Chip for dielectrophoretic microbial capture, separation and detection II: experimental study

In our previous paper we have modelled a dielectrophoretic force (DEP) and cell particle behavior in a microfluidic channel (Weber MUet al2023 Chip for dielectrophoretic microbial capture, separation and detection I: theoretical basis of electrode designNanotechnologythis issue). Here we test and confirm the results of our modeling work by experimentally validating the theoretical design constraints of the ring electrode architecture. We have compared and tested the geometry and particle capture and separation performance of the two separate electrode designs (the ring and dot electrode structures) by investigating bacterial motion in response to the applied electric field. We have quantitatively evaluated the electroosmosis (EO) to positive DEP (PDEP) transition in both electrode designs and explained the differences in capture efficiency of the ring and dot electrode systems. The ring structure shows 99% efficiency of bacterial capture both for PDEP and for EO. Moreover, the ring structure shows an over 200 faster bacterial response to the electric field. We have also established that the ring electrode architecture, with appropriate structure periodicity and spacing, results in efficient capture and separation of microbial cells. We have identified several critical design constraints that are required to achieve high efficiency bacterial capture. We have established that the spacing between consecutive DEP traps smaller than the length of the depletion zone will ensure that the DEP force dominates bacterial motion over motility and Brownian motion.

Chip for dielectrophoretic microbial capture, separation and detection I: theoretical basis of electrode design

We model the dielectrophoretic response ofE. colibacterial cells and red blood cells, upon exposure to an electric field. We model the separation, capture, and release mechanisms under flow conditions in a microfluidic channel and show under which conditions efficient separation of different cell types occurs. The modelling work is aimed to guide the separation electrode architecture and design for experimental validation of the model. The dielectrophoretic force is affected both by the geometry of the electrodes (the gradient of the electric field), the Re{CM(ω)} factor, and the permittivity of the medium ϵ_m. Our modelling makes testable predictions and shows that designing the electrode structure to ensure structure periodicity with spacing between consecutive traps smaller than the length of the depletion zone ensures efficient capture and separation. Such electrode system has higher capture and separation efficiency than systems with the established circular electrode architecture. The simulated, modelled microfluidic design allows for the separated bacteria, concentrated by dedicated dielectrophoretic regions, to be subsequently detected using label-free functionalized nanowire sensors. The experimental validation of the modelling work presented here and the validation of the theoretical design constraints of the chip electrode architecture is presented in the companion paper in the same issue (Weber MUet al2022 Chip for dielectrophoretic Microbial Capture, Separation and Detection II: Experimental Study).

Ultra-deep long-read sequencing detects IS-mediated gene duplications as a potential trigger to generate arrays of resistance genes and a mechanism to induce novel gene variants such as blaCTX-M-243

BACKGROUND

Extended-spectrum β-lactamases (ESBLs) are enzymes that can render their hosts resistant to various β-lactam antibiotics. CTX-M-type enzymes are the most prevalent ESBLs and the main cause of resistance to third-generation cephalosporins in Enterobacteriaceae. The number of described CTX-M types is continuously rising, currently comprising over 240 variants. During routine screening we identified a novel blaCTX-M gene.

OBJECTIVES

To characterize a novel blaCTX-M variant harboured by a multidrug-resistant Escherichia coli isolate of sequence type ST354.

METHODS

Antibiotic susceptibilities were determined using broth microdilution. Genome and plasmid sequences were reconstructed using short- and long-read sequencing. The novel blaCTX-M locus was analysed using long-read and Sanger sequencing. Plasmid polymorphisms were determined in silico on a single plasmid molecule level.

RESULTS

The novel blaCTX-M-243 allele was discovered alongside a nearly identical blaCTX-M-104-containing gene array on a 219 kbp IncHI2A plasmid. CTX-M-243 differed from CTX-M-104 by only one amino acid substitution (N109S). Ultra-deep (2300-fold coverage) long-read sequencing revealed dynamic scaling of the blaCTX-M genetic contexts from one to five copies. Further antibiotic resistance genes such as blaTEM-1 also exhibited sequence heterogeneity but were stable in copy number.

CONCLUSIONS

We identified the novel ESBL gene blaCTX-M-243 and illustrate a dynamic system of varying blaCTX-M copy numbers. Our results highlight the constant emergence of new CTX-M family enzymes and demonstrate a potential evolutionary platform to generate novel ESBL variants and possibly other antibiotic resistance genes.

