Intersubunit circular permutation of human hemoglobin

The mechanism of formation, structure and physiological relevance of covalent hemoglobin attachment to the erythrocyte membrane

Covalent hemoglobin binding to membranes leads to band 3 (AE1) clustering and the removal of erythrocytes from the circulation; it is also implicated in blood storage lesions. Damaged hemoglobin, with the heme being in a redox and oxygen-binding inactive hemichrome form, has been implicated as the binding species. However, previous studies used strong non-physiological oxidants. In vivo hemoglobin is constantly being oxidised to methemoglobin (ferric), with around 1% of hemoglobin being in this form at any one time. In this study we tested the ability of the natural oxidised form of hemoglobin (methemoglobin) in the presence or absence of the physiological oxidant hydrogen peroxide to initiate membrane binding. The higher the oxidation state of hemoglobin (from Fe(III) to Fe(V)) the more binding was observed, with approximately 50% of this binding requiring reactive sulphydryl groups. The hemoglobin bound was in a high molecular weight complex containing spectrin, ankyrin and band 4.2, which are common to one of the cytoskeletal nodes. Unusually, we showed that hemoglobin bound in this way was redox active and capable of ligand binding. It can initiate lipid peroxidation showing the potential to cause cell damage. In vivo oxidative stress studies using extreme endurance exercise challenges showed an increase in hemoglobin membrane binding, especially in older cells with lower levels of antioxidant enzymes. These are then targeted for destruction. We propose a model where mild oxidative stress initiates the binding of redox active hemoglobin to the membrane. The maximum lifetime of the erythrocyte is thus governed by the redox activity of the cell; from the moment of its release into the circulation the timer is set.

Circular permutation directs orthogonal assembly in complex collagen peptide mixtures

Multiple types of natural collagens specifically assemble and co-exist in the extracellular matrix. Although noncollagenous trimerization domains facilitate the folding of triple-helical regions, it is intriguing to ask whether collagen sequences are also capable of controlling heterospecific association. In this study, we designed a model system mimicking simultaneous specific assembly of two collagen heterotrimers using a genetically inspired operation, circular permutation. Previously, surface charge-pair interactions were optimized on three collagen peptides to promote the formation of an abc-type heterotrimer. Circular permutation of these sequences retained networks of stabilizing interactions, preserving both triple-helical structure and heterospecificity of assembly. Combining original peptides A, B, and C and permuted peptides D, E, and F resulted primarily in formation of A:B:C and D:E:F, a heterospecificity of 2 of 56 possible stoichiometries. This degree of specificity in collagen molecular recognition is unprecedented in natural or synthetic collagens. Analysis of natural collagen sequences indicates low similarity between the neighboring exons. Combining the synthetic collagen model and bioinformatic analysis provides insight on how fibrillar collagens might have arisen from the duplication of smaller domains.

Polymer nanoreactors with dual functionality: simultaneous detoxification of peroxynitrite and oxygen transport

The design of multifunctional systems is in focus today as a key strategy for coping with complex challenges in various domains that include chemistry, medicine, environmental sciences, and technology. Herein, we introduce protein-containing polymer nanoreactors with dual functionality: peroxynitrite degradation and oxygen transport. Vesicles made of poly-(2-methyloxazoline)-poly(dimethylsiloxane)-poly(2-methyloxazoline) successfully encapsulated hemoglobin (Hb), which serves as a model protein because of its dual function in oxygen transport and peroxynitrite degradation. By inserting channel proteins, the polymer membranes of vesicles permitted passage of various compounds that served for the assessment of in situ Hb activity. The requisite conformational changes in the protein structure and the change in oxidation states that took place within the confined space of the vesicle cavity demonstrated that Hb preserved its dual functionality: peroxynitrite degradation and oxygen transport. The functionality of our nanoreactor, combined with its simple procedure of production and extensive stability over several months, supports it as a promising system for further medical applications.

