Phthalides serve as potent modulators to boost fetal hemoglobin induction therapy for β-hemoglobinopathies

Fetal hemoglobin (HbF) induction therapy has become the most promising strategy for treating β-hemoglobinopathies, including sickle-cell diseases and β-thalassemia. However, subtle but critical structural difference exists between HbF and normal adult hemoglobin (HbA), which inevitably leads to reduced binding of the endogenous modulator 2,3-bisphosphoglycerate (2,3-BPG) to HbF and thus increased oxygen affinity and decreased oxygen transport efficiency of HbF. We combined the oxygen equilibrium experiments, resonance Raman (RR) spectroscopy, and molecular docking modeling, and we discuss 2 phthalides, z-butylidenephthalide and z-ligustilide, that can effectively lower the oxygen affinity of HbF. They adjust it to a level closer to that of HbA and make it a more satisfactory oxygen carrier for adults. From the oxygen equilibrium curve measurements, we show that the 2 phthalides are more effective than 2,3-BPG for modulating HbF. The RR spectra show that phthalides allosterically stabilize the oxygenated HbF in the low oxygen affinity conformation, and the molecular docking modeling reveals that the 2 chosen phthalides interact with HbF via the cleft around the γ1/γ2 interface with a binding strength ∼1.6 times stronger than that of 2,3-BPG. We discuss the implications of z-butylidenephthalide and z-ligustilide in boosting the efficacy of HbF induction therapy to mitigate the clinical severities of β-hemoglobinopathies.

Fetal blood sampling in prenatal diagnosis of thalassemia at late pregnancy

Fetal blood sampling is a procedure that involves the drawing of a blood sample from the umbilical vein of the umbilical cord, which can be performed after 18 weeks gestation. Fetal blood sampling is a preferable method for prenatal diagnosis of thalassemia in second trimester or late pregnancy. Additionally, it is suggested to be performed in cases in which mosaicisms are identified by amniocentesis or chorionic villus sampling (CVS), areas where DNA analysis is not available, and when mutations of the parents are not known. Laboratory steps regarding prenatal diagnosis by fetal blood sampling were summarized, including the ensuring of fetal origin, determination of red blood cell parameters, fetal hemoglobin analysis, and finally fetal DNA analysis. The objective of this review is to present an overview of procedures in terms of benefits, laboratory interpretations, and some limitations.

Glutathionylated γG and γA subunits of hemoglobin F: a novel post-translational modification found in extremely premature infants by LC-MS and nanoLC-MS/MS

Oxidative stress plays an important role in the development of various disease processes and is a putative mechanism in the development of bronchopulmonary dysplasia, the most common complication of extreme preterm birth. Glutathione, a major endogenous antioxidant and redox buffer, also mediates cellular functions through protein thiolation. We sought to determine if post-translational thiol modification of hemoglobin F occurs in neonates by examining erythrocyte samples obtained during the first month of life from premature infants, born at 23 0/7 - 28 6/7 weeks gestational age, who were enrolled at our center in the Prematurity and Respiratory Outcomes Program (PROP). Using liquid chromatography-mass spectrometry (LC-MS), we report the novel finding of in vivo and in vitro glutathionylation of γG and γA subunits of Hgb F. Through tandem mass spectrometry (nanoLC-MS/MS), we confirmed the adduction site as the Cys-γ94 residue and through high-resolution mass spectrometry determined that the modification occurs in both γ subunits. We also identified glutathionylation of the β subunit of Hgb A in our patient samples; we did not find modified α subunits of Hgb A or F. In conclusion, we are the first to report that glutathionylation of γG and γA of Hgb F occurs in premature infants. Additional studies of this post-translational modification are needed to determine its physiologic impact on Hgb F function and if sG-Hgb is a biomarker for clinical morbidities associated with oxidative stress in premature infants.

Jaundice and alpha gene triplication in beta-thalassemia: Association or causation?

There are few studies investigating alpha globin gene triplications in beta-thalassemia in Asian Indians and its effect on phenotype, which was the primary aim of this study. Gap-PCR was performed in order to detect common alpha thalassemia determinants (-alpha(3.7), -alpha(4.2) and alpha alpha alpha(anti 3.7) triplication). Alpha-triplication was detected in 15.4% (10/65) of patients with thalassemia intermedia, 8.8% (4/45) of those with thalassemia minor and in 2.7% (2/74) of healthy controls. The severity of jaundice was higher in thalassemia intermedia cases with alpha-triplication and two of the alpha-triplication cases had a marked increase in serum bilirubin following intercurrent illness. Thus, alpha globin gene triplication is important genetic determinant underlying thalassemia intermedia in North Indians. Patients with alpha-triplication may develop prominent jaundice with marked increase in serum bilirubin following antecedent aggravating factors.

Functional properties of the newly observed (G)γ-chain fetal hemoglobin variant Hb F-Monserrato-Sassari (HBG2:c.280T>C) or [(G)γ93 (F9) Cys→Arg]

BACKGROUND

HbF-Monserrato-Sassari is a newly discovered abnormal fetal hemoglobin observed in an apparently normal newborn baby during a hemoglobinopathies survey at birth in North Sardinian population.

METHODS

Electrophoretic analysis of the cord blood lysate evidenced for an abnormal tetramer due to a mutated fetal globin chain. Electrospray ionisation-mass spectrometry and gene sequencing were used to identify the mutation. Oxygen binding ability of the variant Hb was determined.

RESULTS

Sequencing of the γ globin genes revealed the TGT→CGT transition at codon 93 in one of the two (G)γ genes, which leads to the Arg for Cys amino acid replacement at position 9 of the F α-helix. The amino acid substitution was confirmed by mass spectrometric analysis of the globin chains. Since modifications or substitutions at position β93 are known to affect the arrangement of a salt bridge at the α1β2 sliding contacts that are crucial for subunit cooperativity, the functional properties of the variant were studied to evaluate the effect of the replacement at the same position in the γ globin chain. With respect to normal HbF, the variant showed a significant increase in oxygen affinity and a slight decrease of both Bohr effect and cooperativity.

GENERAL SIGNIFICANCE

Result indicates a key role of the Cys γ93 residue for subunit cooperativity in the T→R transition of the HbF tetramer. Substitutions at the F9 position of the (G)γ globin may result in stabilization of the high affinity R-state of the Hb tetramer. Because of the loss of Cys γ93 residue, this variant is considered to be potentially compromised in nitric oxide transport.

3-years experience review of neonatal screening for hemoglobin disorders using tandem mass spectrometry

BACKGROUND

Neonatal screening programs for sickle cell disease are common in North America and in some European countries. Isoelectric Focusing or High Performance Liquid Chromatography is the main technique used for hemoglobin variant detection.

METHODS

Since tandem mass spectrometry is being used for screening of inherited metabolic disorders and allows protein identification, we had developed an application to identify the most relevant hemoglobin mutations with this technology.

RESULTS

This approach had been previously validated and has been routinely applied in our laboratory for the last three years. We report here our experience with this new method in the field, applied to our East-Belgian population.

CONCLUSIONS

To conclude, mass spectrometry provides an efficient alternative approach for laboratories performing neonatal screening of hemoglobin disorders.