Mutations in the gdpP gene are a clinically relevant mechanism for β-lactam resistance in meticillin-resistant Staphylococcus aureus lacking mec determinants

In Staphylococcus aureus, resistance to β-lactamase stable β-lactam antibiotics is mediated by the penicillinbinding protein 2a, encoded by mecA or by its homologues mecB or mecC. However, a substantial number of meticillin-resistant isolates lack known mec genes and, thus, are called meticillin resistant lacking mec (MRLM). This study aims to identify the genetic mechanisms underlying the MRLM phenotype. A total of 141 MRLM isolates and 142 meticillin-susceptible controls were included in this study. Oxacillin and cefoxitin minimum inhibitory concentrations were determined by broth microdilution and the presence of mec genes was excluded by PCR. Comparative genomics and a genome-wide association study (GWAS) approach were applied to identify genetic polymorphisms associated with the MRLM phenotype. The potential impact of such mutations on the expression of PBP4, as well as on cell morphology and biofilm formation, was investigated. GWAS revealed that mutations in gdpP were significantly associated with the MRLM phenotype. GdpP is a phosphodiesterase enzyme involved in the degradation of the second messenger cyclic-di-AMP in S. aureus. A total of 131 MRLM isolates carried truncations, insertions or deletions as well as amino acid substitutions, mainly located in the functional DHH-domain of GdpP. We experimentally verified the contribution of these gdpP mutations to the MRLM phenotype by heterologous complementation experiments. The mutations in gdpP had no effect on transcription levels of pbp4; however, cell sizes of MRLM strains were reduced. The impact on biofilm formation was highly strain dependent. We report mutations in gdpP as a clinically relevant mechanism for β-lactam resistance in MRLM isolates. This observation is of particular clinical relevance, since MRLM are easily misclassified as MSSA (meticillin-susceptible S. aureus), which may lead to unnoticed spread of β-lactam-resistant isolates and subsequent treatment failure.

Failure of Vitek2 to reliably detect vanB-mediated vancomycin resistance in Enterococcus faecium

OBJECTIVES

The increasing prevalence of VRE necessitates their reliable detection, especially for low-level resistance mediated by vanB in Enterococcus faecium. In this prospective study we analysed if vanB-mediated vancomycin resistance can be reliably detected by Vitek2.

METHODS

One thousand, three hundred and forty-four enterococcal isolates from routine clinical specimens were tested by Vitek2 (bioMérieux, Nürtingen, Germany). Additionally, a bacterial suspension (with a turbidity equivalent to that of a 0.5 McFarland standard) was inoculated on chromID VRE screening agar (bioMérieux) and incubated for 48 h. If vancomycin tested susceptible by Vitek2 but growth was detected on the screening agar, PCR for vanA/vanB was performed (GeneXpert vanA/B test, Cepheid, Frankfurt, Germany). For isolates that tested susceptible to vancomycin by Vitek2 but were vanA/B positive, MICs were determined before and after cultivation in broth with increasing concentrations of vancomycin.

RESULTS

One hundred and fifty-six out of 491 E. faecium were VRE and were predominantly vanB positive (81.0%). Of these, Vitek2 did not identify 14 as VRE (sensitivity 91.0%). By broth microdilution 9/14 isolates demonstrated high MICs (≥32 mg/L) and 5/14 showed low vancomycin MICs, which did not increase despite vancomycin exposure. Three of the 14 isolates demonstrated growth on chromID VRE; after vancomycin exposure seven additional isolates were able to grow on chromID VRE.

CONCLUSIONS

Vitek2 fails to detect vanB-mediated vancomycin resistance consistently, especially, but not limited to, low-level resistance. As this may lead to treatment failure and further dissemination of vanB VRE, additional methods (e.g. culture on VRE screening agar or PCR) are necessary to reliably identify vanB-positive enterococci in clinical routine.

Genome-Wide Association Studies for the Detection of Genetic Variants Associated With Daptomycin and Ceftaroline Resistance in Staphylococcus aureus

Background

As next generation sequencing (NGS) technologies have experienced a rapid development over the last decade, the investigation of the bacterial genetic architecture reveals a high potential to dissect causal loci of antibiotic resistance phenotypes. Although genome-wide association studies (GWAS) have been successfully applied for investigating the basis of resistance traits, complex resistance phenotypes have been omitted so far. For S. aureus this especially refers to antibiotics of last resort like daptomycin and ceftaroline. Therefore, we aimed to perform GWAS for the identification of genetic variants associated with DAP and CPT resistance in clinical S. aureus isolates.

Materials/methods

To conduct microbial GWAS, we selected cases and controls according to their clonal background, date of isolation, and geographical origin. Association testing was performed with PLINK and SEER analysis. By using in silico analysis, we also searched for rare genetic variants in candidate loci that have previously been described to be involved in the development of corresponding resistance phenotypes.

Results

GWAS revealed MprF P314L and L826F to be significantly associated with DAP resistance. These mutations were found to be homogenously distributed among clonal lineages suggesting convergent evolution. Additionally, rare and yet undescribed single nucleotide polymorphisms could be identified within mprF and putative candidate genes. Finally, we could show that each DAP resistant isolate exhibited at least one amino acid substitution within the open reading frame of mprF. Due to the presence of strong population stratification, no genetic variants could be associated with CPT resistance. However, the investigation of the staphylococcal cassette chromosome mec (SCCmec) revealed various mecA SNPs to be putatively linked with CPT resistance. Additionally, some CPT resistant isolates revealed no mecA mutations, supporting the hypothesis that further and still unknown resistance determinants are crucial for the development of CPT resistance in S. aureus.

Conclusion

We hereby confirmed the potential of GWAS to identify genetic variants that are associated with antibiotic resistance traits in S. aureus. However, precautions need to be taken to prevent the detection of spurious associations. In addition, the implementation of different approaches is still essential to detect multiple forms of variations and mutations that occur with a low frequency.