Coexpression of human alpha- and circularly permuted beta-globins yields a hemoglobin with normal R state but modified T state properties

For the first time, a circularly permuted human beta-globin (cpbeta) has been coexpressed with human alpha-globin in bacterial cells and shown to associate to form alpha-cpbeta hemoglobin in solution. Flash photolysis studies of alpha-cpbeta show markedly biphasic CO and O(2) kinetics with the amplitudes for the fast association phases being dominant due the presence of large amounts of high-affinity liganded hemoglobin dimers. Extensive dimerization of liganded but not deoxygenated alpha-cpbeta was observed by gel chromatography. The rate constants for O(2) and CO binding to the R state forms of alpha-cpbeta are almost identical to those of native HbA (k'(R(CO)) approximately 5.0 microM(-1) s(-1); k'(R(O(2))) approximately 50 microM(-1) s(-1)), and the rate of O(2) dissociation from fully oxygenated alpha-cpbeta is also very similar to that observed for HbA (k(R(O(2))) approximately 21-28 s(-1)). When the equilibrium deoxyHb form of alpha-cpbeta is reacted with CO in rapid mixing experiments, the observed time courses are monophasic and the observed bimolecular association rate constant is approximately 1.0 microM(-1) s(-1), which is intermediate between the R state rate measured in partial photolysis experiments (approximately 5 microM(-1) s(-1)) and that observed for T state deoxyHbA (k'(T(CO)) approximately 0.1 to 0.2 microM(-1) s(-1)). Thus the deoxygenated permutated beta subunits generate an intermediate, higher affinity, deoxyHb quaternary state. This conclusion is supported by equilibrium oxygen binding measurements in which alpha-cpbeta exhibits a P(50) of approximately 1.5 mmHg and a low n-value (approximately 1.3) at pH 7, 20 degrees C, compared to 8.5 mmHg and n approximately 2.8 for native HbA under identical, dilute conditions.

The status of new anticoagulants

Currently available anticoagulants include heparin, low-molecular weight heparin, fondaparinux and warfarin. Despite advances with low-molecular weight heparin and fondaparinux, the currently available agents have limitations that have provided the impetus for the development of new drugs for prevention and treatment of both venous and arterial thromboembolism. Novel anticoagulants targeting specific steps in coagulation are in various stages of development. This paper reviews the pharmacology of these new agents and describes the results of clinical trials with new anticoagulants in more advanced stages of clinical testing.

New anticoagulants

The limitations of heparin and warfarin have prompted the development of new anticoagulant drugs for prevention and treatment of venous and arterial thromboembolism. Novel parenteral agents include synthetic analogs of the pentasaccharide sequence of heparin that mediates its interaction with antithrombin. Fondaparinux, the first synthetic pentasaccharide, is licensed for prevention of venous thromboembolism (VTE) after major orthopedic surgery and for initial treatment of patients with VTE. Idraparinux, a long-acting pentasaccharide that is administered subcutaneously once-weekly, is being compared with warfarin for treatment of VTE and for prevention of cardioembolic events in patients with atrial fibrillation. New oral anticoagulants include direct inhibitors of thrombin, factor Xa and factor IXa. Designed to provide more streamlined anticoagulation than warfarin, these agents can be given without routine coagulation monitoring. Ximelagatran, the first oral direct thrombin inhibitor, is as effective and safe as warfarin for prevention of cardioembolic events in patients with atrial fibrillation. However, ximelagatran produces a three-fold elevation in alanine transaminase levels in 7.9% of patients treated for more than a month, the long-term significance of which is uncertain. Whether other direct thrombin inhibitors or inhibitors of factors Xa or IXa also have this problem is under investigation. After a brief review of coagulation pathways, this paper focuses on new anticoagulants in advanced stages of clinical testing.

Conditioning Regimens Including High-Dose Busulfan Cause a High Incidence of Transplant-Related Mortality after Myeloablative Stem Cell Transplantation

BACKGROUND

Transplant-related mortality (TRM) significantly decreases the long-term survival of patients with hematopoietic diseases who undergo hematopoietic stem cell transplantation (HSCT).