Accurate Measurements of Fetal Hemoglobin for Neonates with Different Gestational Ages

The purposes of this study were to examine the accuracy of fetal hemoglobin (Hb F, alpha2gamma2) as quickly measured by a hemoximeter but verified by high performance liquid chromatography [HPLC, including Hb F total (Hb Ft), acetylated Hb F (Hb F1), and non acetylated Hb F ( Hb F*)], and to predict the Hb F levels for different gestational weeks of neonates. Thirty-nine neonates of predominantly Hispanic and African American ethnicity, with gestational ages ranging from 25 to 38 weeks, were investigated. Analyses were performed on 163 blood samples that were pure neonates' blood before the transfusion of any adult blood. Two neonates had increased Hb C [beta6(A3)Glu-->Lys, GAC-->AAG] levels (1.67-2.79%) and one neonate whose mother drank alcohol during pregnancy, had elevated Hb A2 levels (0.12-0.14%). After excluding these data points, the mean Hb F were overestimated by hemoximeter, 118.4 +/- 8.77% vs. 92.6 +/- 2.77% by HPLC (mean difference: 25.8 +/- 7.71%, p = <0.001). Mean Hb F1 was 10.5 +/- 2.28%. Hb F levels decreased as gestational age increased (p <0.001 for Hb Ft and Hb F*; p = <0.05 for Hb F1). A multivariate regression model for Hb F prediction was established with the best R2. The gestational age and post birth hours in the prediction of Hb Ft was included when Hb F could be determined at the clinical settings. Future studies may be needed to account for Hb F1 when measuring Hb F levels to assess oxygenation status in (pre term) neonates.

Hemoglobin Kenya composed of alpha- and ((A)gammabeta)-fusion-globin chains, associated with hereditary persistence of fetal hemoglobin

Hb Kenya is made up of two normal alpha-globin chains and two (A)gammabeta-fusion globin chains. The latter are the product of an (A)gammabeta-hybrid globin gene formed as a result of misalignment during meiosis and nonhomologous crossing over. It is associated with a deletion of 22.7 kb including the delta-globin gene, between the (A)gamma- and beta-globin genes. Hb Kenya is found in Kenyans and Ugandans. Heterozygotes have moderately increased Hb F, and this mutation has been known as an ((A)gammabeta)(+) hereditary persistence of fetal hemoglobin (HPFH). Compound heterozygotes for Hb Kenya/Hb S are thought to be asymptomatic, but reports of long term follow-up of these patients are lacking. The correct identification of Hb Kenya is sometimes problematic. In cation exchange high performance liquid chromatography, Hb Kenya elutes in similar position as Hb A(2), Hb Lepore, Hb E, and several other variant hemoglobins. Definitive diagnosis that is necessary for proper patient management is best done by DNA-based gap-PCR tests.

Human embryonic, fetal, and adult hemoglobins have different subunit interface strengths. Correlation with lifespan in the red cell

The different types of naturally occurring, normal human hemoglobins vary in their tetramer-dimer subunit interface strengths (stabilities) by three orders of magnitude in the liganded (CO or oxy) state. The presence of embryonic zeta-subunits leads to an average 20-fold weakening of tetramer-dimer interfaces compared to corresponding hemoglobins containing adult alpha-subunits. The dimer-monomer interfaces of these hemoglobins differ by at least 500-fold in their strengths; such interfaces are weak if they contain zeta-subunits and exchange with added beta-subunits in the form of beta(4) (HbH) significantly faster than do those with alpha-subunits. Subunit exchange occurs at the level of the dimer, although tetramer formation reciprocally influences the amount of dimer available for exchange. Competition between subunit types occurs so that pairs of weak embryonic hemoglobins can exchange subunits to form the stronger fetal and adult hemoglobins. The dimer strengths increase in the order Hb Portland-2 (zeta(2)beta(2)) < Hb Portland-1 (zeta(2)gamma(2)) approximately equal Hb Gower-1 (zeta(2)epsilon(2)) < Hb Gower-2 (alpha(2)epsilon(2)) < HbF(1) < HbF (alpha(2)gamma(2)) < HbA(2) (alpha(2)delta(2)), i.e., from embryonic to fetal to adult types, representing maturation from weaker to stronger monomer-monomer subunit contacts. This increasing order recapitulates the developmental order in which globins are expressed (embryonic --> fetal --> adult), suggesting that the intrinsic binding properties of the subunits themselves regarding the strengths of interfaces they form with competing subunits play an important role in the dynamics of protein assemblies and networks.

Molecular Crowding Limits the Role of Fetal Hemoglobin in Therapy for Sickle Cell Disease

The dominant assumption central to most treatments for sickle cell anemia has been that replacement of sickle hemoglobin (HbS) by fetal hemoglobin (HbF) would have major clinical benefit. Using laser photolysis, we have measured polymerization kinetics including rates of homogeneous and heterogeneous nucleation on mixtures of 20% and 30% HbF with HbS. We find that the present model for polymerization, including molecular crowding, can accurately predict the rates of such mixtures, by using the single assumption that no significant amount of HbF enters the polymer. The effects of replacing HbS by HbF on the rates of polymer formation are found to be significantly lower than previous measurements appeared to indicate because the impact of the replacement is also highly dependent on the total hemoglobin concentration. This is because the molecular crowding of non-polymerizing HbF offsets substantially the effects of decreasing the concentration of HbS concentration, an effect that increases with concentration. Most strikingly, the demonstrated benefit of hydroxyurea therapy in slowing the kinetics of intracellular polymerization cannot be primarily due to enhanced HbF, but must have some other origin, which could itself represent a promising therapeutic approach.

From the Arctic to fetal life: physiological importance and structural basis of an 'additional' chloride-binding site in haemoglobin

Haemoglobins from mammals of sub-Arctic and Arctic species, as well as fetal human Hb, are all characterized by a significantly lower Delta H of oxygenation compared with the majority of mammalian haemoglobins from temperate species (exceptions are represented by some cold-resistant species, such as cow, horse and pig). This has been interpreted as an adaptive mechanism of great importance from a physiological point of view. To date, the molecular basis of this thermodynamic characteristic is still not known. In the present study, we show that binding of extra chloride (with respect to adult human Hb) ions to Hb would significantly contribute to lowering the overall heat of oxygenation, thus providing a molecular basis for the low effect of temperature on the oxygenation-deoxygenation cycle. To this aim, the oxygen binding properties of bovine Hb, bear (Ursus arctos) Hb and horse Hb, which are representative of this series of haemoglobins, have been studied with special regard to the effect of heterotropic ligands, such as organic phosphates (namely 2,3-diphosphoglycerate) and chloride. Functional results are consistent with a mechanism for ligand binding that involves an additional binding site for chloride ion. Analysis of computational chemistry results, obtained by the GRID program, further confirm the hypothesis that the reason for the lower Delta H of oxygenation is mainly due to an increase in the number of the oxygen-linked chloride-binding sites.