Excellent performance of CHROMagarTM LIN-R to selectively screen for linezolid-resistant enterococci and staphylococci

The increasing number of nosocomial pathogens with resistances against last resort antibiotics like linezolid leads to a pressing need for the reliable detection of these drug-resistant bacteria. National guidelines on infection prevention, e.g., in Germany, have already recommend screening for linezolid-resistant bacteria, although a corresponding screening agar medium has not been provided. In this study we analyzed the performance and reliability of a commercial, chromogenic linezolid screening agar. The medium was capable to predict more than a hundred linezolid-resistant isolates of E. faecium, E. faecalis, S. aureus, S. epidermidis, and S. hominis with excellent sensitivity and specificity. All isolates were collected at the National Reference Centre between 2010 and 2020.

Thirty years of VRE in Germany - "expect the unexpected": The view from the National Reference Centre for Staphylococci and Enterococci

Enterococci are commensals of the intestinal tract of many animals and humans. Of the various known and still unnamed new enterococcal species, only isolates of Enterococcus faecium and Enterococcus faecalis have received increased medical and public health attention. According to textbook knowledge, the majority of infections are caused by E. faecalis. In recent decades, the number of enterococcal infections has increased, with the increase being exclusively associated with a rising number of nosocomial E. faecium infections. This increase has been accompanied by the dissemination of certain hospital-acquired strain variants and an alarming progress in the development of antibiotic resistance namely vancomycin resistance. With this review we focus on a description of the specific situation of vancomycin resistance among clinical E. faecium isolates in Germany over the past 30 years. The present review describes three VRE episodes in Germany, each of which is framed by the beginning and end of the respective decade. The first episode is specified by the first appearance of VRE in 1990 and a country-wide spread of specific vanA-type VRE strains (ST117/CT24) until the late 1990s. The second decade was initially marked by regional clusters and VRE outbreaks in hospitals in South-Western Germany in 2004 and 2005, mainly caused by vanA-type VRE of ST203. Against the background of a certain "basic level" of VRE prevalence throughout Germany, an early shift from the vanA genotype to the vanB genotype in clinical isolates already occurred at the end of the 2000s without much notice. With the beginning of the third decade in 2010, VRE rates in Germany have permanently increased, first in some federal states and soon after country-wide. Besides an increase in VRE prevalence, this decade was marked by a sharp increase in vanB-type resistance and a dominance of a few, novel strain variants like ST192 and later on ST117 (CT71, CT469) and ST80 (CT1065). The largest VRE outbreak, which involved about 2,900 patients and lasted over three years, was caused by a novel and until that time, unknown strain type of ST80/CT1013 (vanB). Across all periods, VRE outbreaks were mainly oligoclonal and strain types varied over space (hospital wards) and time. The spread of VRE strains obviously respects political borders; for instance, both vancomycin-variable enterococci which were highly prevalent in Denmark and ST796 VRE which successfully disseminated in Australia and Switzerland, were still completely absent among German hospital patients, until to date.

Validating a screening agar for linezolid-resistant enterococci

BACKGROUND

Linezolid is an alternative treatment option for infections with multidrug-resistant Gram-positive bacteria including vancomycin-resistant enterococci. Some countries report an increasing number of isolates with resistance to linezolid. The recent publication of the Commission for Hospital Hygiene in Germany on enterococci/VRE recommends screening for linezolid-resistant enterococci (LRE). However, a suitable selective medium or a genetic test is not available. Our aim was to establish a selective screening agar for LRE detection and validate its application with a comprehensive collection of clinical LRE and linezolid-susceptible enterococci.

METHODS

We decided to combine the selective power of an enterococcal screening agar with a supplementation of linezolid. Several rounds of analyses with reference, control and test strains and under varying linezolid concentrations of a wider and a smaller range were investigated and assessed. The collection of linezolid-resistant enterococcal control strains included isolates with different resistance mechanisms (23S rDNA mutations, cfr(B), optrA, poxtA). Finally, we validated our LRE screening agar with 400 samples sent to our National Reference Centre in 2019.

RESULTS

Several rounds of pre-tests and confirmatory analyses favored Enterococcosel® Agar supplemented with a concentration of 2 mg/L linezolid. A 48 h incubation period was essential for accurate identification of LRE strains. Performance of the LRE screening agar revealed a sensitivity of 96.6% and a specificity of 94.4%.

CONCLUSIONS

Here we describe preparation of a suitable screening agar and a procedure to identify LRE isolates with high accuracy.