METHODS

We retrospectively evaluated the risk factors for TRM of 58 consecutive patients with hematopoietic disease who underwent allogeneic myeloablative HSCT between April 1994 and June 2002.

RESULTS

Thirty-one patients died after HSCT, 16 of whom were diagnosed as TRM. The actuarial incidence of TRM was 57.6 % in patients who received conditioning chemotherapies including high-dose busulfan and 19.4 % in those who did not (p = 0.008). Multivariate Cox model analysis revealed that disease status, graft versus host disease (GVHD) grades III-IV and the use of high-dose busulfan were significant and independent risk factors for TRM.

CONCLUSION

These results suggest that high-dose busulfan as a conditioning therapy for allogeneic HSCT should be used cautiously, especially in patients with advanced disease.

Construction of a dual chain pseudotetrameric chicken avidin by combining two circularly permuted avidins

Two distinct circularly permuted forms of chicken avidin were designed with the aim of constructing a fusion avidin containing two biotin-binding sites in one polypeptide. The old N and C termini of wild-type avidin were connected to each other via a glycine/serine-rich linker, and the new termini were introduced into two different loops. This enabled the creation of the desired fusion construct using a short linker peptide between the two different circularly permuted subunits. The circularly permuted avidins (circularly permuted avidin 5 --> 4 and circularly permuted avidin 6 --> 5) and their fusion, pseudotetrameric dual chain avidin, were biologically active, i.e. showed biotin binding, and also displayed structural characteristics similar to those of wild-type avidin. Dual chain avidin facilitates the development of dual affinity avidins by allowing adjustment of the ligand-binding properties in half of the binding sites independent of the other half. In addition, the subunit fusion strategy described in this study can be used, where applicable, to modify oligomeric proteins in general.

AML1/RUNX1 increases during G1 to S cell cycle progression independent of cytokine-dependent phosphorylation and induces cyclin D3 gene expression

AML1/RUNX1, a member of the core binding factor (CBF) family stimulates myelopoiesis and lymphopoiesis by activating lineage-specific genes. In addition, AML1 induces S phase entry in 32Dcl3 myeloid or Ba/F3 lymphoid cells via transactivation. We now found that AML1 levels are regulated during the cell cycle. 32Dcl3 and Ba/F3 cell cycle fractions were prepared using elutriation. Western blotting and a gel shift/supershift assay demonstrated that endogenous CBF DNA binding and AML1 levels were increased 2-4-fold in S and G(2)/M phase cells compared with G(1) cells. In addition, G(1) arrest induced by mimosine reduced AML1 protein levels. In contrast, AML1 RNA did not vary during cell cycle progression relative to actin RNA. Analysis of exogenous Myc-AML1 or AML1-ER demonstrated a significant reduction in G(1) phase cells, whereas levels of exogenous DNA binding domain alone were constant, lending support to the conclusion that regulation of AML1 protein stability contributes to cell cycle variation in endogenous AML1. However, cytokine-dependent AML1 phosphorylation was independent of cell cycle phase, and an AML1 mutant lacking two ERK phosphorylation sites was still cell cycle-regulated. Inhibition of AML1 activity with the CBFbeta-SMMHC or AML1-ETO oncoproteins reduced cyclin D3 RNA expression, and AML1 bound and activated the cyclin D3 promoter. Signals stimulating G(1) to S cell cycle progression or entry into the cell cycle in immature hematopoietic cells might do so in part by inducing AML1 expression, and mutations altering pathways regulating variation in AML1 stability potentially contribute to leukemic transformation.

Emerging anticoagulant drugs

The limitations of traditional anticoagulants, heparin and warfarin, have prompted the development of new anticoagulant drugs for prevention and treatment of both venous and arterial thromboembolism. After a brief review of thrombogenesis and its regulation, this study focuses on new anticoagulant agents in more advanced stages of clinical testing.