Quantitative trait locus on chromosome 8q influences the switch from fetal to adult hemoglobin

The switch from fetal to adult hemoglobin is incomplete; the residual fetal hemoglobin in adults is restricted to a subset of erythrocytes called F cells. F-cell levels are influenced by a sequence variant (C → T) at position -158 upstream of the γ-globin gene, termed the XmnI-Gγ polymorphism. How the Gγ-158 C → T variant influences the expression of the Gγ-globin gene is unknown but is likely to involve the interaction of a multiprotein transcription complex. In a recent genome-wide linkage study of a large Asian Indian kindred, a genetic interaction between the XmnI-Gγ site and a locus on chromosome 8q was reported to influence adult F-cell levels. We report the replication of linkage to chromosome 8q in a sample of European twin pairs. This result provides strong evidence that a quantitative trait locus exists on chromosome 8q that influences the developmental switch from fetal to adult hemoglobin. (Blood. 2004;104:2184-2186)

Three new structural variants of fetal hemoglobin: Hb F-Campinas [Agamma g121(GH4)Glu --> Gln], Hb F-Paulinia [Ggamma 80(EF4)Asp --> Tyr] and Hb F-Joanopolis [Ggamma73(E17) Asp -->Ala]

Three new structural variants of fetal hemoglobin were detected in newborns during a neonatal screening for Hb S in the southeast of Brazil: Hb F-Campinas [AgammaI121 (GH4)Glu --> Gln], Hb F-Paulinia [Ggamma80(EF4)Asp -->Tyr] and Hb F-Joanopolis [Ggamma73(E17) Asp -->Ala]. These variants were not related to clinical abnormalities

Significance of beta116 His (G18) at alpha1beta1 contact sites for alphabeta assembly and autoxidation of hemoglobin

The role of heterotetramer interaction sites in assembly and autoxidation of hemoglobin is not clear. The importance of beta(116His) (G-18) and gamma(116Ile) at one of the alpha1beta1 or alpha1gamma1 interaction sites for homo-dimer formation and assembly in vitro of beta and gamma chains, respectively, with alpha chains to form human Hb A and Hb F was assessed using recombinant beta(116His)(-->)(Asp), beta(116His)(-->)(Ile), and beta(112Cys)(-->)(Thr,116His)(-->)(Ile) chains. Even though beta chains (e.g., 116 His) are in monomer/tetramer equilibrium, beta(116Asp) chains showed only monomer formation. In contrast, beta(116Ile) and beta(112Thr,116Ile) chains showed homodimer and homotetramer formation like gamma-globin chains which contain 116 Ile. Assembly rates in vitro of beta(116Ile) or beta(112Thr,116Ile) chains with alpha chains were 340-fold slower, while beta(116Asp) chains promoted assembly compared to normal beta-globin chains. These results indicate that amino acid hydrophobicity at the G-18 position in non-alpha chains plays a key role in homotetramer, dimer, and monomer formation, which in turn plays a critical role in assembly with alpha chains to form Hb A and Hb F. These results also suggest that stable dimer formation of gamma-globin chains must not occur in vivo, since this would inhibit association with alpha chains to form Hb F. The role of beta(116His) (G-18) in heterotetramer-induced stabilization of the bond with oxygen in hemoglobin was also assessed by evaluating autoxidation rates using recombinant Hb tetramers containing these variant globin chains. Autoxidation rates of alpha(2)beta(2)(116Asp) and alpha(2)beta(2)(116Ile) tetramers showed biphasic kinetics with the faster rate due to alpha chain oxidation and the slower to the beta chain variants whose rates were 1.5-fold faster than that of normal beta-globin chains. In addition, NMR spectra of the heme area of these two hemoglobin variant tetramers showed similar resonance peaks, which are different from those of Hb A. Oxygen-binding properties of alpha(2)beta(2)(116His)(-->)(Asp) and alpha(2)beta(2)(116His)(-->)(Ile), however, showed slight alteration compared to Hb A. These results suggest that the beta116 amino acid (G18) plays a critical role in not only stabilizing alpha1beta1 interactions but also in inhibiting hemoglobin oxidation. However, stabilization of the bonds between oxygen and heme may not be dependent on stabilization of alpha1beta1 interactions. Tertiary structural changes may lead to changes in the heme region in beta chains after assembly with alpha chains, which could influence stability of dioxygen binding of beta chains.

Two new mutations in cis on (G)gamma chain of fetal hemoglobin: Hb F-Madrid [G gamma 50(D1)Ser-->Cys] and [G gamma 75(E19)Ile-->Thr]

We describe a new structural hemoglobin variant of (G)gamma with two amino acid replacements in cis found in the umbilical cord blood of a neonate in Madrid, Spain. The substitutions were identified on exon 2 of the (G)gamma globin gene, at codon 50 (T CT-->T GT) and at codon 75 (A TA-->A CA). We have named it Hb F-Madrid. The father of the propositus was the carrier of the same (G)gamma chain variant and, moreover, molecular study of alpha genes revealed the loss of an alpha gene (-alpha(3.7)/alpha alpha) both in the propositus and his mother.

The cooperativity of human fetal and adult hemoglobins is optimized: a consideration based on the effectiveness of the Bohr shift

The physiological significance of the cooperativity of human hemoglobin (Hb) is considered from the viewpoint of the effectiveness of the Bohr shift at the sites of O(2) release and uptake across the placental membrane. The effects of the Bohr shift was examined by changing the O(2) saturation of Hb (S(pO2)) per unit change in P(50), -dS(PO2)/d P(50), where P(50) is partial pressure of O(2) at half saturation. The Bohr shift at the sites of O(2) uptake and release was found to be highly effective in both fetal and maternal bloods at physiological degree of cooperativity (Hill's coefficient, n=2.65). From the results obtained in this paper, it is concluded that the positions of OECs of fetal and maternal Hbs are regulated to receive a maximal benefit from the Bohr shift, and that a relatively low n value of human tetrameric Hb is adequate for the O(2) and CO(2) exchange across the placental membrane.

Assembly of human hemoglobin (Hb) beta- and gamma-globin chains expressed in a cell-free system with alpha-globin chains to form Hb A and Hb F

Rates of in vitro synthesis of radiolabeled gamma and beta chains made in a cell-free transcription/translation system were similar, but expressed globin chains were unstable. The addition of unlabeled beta or gamma chains at the start of chain synthesis generated radiolabeled beta(4) or gamma(2) and gamma(4) chains, respectively. If unlabeled alpha-globin chains were added at the start of chain synthesis, then approximately equal amounts of radiolabeled alphabeta or alphagamma bands were generated. If unlabeled Hb A or Hb F was added to reactions containing radiolabeled alphabeta or alphagamma prior to electrophoresis, then radiolabeled Hb A or Hb F tetramers, respectively, were generated. If alpha chains were added after synthesis of radiolabeled gamma chains made in the presence of unlabeled gamma chains, then little radiolabeled alphagamma formed. In contrast, if alpha chains were added after synthesis of radiolabeled beta chains made in the presence of unlabeled beta chains, then radiolabeled alpha(2)beta(2) formed. These findings suggest that beta and gamma chains associate with alpha chains during or soon after translation. This would prevent the formation of unstable monomers as well as stable gamma(2) dimers and suggests that alpha chains may bind to nascent non-alpha chains, acting as folding catalysts to promote functional tetrameric hemoglobin formation in vivo.

Hemoglobin equilibrium analysis by the multiangle laser light-scattering method

Dimer-tetramer and monomer-dimer-tetramer equilibria of tetrameric hemoglobins and their single chains in the CO form, respectively, were evaluated using the microbatch multiangle light-scattering (MALS) analysis system. The molecular weights of human Hb A and Hb F in the CO form were dependent on concentration. The dissociation constants to dimers of Hb A and Hb F were 2.58 x 10(-6) and 0.66 x 10(-6), respectively. Equilibration of single globin chains, including alpha, beta, and gamma chains, was also evaluated by the same method. The dissociation constants of alpha-chain dimers to monomers, of beta-chain tetramers to monomers, and of gamma-chain tetramers to dimers were 14 x 10(-6), 25 x 10(-17), and 6.86 x 10(-6) M, respectively. These results indicate that the MALS analysis system can not only determine molecular weight but also characterize protein-protein interactions of multi-subunit proteins.