IS26-Mediated Transfer of bla NDM-1 as the Main Route of Resistance Transmission During a Polyclonal, Multispecies Outbreak in a German Hospital

One of the most demanding challenges in infection control is the worldwide dissemination of multidrug-resistant (MDR) bacteria in clinical settings. Especially the increasing prevalence of carbapenemase producing Gram-negative pathogens poses an urgent threat to public health, as these enzymes confer resistance to almost all β-lactam antibiotics including carbapenems. In this study, we report a prolonged nosocomial outbreak of various NDM-1-producing Enterobacterales species due to clonal spread and cross-species exchange of plasmids and possibly transposons. Between July 2015 and September 2017, a total of 51 carbapenemase-positive isolates were collected from 38 patients and three environmental sources in a single German hospital. Combining molecular typing methods and whole genome sequencing, the metallo-β-lactamase gene bla _NDM-1 was found to be present in 35 isolates of which seven additionally carried the carbapenemase gene bla _KPC-2. Core genome MLST (cgMLST) revealed different clusters of closely related isolates of Escherichia coli, Klebsiella pneumoniae, Citrobacter freundii, Morganella morganii or Enterobacter cloacae indicating clonal spread. The detailed reconstruction of the plasmid sequences revealed that in all outbreak-associated isolates bla_NDM-1 was located on similar composite transposons, which were also very similar to Tn125 previously described for Acinetobacter baumannii. In contrast to Tn125, these structures were flanked by IS26 elements, which could facilitate horizontal gene transfer. Moreover, the identical plasmid was found to be shared by E. coli and M. morganii isolates. Our results highlight the importance of detailed genome-based analyses for complex nosocomial outbreaks, allowing the identification of causal genetic determinants and providing insights into potential mechanisms involved in the dissemination of antibiotic resistances between different bacterial species.

Comparative evaluation of VITEK® 2 and three commercial gradient strip assays for daptomycin susceptibility testing of Staphylococcus aureus

Objectives

MRSA remains a major cause of severe nosocomial infections and the increased use of vancomycin and daptomycin for MRSA treatment over the last decade has led to the isolation of MRSA strains with decreased daptomycin susceptibility. In addition, a growing number of MSSA isolates with reduced susceptibility to daptomycin have been described lately. Surveillance of the emergence of such a daptomycin-non-susceptible MSSA population requires prompt and reliable daptomycin susceptibility testing. Therefore, this work aimed to evaluate the ability of commonly used methods to detect daptomycin resistance in clinical microbiological laboratories.

Methods

We used commercially available manual and automated test systems, including VITEK® 2 and three gradient strip assays, in comparison with broth microdilution, to detect daptomycin resistance in a representative Staphylococcus aureus strain collection.

Results

We found high inter-assay concordance as well as congruence with the reference method. This is demonstrated by essential agreement between commercial test systems and reference broth microdilution ranging from 98.1% to 100% and by categorical agreement from 98.2% to 99.1%. Thus, all systems used were able to detect daptomycin non-susceptibility in MRSA and MSSA isolates.

Conclusions

Our data indicate that routine laboratories are at limited risk of overlooking further daptomycin resistance development, as long as commercially available test systems are used according to the manufacturer's recommendations. However, laboratories must be aware of an increasing number of daptomycin-non-susceptible MSSA isolates, including those exhibiting elevated MICs of glycopeptides.

Temperature dependence of haemoglobin-oxygen affinity in heterothermic vertebrates: mechanisms and biological significance

As demonstrated by August Krogh et al. a century ago, the oxygen-binding reaction of vertebrate haemoglobin is cooperative (described by sigmoid O(2) equilibrium curves) and modulated by CO(2) and protons (lowered pH) that - in conjunction with later discovered allosteric effectors (chloride, lactate and organic phosphate anions) - enhance O(2) unloading from blood in relatively acidic and oxygen-poor tissues. Based on the exothermic nature of the oxygenation of the haem groups, haemoglobin-O(2) affinity also decreases with rising temperature. This thermal sensitivity favours oxygen unloading in warm working muscles, but may become detrimental in regionally heterothermic animals, for example in cold-tolerant birds and mammals and warm-bodied fish, where it may perturb the balance between O(2) unloading and O(2) requirement in organs with substantially different temperatures than at the respiratory organs and thus commonly is reduced or obliterated. Given that the oxygenation of haemoglobin is linked with the endothermic release of allosteric effectors, increased effector interaction is an effective strategy that is widely exploited to achieve adaptive reductions in the temperature dependence of blood-O(2) affinity. The molecular mechanisms implicated in heterothermic vertebrates from different taxonomic groups reveal remarkable variability, both as regards the effectors implicated (protons in tunas, organic phosphates in sharks and billfish, chloride ions in ruminants and chloride and phosphate anions in the extinct woolly mammoth, etc.) and binding sites for the same effectors, indicating multiple evolutionary origins, but convergent physiological functionality (reductions in temperature dependence of O(2) -binding affinity that safeguard tissue O(2) supply).

Lacking deoxygenation-linked interaction between cytoplasmic domain of band 3 and HbF from fetal red blood cells

AIM

Several of the red blood cell's metabolic and membrane functions display dependence on haemoglobin oxygenation. In adult human red cells, the increased glycolytic rate at low O2 tension results from binding of deoxygenated HbA at negatively charged, N-terminal, cytoplasmic domain of the membrane protein band 3, which liberates glycolytic enzymes from this site. This study aims to investigate the role of fetal HbF (that has lower anion-binding capacity than HbA) in fetal red cells (that are subjected to low O2 tensions), and to elucidate possible linkage (e.g. via the major red cell membrane organising centre, band 3) between the individual oxygenation-linked reactions encountered in red cells.

