less clear. Comparison with other polyamide-DNA cocrystal structures reveals structural themes and differences that may relate to sequence preference.