N-terminal contributions of the gamma-subunit of fetal hemoglobin to its tetramer strength: remote effects at subunit contacts

The greatly increased tetramer strength of liganded fetal hemoglobin compared with adult hemoglobin is shown by its 70-fold smaller tetramer-dimer dissociation constant. This property has been shown previously to be only partially caused by the 5-amino-acid differences at both types of interfaces in each hemoglobin. A major contributor to tetramer strengthening is the 18-amino-acid N-terminal A helix of the gamma-subunit of fetal hemoglobin, which differs from the beta-subunit of adult hemoglobin at eight amino acid residues. This long-distance communication between the A helix and the distant C helix and FG helical corner comprising the subunit contacts at the allosteric interface represents internal signaling. Physiologically, its greater tetramer strength endows fetal hemoglobin with the capacity to abstract oxygen from maternal adult hemoglobin. It also leads to resistance of fetal red cells to the malaria parasite because the HbF tetramer does not dissociate to dimers as readily as HbA; dimers are digested by malaria proteases but tetramers are not. In this communication, we report which sites on the A helix of the gamma-subunit are important for tetramer strengthening in HbF by substituting certain amino acids in the beta-subunit by the corresponding residues in the gamma-subunit. The recombinant hemoglobins containing up to five replacements together have been extensively characterized. Mass values were within 1 unit of theory. Gly 1 (gamma) of HbF with its high pK(a) of 8.1 compared with a 7.1 value for Val 1 (beta) of HbA creates a highly electropositive N terminus that may couple with the electronegative sequence just after it on the gamma-subunit. The Leu 3 to Phe replacement has no apparent role; however, position 5 is important because replacement of Pro 5 (beta) by Glu 5 (gamma) promotes tetramer strengthening. The Glu --> Asp replacement at position 7 enhances this effect because of the lower pK(a) of Asp but the Val --> Ile substitution at position 11 has no effect. Thus, the three positive/negative sites at positions 1, 5, and 7 account for practically all of the tetramer strength of HbF, as illustrated by an electrostatic surface potential analysis. The pathway by which information is transmitted to the distant allosteric subunit interfaces is currently under study. Oxygen-binding properties of the hemoglobins with charged substitutions more closely resemble those of HbA rather than those of HbF. Thus, whereas the A helix has a major role in controlling the strength of interactions at the tetramer-dimer allosteric interface, oxygen-binding properties of HbA and HbF are influenced by sequences in the C helix and at the FG helical corner constituting the allosteric interface.

Haem disorder in recombinant- and reticulocyte-derived haemoglobins: evidence for stereoselective haem insertion in eukaryotes

We have used NMR spectroscopy to measure haem disorder in adult human haemoglobin (HbA) obtained from mature erythrocyte cells and from yeast expressing recombinant HbA. Reticulocyte-derived HbA contained much higher levels of haem disorder (11% alpha- and 28% beta-subunit disorder) than observed for HbA from mature erythrocytes (1.5% alpha- and 8% beta-subunit disorder). Thus, unlike in vitro combination of haem and apoHb, biosynthetic haem insertion is not random with respect to orientation, but appears to show stereoselectivity. Recombinant HbA isolated from yeast showed 32% alpha- and 45% beta-subunit haem disorder. These levels relaxed to their equilibrium positions after incubating the Hb in the ferric form. Recombinant embryonic human Hbs showed less haem disorder than recombinant HbA. The levels of haem disorder in embryonic Hbs zeta(2)epsilon(2) and zeta(2)gamma(2) appear to have their equilibrium values. We propose that, in eukaryotes, in vivo haem insertion occurs via both co-translational mechanisms and insertion via semiHb-beta.

The acetylation state of human fetal hemoglobin modulates the strength of its subunit interactions: long-range effects and implications for histone interactions in the nucleosome

The source of the 70-fold increased tetramer strength of liganded fetal hemoglobin relative to that of adult hemoglobin between pH 6.0 and 7.5 reported earlier [Dumoulin et al. (1997) J. Biol. Chem. 272, 31326] has been identified as the N-terminal Gly residue of the gamma-chain, which is replaced by Val in adult hemoglobin. This was revealed by extending the study of the pH dependence of the tetramer-dimer equilibrium of these hemoglobins into the alkaline range as far as pH 9. From pH 7.5 to 9.0, the 70-fold difference in the association equilibrium constant between hemoglobins F and A lessened progressively. This behavior was attributed to the difference in the pK(a) 8.1 of Gly-1(gamma) compared to the pK(a) 7.1 value of Val-1(beta) of hemoglobins F and A, respectively. Evidence for this conclusion was obtained by demonstrating that natural hemoglobin F(1), which is specifically acetylated at Gly-1(gamma) and hence unable to be protonated, behaves like HbA and not HbF in its tetramer-dimer association properties over the pH range studied. An increased degree of protonation of the gamma-chain N-terminus of hemoglobin F from pH 9.0 to 8.0 is therefore suggested as responsible for its increased tetramer strength representing an example of transmission of a signal from its positively charged N-terminal tail to the distant subunit allosteric interface where the equilibrium constant is measured. An analogy is made between the effects of acetylation of the fetal hemoglobin tetramer on the strength of its subunit interactions and acetylation of some internal Lys residues within the N-terminal segments of the histone octamer around which DNA is wrapped in the nucleosome.

HbF in the adult: could its composition discriminate normal from abnormal foetal globin gene expression?

The relative proportions of the two gamma chain species (G gamma and A gamma) have been reinvestigated in newborns, during the physiological switch from foetal to adult haemoglobin, and in adults with some persistent expression of HbF. In newborns, with about 80% HbF, the G gamma percentage was close to 70% while in adult RBC, with less than 0.5% HbF, the G gamma chain was almost non-detectable and may reflect the completion of the foetal to adult switch. Conversely, in adult patients with HbF above 0.6%, usually accompanying some degree of marrow stimulation, the relative ratio of G gamma varied between 40 and 60%, independently of HbF level. This ratio corresponds to what has been described in the literature as being the adult type of HbF. In all the cases where we found higher levels of G gamma, the results could be explained by the presence of a specific genetic background such as the Senegalese haplotype in sickle cell disease.

The ferric form of the three human embryonic hemoglobins and their reactions with azide ions

The ferric forms of the three human embryonic hemoglobins exhibit pH titration's with pKa values in the range 8.25-8.6, which correlate with the reduced proteins affinity for oxygen. Azide ions bind to each ferric protein in a process consisting of two separate binding steps. The equilibrium constants for this process are in the 10-30 microM range and can be assigned to the individual chain within the proteins as reactivity appears independent of the nature of the partner chain. The kinetics of the azide binding reactions occur on the second time scale at mM ligand concentrations and also indicate the presence of two distinct processes that have been assigned to each of the two chains within the proteins. Non of these processes indicate any evidence of heme-heme interactions within the ferric form of the proteins. These findings are discussed in comparison with previously reported properties of adult ferric hemoglobin.

Fetal and embryonic hemoglobins in erythroblasts of chromosomally normal and abnormal fetuses at 10-40 weeks of gestation

BACKGROUND AND OBJECTIVES

During fetal development a change in erythropoiesis from hepatic to medullary site occurs. In chromosomally abnormal fetuses this change is delayed. Hemoglobin production also undergoes developmental switches from embryonic to fetal hemoglobins in the first trimester of pregnancy. The aim of study was to determine the proportion of embryonic and fetal hemoglobins in fetal erythroblasts of chromosomally normal and abnormal fetuses at 10-40 weeks of gestation.

DESIGN AND METHODS

Fetal blood was obtained from 93 chromosomally normal and 19 abnormal fetuses at 10-40 weeks of gestation. Fetal erythroblasts were isolated by triple density gradient centrifugation and magnetic cell sorting with CD71 antibody. Fluorescent antibodies were used to immuno-stain for zeta (zeta), epsilon (epsilon) and gamma (gamma) hemoglobin chains.