METHODS

The interaction between band 3 and Hb is analysed in terms of the effects, measured under different conditions, of a 10-mer peptide that corresponds to the N-terminus of human band 3 protein, on the oxygenation reaction of HbF and HbA, isolated from umbilical chord red cells.

RESULTS

Contrasting with the unequivocal interaction of the peptide with HbA that with fetal HbF is weak, and annihilated in the presence of autochthonous red cell O2 affinity modulators (chloride and organic phosphates).

CONCLUSION

The data indicate that HbF does not function as a transducer mediating O2 dependence of glycolysis in fetal red cells, in accordance with the different O2 and metabolic profiles compared to those in HbA-bearing adult red cells. In conjunction with the previously discovered O2 dependence of K+ transport in HbF-rich fetal cells, they moreover argue against linkage between different, physiologically relevant, O2-dependent red cell functions.

Multiple strategies of Lake Victoria cichlids to cope with lifelong hypoxia include hemoglobin switching

Many fish species adapt to hypoxia by reducing their metabolic rate and increasing hemoglobin-oxygen (Hb-O(2)) affinity. Pilot studies with young broods of cichlids showed that the young could survive severe hypoxia in contrast with the adults. It was therefore hypothesized that early exposure results in improved oxygen transport. This hypothesis was tested using split brood experiments. Broods of Astatoreochromis alluaudi, Haplochromis ishmaeli, and a tilapia hybrid (Oreochromis) were raised either under normoxia (NR; 80-90% air saturation) or hypoxia (HR; 10% air saturation). The activity of the mitochondrial citrate synthase was not different between NR and HR tilapia, but was significantly decreased in HR A. alluaudi and H. ishmaeli, indicating lowered maximum aerobic capacities. On the other hand, hemoglobin and hematocrit levels were significantly higher in all HR fish of the three species, reflecting a physiological adaptation to safeguard oxygen transport capacity. In HR tilapia, intraerythrocytic GTP levels were decreased, suggesting an adaptive increase of blood-O(2) affinity. Similar changes were not found in HR H. ishmaeli. In this species, however, all HR specimens exhibited a distinctly different iso-Hb pattern compared with their NR siblings, which correlated with a higher intrinsic Hb-O(2) affinity in the former. All HR cichlids thus reveal left-shifted Hb-O(2) equilibrium curves, mediated by either decreased allosteric interaction or, in H. ishmaeli, by the production of new hemoglobins. It is concluded that the adaptation to lifelong hypoxia is mainly due to improved oxygen transport.

Effects of short-term hypoxia on neuroglobin levels and localization in mouse brain tissues

Nerve cells are highly susceptible to ischemic and hypoxic injuries. The neuroglobin (Ngb), found in vertebrate nerve cells, has been suggested to protect nerve cells from ischemic episodes by a yet unknown mechanism. However, contradicting reports exist regarding localization and up-regulation of Ngb in response to hypoxia. The aim of the present study was to probe the distribution of Ngb proteins in mouse brain and retina by immunohistochemistry, and to quantify the levels of Ngb mRNA by reverse-transcription-polymerase chain reaction (RT-PCR) after short-term (2 h) exposure to 7.6% oxygen. We found Ngb to be present throughout the neocortex, most abundantly in the perirhinal, entorhinal and temporal cortical areas, the thalamus and hypothalamus, the choroid plexus, the olfactory bulb and the cranial nerve nuclei in the brainstem. Intense staining was observed in the mesencephalic central grey area and the Purkinje cells. Two-hour hypoxic exposure caused no detectable changes in staining intensity or spatial distribution of Ngb neither in the Purkinje cells nor in any other brain areas observed. The RT-PCR data supported the lack of differences in brain Ngb levels between normal and oxygen-deprived animals. In the retina, Ngb localization by immunohistochemistry was confined to the inner segments of the photoreceptors, the plexiform layers and the ganglion cells. Short-termed hypoxia did not change retinal Ngb levels as assessed by both techniques. The lack of Ngb up-regulation in the brain is consistent with results from previous long-term hypoxic experiments, suggesting that Ngb is not regulated by pure hypoxia in vivo.

New insights into the proton-dependent oxygen affinity of Root effect haemoglobins

A long-standing puzzle with regard to protein structure/function relationships is the proton-dependent modification of haemoglobin (Hb) structure that causes oxygen to be unloaded from Root effect Hbs into the swim bladders and eyes of fish even against high oxygen pressure gradients. Although oxygen unloading in Root effect Hbs has generally been attributed to proton-dependent stabilization of the T-state, protonation of Root effect Hbs can alter their ligand affinities in both R- and T-state conformations and either stabilize the T-state or destabilize the R-state. The C-terminal residues that are so important in the Bohr effect of human Hb appear to be involved in the Root effects of some fish Hbs and not in others, indicating that several evolutionary pathways have resulted in expression of highly pH-dependent Hbs. New data are presented that show surprising similarities in the pH- and anion-dependence of sulfhydryl group reactivity and anaerobic oxidation of human and fish Hbs. The available evidence supports the concept that in both Bohr effect and Root effect Hbs a large steric component acts in addition to quaternary shifts between R and T conformations to regulate ligand affinity. Allosteric effectors moderate these steric effects within both R- and T-state conformations and allow for an elegant match between Hb function and the wide-ranging physiological needs of diverse organisms.

Comparison of stone size and response to analgesic treatment in predicting outcome of patients with renal colic

The aim of this study was to compare the prognostic value of stone size and response to analgesic treatment in patients with renal colic. We reviewed the charts of patients treated for renal colic in our Emergency Department. The eligibility criteria were a radiological examination demonstrating direct or indirect signs of ureteral obstruction and/or a stone. The primary endpoint was the requirement for surgical treatment. The parameters considered as prognostic factors were pain relief with ketorolac (K) or ketorolac plus opiate treatment (KO), and stone size (>or= or <6 mm). Ninety-five patients were considered for analysis. Of these, 49 (52%) had a stone demonstrated radiologically. Four out of 27 patients (15%) in the KO group and six out of 68 patients (8.8%) in the K group required a surgical procedure to relieve the obstruction (NS). Four out of five patients (80%) with a stone >or=6 mm required a surgical procedure, compared with one out of 44 (2.2%) who had a stone smaller than 6 mm (P<0.001). In conclusion, stone size is a better prognostic factor than the response to analgesic treatment in predicting the clinical outcome of patients with renal colic. A stone >or=6 mm in patients with renal colic should alert the emergency physician that urological complications requiring surgical intervention may occur and that urological management may be warranted.

Evaluation of emergency air evacuation of critically ill patients from cruise ships

BACKGROUND

The study objectives were to assess the ship physician's diagnostic accuracy in making the decision to air evacuate critically ill patients from cruise ships, to determine the outcome of these patients, and the overall benefit of air evacuation.

METHODS

From October 1999 to May 2000, we performed a prospective study of critically ill patients coming from cruise ships in the Caribbean and transported to our institution by air ambulance. Demographics, initial diagnosis, and treatment on board were collected by the triage officer at the time of the cruise physician's first call. In route complications and flight team composition were obtained from the air ambulance monitoring log. Patients were followed-up in the hospital for complications, outcome, and final diagnosis.

RESULTS

A consecutive series of 104 patients were considered for analysis. There were 65 men and 39 women (mean age: 68.7 years). Cruise physician's diagnosis was correct in more than 90% of the cases. Internal medicine and surgical conditions represented 80.8% and 19.2% of the cases respectively, falling mainly into three categories: cardiac (34.6%), neurological (20.2%), and digestive (14%). Two cardiac arrests and 1 ventricular fibrillation were successfully resuscitated and 5 of 15 myocardial infarctions received thrombolytic therapy on board. Air transfers were warranted in 96.1% of the cases and physician presence in the flight was considered appropriate in 97.6%. In route complications and mortality rate were 5.8% and 2.9% respectively, related to serious cardiac events. Among the 98 hospitalized patients, 10 patients developed new complications and 5 died. The overall mortality rate was 7.7%.

CONCLUSION

The cruise industry appears off to a good start in the medical treatment of passengers needing air evacuation to a land based medical facility. There is room for improvement and adoption of American College of Emergency Physicians (ACEP) and International Council of Cruise Lines (ICCL) Health Care Guidelines are meaningful first steps. Analysis of Caribbean medical facilities and implementation of active telemedicine conferencing represent alternatives to air evacuation that need to be studied.

Oxygen binding by single red blood cells from the red-eared turtle Trachemys scripta

Oxygen-binding properties of single red blood cells from the red-eared turtle Trachemys scripta were measured by microspectrophotometry to describe the variation in oxygen affinity of red blood cells and to gain insight into the distribution of functionally different hemoglobins among red blood cells. Methodologically, this study represents the first report on the cell-to-cell variation in oxygen-binding properties based on oxygen-binding curves of single vertebrate red blood cells. The cells differed significantly with respect to oxygen affinity. Mean oxygen pressure at half saturation of the cells in a blood sample was found to be 20.1 +/- 3.3 (SD) Torr. The distribution of oxygen affinities among red blood cells is unimodal, indicating that the two hemoglobins found in turtle blood are not segregated in distinct cells. Therefore, the functional interaction shown by these hemoglobins in vitro is likely to take place in vivo. The considerable variation in oxygen affinity between individual red blood cells calls for its incorporation in models of tissue oxygenation.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