RESULTS

The percentages of the positively stained cells were calculated. In chromosomally normal fetuses the percentage of erythroblasts expressing the zeta chain was 25% at 10 weeks but this decreased exponentially with gestation to less than 1% by 17 weeks. Similarly, the percentage of cells expressing the epsilon chain decreased from 97% at 10 weeks to less than 1% by 25 weeks. In contrast, expression of the gamma chain increased from about 30% at 10 weeks to 90% by 16 weeks and decreased thereafter to 60% at 40 weeks. In the abnormal fetuses, the percentage of erythroblasts expressing the zeta chain and the epsilon chain decreased to less than 1% by 23 and 28 weeks respectively, while maximum expression of the gamma chain was at about 22 weeks.

INTERPRETATION AND CONCLUSIONS

In the chromosomally abnormal group the pattern of change in the expression of the various hemoglobin chains during gestation was similar to that in the normal fetuses but was delayed by three to six weeks. These findings suggest that in fetuses with chromosomal abnormalities there is a developmental delay in the switch from embryonic to fetal hemoglobin chains.

Functional effects of replacing human alpha- and beta-globins with their embryonic globin homologues in defined haemoglobin heterotetramers

Embryonic- and adult-stage globin subunits assemble into haemoglobin (Hb) heterotetramers that are expressed at low levels throughout human intrauterine development. These haemoglobins differ from adult Hb A (alpha2beta2) by the substitution of embryonic zeta for adult alpha globin (Hb zeta2beta2), or embryonic epsilon for adult beta globin (Hb alpha2epsilon2). Several key physiological properties of these 'semiembryonic' haemoglobins remain undefined, as ethical and methodological considerations have limited their availability from both human sources and conventional expression systems. The current study attempts to estimate how the physiological properties of semiembryonic and adult haemoglobins may differ, by determining whether the O2-binding characteristics of hybrid human/mouse haemoglobins change when human alpha- or beta-globin subunits are replaced by human embryonic zeta- or epsilon-globin subunits respectively. Each of the four human globins is expressed in transgenic mice that are nullizygous for either the endogenous mouse alpha- or beta-globin genes, resulting in the high-level expression of haemoglobins that can be studied either in situ in intact erythrocytes or in vitro. We showed that the exchange of human zeta-globin for human alpha-globin chains increased haemoglobin O2 affinity, both in the presence and in the absence of 2, 3-bisphosphoglycerate (2,3-BPG), and reduced the pH-dependent shift in its oxygen equilibrium curve (Bohr effect). By comparison, hybrid haemoglobins containing either human epsilon-globin or human beta-globin exhibited nearly identical O2-binding properties, both in situ and in vitro, regardless of 2,3-BPG levels or ambient pH. Neither the zeta-for-alpha nor the epsilon-for-beta substitutions substantially altered binding affinity for 2,3-BPG or cooperativity between globin subunits. These studies suggest that semiembryonic haemoglobins that assemble entirely from human subunits may exhibit properties that are similar to those of human Hb A.

Assembly of gamma- with alpha-globin chains to form human fetal hemoglobin in vitro and in vivo

Soluble gamma-globin chains were expressed in bacteria and purified to assess the mechanism of gamma- and alpha-chain assembly to form Hb F. Formation of Hb F in vitro following incubation of equimolar mixtures of gamma and alpha chains was about 4 x 10(5)-fold slower than assembly of alpha and beta chains to form Hb A in vitro. Results of assembly for gamma(116Ile-->His) and gamma(112Thr-->Asp) chains with alpha chains were similar to that of beta chains, whereas assembly of gamma(112Thr-->Cys) and alpha chains was similar to wild type gamma chains, indicating that amino acid differences at alpha1beta1 and alpha1gamma1 interaction sites between gamma116 Ile and beta116 His are responsible for the different assembly rates in vitro in the formation of Hb F and Hb A. Homoassembly in vitro of individual gamma chains as assessed by size-exclusion chromatography shows that gamma and gamma(112Thr-->Cys) chains form stable dimers like alphabeta and alphagamma that do not dissociate readily into monomers like beta chains. In contrast, gamma(116Ile-->His) chains form monomers and dimers upon dilution. These results are consistent with the slower assembly rate in vitro of gamma and gamma(112Thr-->Cys) with alpha chains, whereas the faster rate of assembly of gamma(116Ile-->His) and gamma(112Thr-->Asp) chains with alpha chains, like beta chains, may be caused by dissociation to monomers. These results suggest that dissociation of gamma(2) dimers to monomers limits formation of Hb F in vitro. However, yields of soluble Hb F expressed in bacteria were similar to Hb A, and no unassembled alpha and gamma chains were detected. These results indicate that gamma chains assemble in vivo with alpha chains prior to forming stable gamma(2) dimers, possibly binding to alpha chains as partially folded nascent gamma-globin chains prior to release from polyribosomes.

Transposing sequences between fetal and adult hemoglobins indicates which subunits and regulatory molecule interfaces are functionally related

To correlate amino acid sequence changes with hemoglobin function we are carrying out a detailed recombinant analysis of the adult hemoglobin/fetal hemoglobin (HbA/HbF) systems. The important physiological differences between these two tetramers lie at unspecified sites in the 39 sequence substitutions of the 146 amino acids in their beta and gamma chains. In this paper, significant differences in the tetramer-dimer dissociation constants (referred to as tetramer "strength" or "stability") of adult (HbA) and fetal (HbF) hemoglobin tetramers have been used to probe the relationship between the allosteric, sliding interface and the effects of the allosteric regulator, 2,3-DPG, in promoting oxygen release. The single amino acid difference at the allosteric interfaces of these two hemoglobins, Glu-43(beta) --> Asp-43(gamma), which is not near the DPG binding site, leads to a significantly lower DPG response, approaching that of HbF. The results are inconsistent with the long-held idea that the replacement of His-143(beta) in HbA to Ser-143(gamma) in HbF is solely responsible for the lowered DPG response in HbF. On the other hand, the Val-1(beta) --> Gly-1(gamma) replacement near the DPG binding site has no effect on the DPG response. The replacement of His-116(beta) by the hydrophobic Ile-116(gamma) at the rigid alpha(1)beta(1) interface has a marginal yet detectable effect on the allosteric alpha(1)beta(2) interface. The results, overall, are interpreted using a model involving electrostatic coupling between certain side chains and extend the concept of a long-range relationship between some distant regions of the tetramer that are likely mediated through the central cavity.

Two new Ggamma chain variants: Hb F-clamart [gamma17(A14)Lys-->Asn] and Hb F-Ouled Rabah [gamma19(B1)Asn-->Lys]

Two new fetal hemoglobin variants affecting the Ggamma chain are reported. Hb F-Clamart was found during investigation of a French newborn who presented with a mild microcytemia. The second variant was found during neonatal screening for hemoglobinopathies of 30,000 babies from a population-at-risk living in the Paris region. It was named Hb F-Ouled Rabah because its structural modification and ethnic distribution is similar to that of Hb D-Ouled Rabah [beta19(B1)Asn-->Lys]. Hb F-Ouled Rabah is clinically silent and occurs at a frequency of ca. 0.1% in newborns originating from Maghreb. Structural characterization of both variants was done by protein chemistry methods, including amino acids analysis and mass spectrometry.