Ligand binding in the ferric and ferrous states of Paramecium hemoglobin

The unicellular protozoan Paramecium caudatum contains a monomeric hemoglobin (Hb) that has only 116 amino acid residues. This Hb shares the simultaneous presence of a distal E7 glutamine and a B10 tyrosine with several invertebrate Hbs. In the study presented here, we have used ligand binding kinetics and resonance Raman spectroscopy to characterize the effect of the distal pocket residues of Paramecium Hb in stabilizing the heme-bound ligands. In the ferric state, the high-spin to low-spin (aquo-hydroxy) transition takes place with a pK(a) of approximately 9.0. The oxygen affinity (P(50) = 0.45 Torr) is similar to that of myoglobin. The oxygen on- and off-rates are also similar to those of sperm whale myoglobin. Resonance Raman data suggest hydrogen bonding stabilization of bound oxygen, evidenced by a relatively low frequency of Fe-OO stretching (563 cm(-1)). We propose that the oxy complex is an equilibrium mixture of a hydrogen-bonded closed structure and an open structure. Oxygen will dissociate preferentially from the open structure, and therefore, the fraction of open structure population controls the rate of oxygen dissociation. In the CO complex, the Fe-CO stretching frequency at 493 cm(-1) suggests an open heme pocket, which is consistent with the higher on- and off-rates for CO relative to those in myoglobin. A high rate of ligand binding is also consistent with the observation of an Fe-histidine stretching frequency at 220 cm(-1), indicating the absence of significant proximal strain. We postulate that the function of Paramecium Hb is to supply oxygen for cellular oxidative processes.

Hemoglobin from a deep-sea hydrothermal-vent copepod

Deep-sea hydrothermal-vent fauna live in a highly variable environment where oxygen levels can be very low, and carbon dioxide and sulfide can reach high concentrations (1). These conditions are harsh for most aerobic metazoans, yet copepods can be abundant at hydrothermal vents. Here we report the structure and functional properties of hemoglobin extracted from the copepod Benthoxynus spiculifer, which was found in large numbers in a paralvinellid/gastropod community collection made during a cruise to the Juan de Fuca Ridge in 1998. Although hemoglobin has been reported in some littoral copepods (2), this is the first study of the structure and functional properties of copepod hemoglobin. Hemoglobin represents about 60% of the total soluble proteins extracted from B. spiculifer, and although it imparts a red color to the copepod, it does not provide a significant storage pool of oxygen. It is a 208-kDa protein, composed of 14 globin chains--7 of 14.3 kDa and 7 of 15.2 kDa. The hemoglobin has a very high and temperature-sensitive oxygen affinity, with no cooperativity or Bohr effect. These properties are adaptive for an animal living in a low-oxygen environment in which the primary function of the hemoglobin is most likely oxygen acquisition to support aerobic respiration.

Isohemoglobin differentiation in the bimodal-breathing amazon catfish Hoplosternum littorale

The bimodal gill(water)/gut(air)-breathing Amazonian catfish Hoplosternum littorale that frequents hypoxic habitats uses "mammalian" 2,3-diphosphoglycerate (DPG) in addition to "piscine" ATP and GTP as erythrocytic O(2) affinity modulators. Its electrophoretically distinct anodic and cathodic hemoglobins (Hb(An) and Hb(Ca)) were isolated for functional and molecular characterization. In contrast to Hb(An), phosphate-free Hb(Ca) exhibits a pronounced reverse Bohr effect (increased O(2) affinity with decreasing pH) that is obliterated by ATP, and opposite pH dependences of K(T) (O(2) association constant of low affinity, tense state) and the overall heat of oxygenation. Dose-response curves indicate small chloride effects and pronounced and differentiated phosphate effects, DPG < ATP < GTP < IHP. Hb(Ca)-O(2) equilibria analyzed in terms of the Monod-Wyman-Changeux model show that small T state bond energy differences underlie the differentiated phosphate effects. Synthetic peptides, corresponding to N-terminal fragment of the cytoplasmic domain of trout band 3 protein, undergo oxygenation-linked binding to Hb(Ca), suggesting a metabolic regulatory role for this hemoglobin. The amino acid sequences for the alpha and beta chains of Hb(Ca) obtained by Edman degradation and cDNA sequencing show unusual substitutions at the phosphate-binding site that are discussed in terms of its reverse Bohr effect and anion sensitivities.

Gas transfer system in Alvinella pompejana (Annelida polychaeta, Terebellida): functional properties of intracellular and extracellular hemoglobins

Alvinella pompejana is a tubicolous polychaete that dwells in the hottest part of the hydrothermal vent ecosystem in a highly variable mixture of vent (350 degrees C, anoxic, CO(2)- and sulfide-rich) and deep-sea (2 degrees C, mildly hypoxic) waters. This species has developed distinct-and specifically respiratory-adaptations to this challenging environment. An internal gas exchange system has recently been described, along with the report of an intracellular coelomic hemoglobin, in addition to the previously known extracellular vascular hemoglobin. This article reports the structure of coelomic hemoglobin and the functional properties of both hemoglobins in order to assess possible oxygen transfer. Coelomocytes contain a unique monomeric hemoglobin with a molecular weight of 14,810+/-1.5 Da, as determined by mass spectrometry. The functional properties of both hemoglobins are unexpectedly very similar under the same conditions of pH (6.1-8.2) and temperature (10 degrees -40 degrees C). The oxygen affinity of both proteins is relatively high (P50=0.66 Torr at 20 degrees C and pH 7), which facilitates oxygen uptake from the hypoxic environment. A strong Bohr effect (Phi ranging from -0.8 to -1.0) allows the release of oxygen to acidic tissues. Such similar properties imply a possible bidirectional transfer of oxygen between the two hemoglobins in the perioesophagal pouch, a mechanism that could moderate environmental variations of oxygen concentration and maintain brain oxygenation.