A new, electrophoretically silent, fetal hemoglobin variant: Hb F-Calabria [Ggamma118(GH1)Phe-->Leu]

Hb F-Calabria [Ggamma118(GH1)Phe-->Leu] is a new fetal hemoglobin variant that was found during routine screening for abnormal hemoglobins in a newborn of Calabrian (Southern Italy) ancestry. The variant chain was identified (acid urea gel electrophoresis of dissociated globin chains in the presence of Triton X-100, and by reversed phase high performance liquid chromatography) as a slightly hydrophilic Ggamma chain. Sequencing of the polymerase chain reaction-amplified exon 3 of the Ggamma-globin gene demonstrated the TTC-->CTC mutation at codon 118 leading to the Phe-->Leu conservative substitution at position GH1. A molecular modeling study supports that the variant might not have clinical implications. This is the 40th example of a Ggamma chain variant.

The role of amino acid alpha38 in the control of oxygen binding to human adult and embryonic haemoglobin Portland

The role of the amino acid at position alpha(38) in haemoglobin has been probed using site-directed mutagenesis. When the Thr residue at position alpha(38) (which is totally conserved in all mammals) is changed to a Gln, the equilibrium properties of the protein are significantly altered. Equilibrium and kinetic data show that the R-state properties of the protein are essentially unaffected by the mutation whilst the allosteric equilibrium and T-state properties are changed. Mutation of the naturally occurring Gln(38) of the human embryonic haemoglobin zeta-chain (the only known non-Thr containing globin) to a Thr residue shows the converse change in properties produced by the adult mutation, although in this case the situation is complicated by significant chain heterogeneity in the T state. An extension of the two-state model of co-operativity is presented to describe quantitatively the equilibrium ligand binding in the presence of T-state chain heterogeneity. A molecular model is described in which the putative interaction of alphaGln(38) and betaTyr(145) is identified which make a significant contribution to the previously reported unusual ligand-binding properties of the zeta-chain containing human embryonic haemoglobins.

Assessment of roles of surface histidyl residues in the molecular basis of the Bohr effect and of beta 143 histidine in the binding of 2,3-bisphosphoglycerate in human normal adult hemoglobin

Site-directed mutagenesis has been used to construct two mutant recombinant hemoglobins (rHbs), rHb(betaH116Q) and rHb(betaH143S). Purified rHbs were used to assign the C2 proton resonances of beta116His and beta143His and to resolve the ambiguous assignments made over the past years. In the present work, we have identified the C2 proton resonances of two surface histidyl residues of the beta chain, beta116His and beta143His, in both the carbonmonoxy and deoxy forms, by comparing the proton nuclear magnetic resonance (NMR) spectra of human normal adult hemoglobin (Hb A) with those of rHbs. Current assignments plus other previous assignments complete the assignments for all 24 surface histidyl residues of human normal adult hemoglobin. The individual pK values of 24 histidyl residues of Hb A were also measured in deuterium oxide (D(2)O) in 0.1 M N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (HEPES) buffer in the presence of 0.1 M chloride at 29 degrees C by monitoring the shifts of the C2 proton resonances of the histidyl residues as a function of pH. Among those surface histidyl residues, beta146His has the biggest contribution to the alkaline Bohr effect (63% at pH 7.4), and beta143His has the biggest contribution to the acid Bohr effect (71% at pH 5.1). alpha20His, alpha112His, and beta117His have essentially no contribution; alpha50His, alpha72His, alpha89His, beta97His, and beta116His have moderate positive contributions; and beta2His and beta77His have a moderate negative contribution to the Bohr effect. The sum of the contributions from 24 surface histidyl residues accounted for 86% of the alkaline Bohr effect at pH 7.4 and about 55% of the acid Bohr effect at pH 5.1. Although beta143His is located in the binding site for 2,3-bisphosphoglycerate (2,3-BPG) according to the crystal structure of deoxy-Hb A complexed with 2, 3-BPG, beta143His is not essential for the binding of 2,3-BPG in the neutral pH range according to the proton NMR and oxygen affinity studies presented here. With the accurately measured and assigned individual pK values for all surface histidyl residues, it is now possible to evaluate the Bohr effect microscopically for novel recombinant Hbs with important functional properties, such as low oxygen affinity and high cooperativity. The present study further confirms the importance of a global electrostatic network in regulating the Bohr effect of the hemoglobin molecule.

A biochemical and biophysical characterization of recombinant mutants of fetal hemoglobin and their interaction with sickle cell hemoglobin

Three recombinant mutants of human fetal hemoglobin (Hb F) have been constructed to determine what effects specific amino acid residues in the gamma chain have on the biophysical and biochemical properties of the native protein molecule. Target residues in these recombinant fetal hemoglobins were replaced with the corresponding amino acids in the beta chain of human normal adult hemoglobin (Hb A). The recombinant mutants of Hb F included rHb F (gamma 112Thr --> Cys), rHb F (gamma 130Trp --> Tyr), and rHb F (gamma 112Thr --> Cys/gamma 130Trp --> Tyr). Specifically, the importance of gamma 112Thr and gamma 130Trp to the stability of Hb F against alkaline denaturation and in the interaction with sickle cell hemoglobin (Hb S) was investigated. Contrary to expectations, these rHbs were found to be as stable against alkaline denaturation as Hb F, suggesting that the amino acid residues mentioned above are not responsible for the stability of Hb F against the alkaline denaturation as compared to that of Hb A. Sub-zero isoelectric focusing (IEF) was employed to investigate the extent of hybrid formation in equilibrium mixtures of Hb S with these hemoglobins and with several other hemoglobins in the carbon monoxy form. Equimolar mixtures of Hb A and Hb S and of Hb A(2) and Hb S indicate that 48-49% of the Hb exists as the hybrid tetramer, which is in agreement with the expected binomial distribution. Similar mixtures of Hb F and Hb S contain only 44% hybrid tetramer. The results for two of our recombinant mutants of Hb F were identical to the results for mixtures of Hb F and Hb S, while the other mutant, rHb F (gamma 130Trp --> Tyr), produced 42% hybrid tetramer. The sub-zero IEF technique discussed here is more convenient than room-temperature IEF techniques, which require Hb mixtures in the deoxy state. These recombinant mutants of Hb F were further characterized by equilibrium oxygen binding studies, which indicated no significant differences from Hb F. While these mutants of Hb F did not have tetramer-dimer dissociation properties significantly altered from those of Hb F, future mutants of Hb F may yet prove useful to the development of a gene therapy for the treatment of patients with sickle cell anemia.

Metal complexes as allosteric effectors of human hemoglobin: an NMR study of the interaction of the gadolinium(III) bis(m-boroxyphenylamide)diethylenetriaminepentaacetic acid complex with human oxygenated and deoxygenated hemoglobin

The boronic functionalities on the outer surface of the Gd(III) bis(m-boroxyphenylamide)DTPA complex (Gd(III)L) enable it to bind to fructosamine residues of oxygenated glycated human adult hemoglobin. The formation of the macromolecular adduct can be assessed by NMR spectroscopy via observation of the enhancement of the solvent water proton relaxation rate. Unexpectedly, a strong binding interaction was also observed for the oxygenated unglycated human adult hemoglobin, eventually displaying a much higher relaxation enhancement. From relaxation rate measurements it was found that two Gd(III)L complexes interact with one hemoglobin tetramer (KD = 1.0 x 10(-5) M and 4.6 x 10(-4) M, respectively), whereas no interaction has been observed with monomeric hemoproteins. A markedly higher affinity of the Gd(III)L complex has been observed for oxygenated and aquo-met human adult hemoglobin derivatives with respect to the corresponding deoxy derivative. Upon binding, a net change in the quaternary structure of hemoglobin has been assessed by monitoring the changes in the high-resolution 1H-NMR spectrum of the protein as well as in the Soret absorption band. On the basis of these observations and the 11B NMR results obtained with the diamagnetic La(III)L complex, we suggest that the interaction between the lanthanide complex and deoxygenated, oxygenated, and aquo-met derivatives of human adult hemoglobin takes place at the 2, 3-diphosphoglycerate (DPG) binding site, through the formation of N-->B coordinative bonds at His143beta and His2beta residues of different beta-chains. The stronger binding to the oxygenated form is then responsible for a shift of the allosteric equilibrium toward the high-affinity R-state. Accordingly, Gd(III)L affinity for oxygenated human fetal hemoglobin (lacking His143beta) is significantly lower than that observed for the unglycated human adult tetramer.

The N-terminal sequence affects distant helix interactions in hemoglobin. Implications for mutant proteins from studies on recombinant hemoglobin felix

The N-terminal 18-amino acid sequence of the beta-chain of hemoglobin, as far as the end of the A helix, has been replaced by the corresponding sequence of the gamma-chain of fetal hemoglobin with the remaining sequence of the beta-chain retained (helices B through H). The gamma-beta-chain had the correct mass, and its entire sequence was established by mass spectrometric analysis of its tryptic peptides; the alpha-chain also had the correct mass. This recombinant hemoglobin (named Hb Felix) retains cooperativity and has an oxygen affinity like that of HbA both in the presence and absence of the allosteric regulators, 2,3-diphosphoglycerate or chloride but differs from HbF in its 2,3-diphosphoglycerate response. However, Hb Felix has some features that resemble fetal hemoglobin, i.e. its significantly decreased tetramer-dimer dissociation and its circular dichroism spectrum, which measure the strength of the tetramer-dimer interface in the oxy conformation and its rearrangement to the deoxy conformation, respectively. Even though Hb Felix contains the HbA amino acids at its tetramer-dimer interface, which is located at a distance from the substitution sites, its interface properties resemble those of HbF. Therefore, the N-terminal sequence and not just those amino acids directly involved at the subunit interface contacts with alpha-chains must have a strong influence on this region of the molecule. The results reinforce the concept of fluid long range relationships among various parts of the hemoglobin tetramer (Dumoulin, A., Manning, L. R., Jenkins, W. T., Winslow, R. M., and Manning, J. M. (1997) J. Biol. Chem. 272, 31326-31332) and demonstrate the importance of the N-terminal sequence, especially in some mutant hemoglobins, in influencing its overall structure by affecting the relationship between helices.

Transgenic mice expressing human fetal globin are protected from malaria by a novel mechanism

Studies in vitro by Pasvol et al (Nature, 270:171, 1977) have indicated that the growth of Plasmodium falciparum in cells containing fetal hemoglobin (HbF = alpha2gamma2) is retarded, but invasion is increased, at least in newborn cells. Normal neonates switch from about 80% HbF at birth to a few percent at the end of the first year of life. Carriers of beta-thalassemia trait exhibit a delay in the normal HbF switch-off, which might partially explain the protection observed in populations with this gene. To study this hypothesis in vivo, we used transgenic (gamma) mice expressing human Agamma and Ggamma chains resulting in 40% to 60% alpha2Mgamma2 hemoglobin, infected with rodent malaria. Two species of rodent malaria were studied. P chabaudi adami causes a nonlethal infection, mainly in mature red blood cells (RBC). P yoelii 17XNL is a nonlethal infection, invading primarily reticulocytes, whereas P yoelii 17XL is a lethal variant of P yoelii 17XNL and causes death of mice in approximately 1 to 2 weeks. Data indicate that this strain may cause a syndrome resembling cerebral malaria caused by P falciparum (Am J Trop Med Hyg, 50:512, 1994). In gamma transgenic mice infected with P chabaudi adami, the parasitemia rose more quickly (in agreement with Pasvol) than in control mice, but was cleared more rapidly. In mice infected with P yoelii 17XNL, a clear reduction in parasitemia was observed. Interestingly, splenectomy before this infection, did not reverse protection. The most striking effect was in lethal P yoelii 17XL infection. Control mice died between 11 to 13 days, whereas gamma mice cleared the infection by day 22 and survived, a phenomenon also observed in splenectomized animals. These results suggest that HbF does indeed have a protective effect in vivo, which is not mediated by the spleen. In terms of mechanisms, light microscopy showed that intraerythrocytic parasites develop slowly in HbF erythrocytes, and electron microscopy showed that hemozoin formation was defective in transgenic mice. Finally, digestion studies of HbF by recombinant plasmepsin II demonstrated that HbF is digested only half as well as hemoglobin A (HbA). We conclude that HbF provides protection from P falciparum malaria by the retardation of parasite growth. The mechanism involves resistance to digestion by malarial hemoglobinases based on the data presented and with the well-known properties of HbF as a super stable tetramer. In addition, the resistance of normal neonates for malaria can now be explained by a double mechanism: increased malaria invasion rates, reported in neonatal RBC, will direct parasites to fetal cells, as well as F cells, and less to the approximately 20% of HbA containing RBC, amplifying the antimalarial effects of HbF.

Gamma chain heterogeneity: determination of Hb F composition by perfusion chromatography

The Ggamma:Agamma ratio is around 70:30 at the time of birth and usually 40:60 in the trace amounts of Hb F found in the adult. Changes in this ratio are observed in several hemoglobin disorders providing insights on the genetics and molecular pathophysiology of these diseases. Several techniques have been proposed to measure the Ggamma:Agamma ratio. We here describe perfusion chromatography which is now in routine use in one of our laboratories. The method involves a high velocity flow of the mobile phase through a porous reversed phase chromatographic stationary bed and allows us to determine this parameter one order of magnitude faster than with conventional high performance liquid chromatography.

Stabilization of the T-state of ferrous human adult and fetal hemoglobin by Ln(III) complexes: a thermodynamic study

The effect of the lanthanide(III) complexes [Gd(1,4,7,10-tetraazacyclododecane-N,N',N", N"'-tetrakis(methylenephosphonate))]5- (Gd-DOTP) and La-DOTP on the oxygen binding and spectroscopic properties of human adult and fetal hemoglobin (HbA and HbF, respectively) has been investigated. The affinity of Gd-DOTP and La-DOTP for oxygenated HbA (HbAO2; KHbAO2 = 2.6 x 10(-3) M) is closely similar to that observed for Ln(III) complexes association to nitrosylated HbA (HbANO KHbANO = 1.8 x 10(-3) M) and to aquo-met HbA (met-HbA; Kmet-HbA = 1.9 x 10(-3) M), being lower than that determined for Gd-DOTP and La-DOTP binding to the deoxygenated form of the tetramer (HbAd; KHbAd = 3.0 x 10(-4) M). The affinity of Gd-DOTP for deoxygenated HbF (HbFd; KHbFd = 9.5 x 10(-4) M) and oxygenated HbF (HbFO2; KHbFO2 = 3.7 x 10(-3) M) is lower than that observed for Ln(III) complexes association to HbAd and HbAO2, respectively. Gd-DOTP and La-DOTP bind to HbA and HbF with a 1:1 stoichiometry per tetramer. Increasing Gd-DOTP and La-DOTP concentration, oxygen affinity for HbA decreases (i.e. P50 increases), this effect being minor for HbF. Upon binding of Ln(III) complexes to HbANO, the X-band EPR spectrum and the absorption spectrum in the Soret region display the characteristics which have been attributed to the T-state of the ligated tetramer. These results represent a clear cut evidence for the specific binding of Gd-DOTP and La-DOTP to the 2,3-D-glycerate bisphosphate (BPG) pocket (i.e. at the dyad axis, in between the beta-chains) of HbA and HbF. The effect of Ln(III) complexes on the ligand binding and spectroscopic properties of HbA and HbF is reminiscent that of BPG, the physiological modulator of human Hb action.

A 1H NMR comparative study of human adult and fetal hemoglobins

The affinities of the individual subunits in human adult and fetal hemoglobins to azide ion have been determined from the combined analysis of NMR and optical titration data. Structural and functional non-equivalence of the constituent subunits, i.e. alpha and beta subunits in human adult hemoglobin and alpha and gamma subunits in human fetal hemoglobin, has been confirmed. The function of the alpha subunits, which are common to both hemoglobins, is essentially identical in these hemoglobins and, in spite of the substitutions of 39 amino acid residues between beta and gamma subunits, they exhibit similar azide ion affinities. The present study also demonstrates that the NMR spectral comparison between the two proteins provides signal assignments to the individual subunits in intact tetramer.

Exchange of subunit interfaces between recombinant adult and fetal hemoglobins. Evidence for a functional inter-relationship among regions of the tetramer

The inter-relationship between the interior subunit interfaces and the exterior diphosphoglycerate (DPG) binding region of the hemoglobin tetramer and the effects of a specific N-terminal acetylation on tetramer assembly have been evaluated. Tetrameric fetal hemoglobin F in the liganded state was found to dissociate to dimers much less than previously appreciated, i.e. about 70 times less than adult hemoglobin A (Kd = 0.01 microM and 0.68 microM, for HbF and HbA, at pH 7.5, respectively) over the pH range 6.2-7.5, whereas HbF1, in which the N termini of the gamma-chains are acetylated, dissociates like HbA. To determine whether this feature of HbF could be transferred to hemoglobin A, the single amino acid difference in their alpha1beta2/alpha1gamma2 interfaces and the 4 amino acid differences in their alpha1beta1/alpha1gamma1 interfaces have been substituted in HbA to those in HbF. This pentasubstituted recombinant HbA/F had the correct molecular weight as determined by mass spectrometry, the expected mobility on isoelectric focusing, the calculated amino acid composition, and normal circular dichroism properties, oxygen binding, and cooperativity. Although HbA/F has the same amino acid side chains that bind DPG as HbA, its diminished response to 2,3-DPG resembled that of HbF. However, its tetramer-dimer dissociation constant (Kd = 0.14 microM) was between that of HbA and HbF despite the fact that it was composed entirely of HbF subunit interfaces. The results indicate that regions of the tetramer distant from the tetramer-dimer interface influence its dissociation and, reciprocally, that the interfaces affect regions involved in the binding of allosteric regulators, suggesting flexible long range inter-relationships in hemoglobin.

Production of human normal adult and fetal hemoglobins in Escherichia coli

A hemoglobin expression system in Escherichia coli is described. In order to produce authentic human hemoglobin, we need to co-express both methionine aminopeptidase and globin genes under the control of a strong promoter. We have constructed three plasmids, pHE2, pHE4 and pHE7, for the expression of human normal adult hemoglobin and a plasmid, pHE9, for the expression of human fetal hemoglobin, in high yields. The globin genes can be derived from either synthetic genes or human globin cDNAs. The extra amino-terminal methionine residues of the expressed globins can be removed by the co-expressed methionine aminopeptidase. The heme is inserted correctly into the expressed alpha-globin from our expression plasmids. A fraction (approximately 25%) of the heme is not inserted correctly into the expressed beta- or gamma-globin. However, the incorrectly inserted hemes can be converted into the correct conformation by carrying out a simple oxidation-reduction process on the purified hemoglobin molecule. We have investigated the functional properties of the expressed hemoglobins by measuring their oxygen-binding properties and their structural features by obtaining their 1H-NMR spectra. Our results show that authentic human normal adult and fetal hemoglobins can be produced from our expression plasmids in E. coli and in high yields. Our expression system allows us to design and to produce any recombinant hemoglobins needed for our research on the structure-function relationship in hemoglobin.

Fetal nucleated erythrocytes (NRBCs) in chorionic villus sample supernatant fluids: an additional source of fetal material for karyotype confirmation

Fetal erythrocytes leak from the fetal capillaries at the time of chorionic villus removal. The purpose of this study was to determine if fetal nucleated erythrocytes (NRBCs) could be isolated from the chorionic villus sampling (CVS) supernatant fluid and used as an additional source of fetal material in order to confirm the fetal karyotype in cases of CVS mosaicism. One hundred CVS supernatant fluids were studied by simultaneous immunophenotyping, using a mouse antifetal haemoglobin antibody, UCH gamma, combined with fluorescent in situ hybridization (FISH) analysis using X- and Y-specific DNA probes. A chromosome 18 probe was also used in the case of a known male fetus with trisomy 18. Fetal haemoglobin (HbF)-positive cells were identified in 73 supernatant fluids and HbF-positive nucleated cells were present in 60 samples. The number of cells detected per sample showed great variation among the individual samples. FISH analysis was performed in 41 cases. FISH prediction of the fetal gender was concordant with the CVS karyotype in all cases, and the fetal trisomy 18 was correctly verified. In five cases in which Y sequences were detected, a small number of HbF-positive cells with two X signals were also identified; interestingly, in three of the five cases, the mother was a beta-thalassaemia carrier. This technique can be used as a quick and accurate method for the immediate verification of CVS results in cases of mosaicism, thus avoiding second-trimester amniocentesis.

Maximal gamma-globin expression in the compound heterozygous state for -175G gamma HPFH and beta degree 39 nonsense thalassaemia: a case study

The -175 (T-->C) G gamma hereditary persistence of fetal haemoglobin is a very rare promoter mutation occurring in Caucasians as well as in African-Americans. Heterozygotes for this non-deletional HPFH show 20% HbF, mostly of G gamma type. We describe here a healthy Sardinian man who coinherited -175 (T-->C) G gamma HPFH with the beta-thalassaemia codon 39 nonsense mutation in trans; he showed 64% HbF, 100% of G gamma type. Although the beta-globin haplotype pattern (II/II) was indicative of the presence of the A gamma T allele on both chromosomes, the A gamma T expression was undetectable by HPLC even in red cell populations separated by age. The proband was, moreover, homozygous for the -4 bp deletion at position -225 to -222 of A gamma promoter which has recently been associated with decreased A gamma T globin expression. These findings suggest that this maximal overexpression of G gamma-globin probably reflects intensified stimulation of the mutated G gamma promoter in this hitherto undescribed genetic condition.

Clinical and hematological response to hydroxyurea in a patient with Hb Lepore/beta-thalassemia

The possibility of increasing Hb F in vivo using drugs like 5-azacytidine, hydroxyurea, and butyrate has been established. However, in many cases this does not entail an increase in total hemoglobin. We report on a patient with Hb Lepore/beta-thalassemia being treated with hydroxyurea (30 mg/Kg/day) because of the presence of erythroid extramedullary masses with severe neurological abnormalities. During therapy the patient showed a remarkable improvement in neurological signs due to the reduction in extra-medullary masses, a significant increase in both total hemoglobin (from 5.8 to 9.7 g/dl) and Hb F (from 4.9 g/dl to 9.1 g/dl). The marked improvement in hemoglobin level in our patient with Hb Lepore/beta-thalassemia suggests gamma-globin gene activation due to the DNA structure determined by the crossover event.