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.

The role of linkers in the reassembly of the 3.6 MDa hexagonal bilayer hemoglobin from Lumbricus terrestris

The extent and kinetics of reassembly of the four groups of linkers L1-L4 with 213 kDa subassemblies of twelve globin chains D, (bac)3(d)3, isolated from the approximately 3.6 MDa hexagonal bilayer (HBL) hemoglobin (Hb) of Lumbricus terrestris, was investigated using gel filtration. The reassembled HBL's were characterized by scanning transmission electron microscopic (STEM) mass mapping and their subunit content determined by reversed-phase chromatography. In reassembly by method (A), the linkers isolated by RP-HPLC at pH approximately 2.2 were added to D at neutral pH; in method (B), the linkers were renatured at neutral pH and then added to D. With method (A) the percentage of HBL reassembly varied from >/=13% in the absence of Ca(II) to </=75% in 1-10 mM Ca(II). Reassembly to HBL structures whose linker contents, STEM images and masses were similar to the native Hb was observed with all the linkers (>/=75%), with ternary and binary linker combinations (40-50%) and with individual linkers producing yields increasing in the following order: L1=1-3%, L2 approximately L3=10-20% and L4=35-55%. The yield was two- to eightfold lower with method (B), except in the case of linkers L1-L3. Although the reassembly kinetics were always biphasic, with t1/2=0.3-3.3 hours and 10-480 hours, the ratio of the amplitudes fast:slow was 1:0.6 with method (A) and 1:2.5 with method (B). These results are consistent with a scheme in which the slow HBL reassembly is dependent on a slow conversion of linker conformation at neutral pH from a reassembly incompetent to a reassembly competent conformation. Although all the linkers self-associate extensively at neutral pH, forming complexes ranging from dimers to >18-mers, the size of the complex does not affect the extent or rate of reassembly. The oxygen binding affinity of reassembled HBLs was similar to that of the native Hb, but their cooperativity was lower. A model of HBL reassembly was proposed which postulates that binding of linker dimers to two of the three T subunits of D causes conformational alterations resulting in the formation of complementary binding sites which permit lateral self-association of D subassemblies, and thus dictate the formation of a hexagonal structure due to the 3-fold symmetry of D.

Functional differentiation in trematode hemoglobin isoforms

The Hbs and the major electrophoretic Hb components (isoHbs) were isolated from three species of the trematodes, Explanatum explanatum (Ee), Gastrothylax crumenifer (Gc) and Paramphistomum epiclitum (Pe), that parasitise the common Indian water buffalo Bubalus bubalis. The Hbs are monomeric and resemble the so-called nonfunctional mutant hemoglobins that have Tyr at B10 or E7 positions (replacing Leu and the His residues, respectively). However, they are capable of binding with O2 and CO. O2 equilibrium studies of trematode Hb isoforms reveal extremely high O2 affinities, with half-saturation O2 tension (P50) values up to 800 times lower than those of human hemoglobins. This correlates with Tyr residues at B10 and at the distal position (E7) that decrease the O2 dissociation rate by contributing hydrogen bonds (H-bonds) to the bound O2. These substitutions also increase the O2 association rates either due to orientation of E7-Tyr towards the solvent and/or by sterically hindering the entry of water molecules into the heme pocket. The latter may account for the low rate of autoxidation of trematode Hbs. The Hbs and their isoforms from different species exhibited pronounced variation in O2 affinity, which may relate to subtle differences in the structure of the heme pocket. The O2 affinities of the composite (unfractionated) Hbs were intermediate to those of the individual Hb isoform. The P50 values of Hbs here obtained by direct O2 equilibrium measurements differed from those calculated from kinetic data already published [Kiger, L., Rashid, A. K., Griffon, N., Haque, M., Moens, L.,Gibson, Q. H., Poyart, C., & Marden, M. C. (1998). Biophys. J. 75, 990-998.] Intermediate state(s) due to slow reorientation of E7-Tyr may account for this difference. Some Hb isoforms showed slight (either normal or reverse) Bohr effects. The hyperbolic O2 equilibrium curve, Hill coefficient (n) values near unity accord with a monomeric nature of trematode Hbs. In marked contrast to vertebrate Hbs, CO does not seem to compete effectively with O2 in trematode Hbs, as evident from partition coefficient values (M) below 1.

Characterization and functional properties of the extracellular coelomic hemoglobins from the deep-sea, hydrothermal vent scaleworm Branchipolynoe symmytilida

Polychaete species belonging to the genus Branchipolynoe are commensal with mussels from deep-sea hydrothermal vents and cold-seeps. Possessing hemoglobins (Hbs), the species B. symmytilida, which is found in the mussel Bathymodiolus thermophilus on the East Pacific Rise, is exceptional in a family normally devoid of respiratory pigments. In a previous paper we described two major coelomic extracellular hemoglobins with unique quaternary structures. Aiming to discern respiratory adaptations to the highly variable hydrothermal environment, this paper characterizes the functional properties of these Hbs and the coelomic fluid. The two major hemoglobins (C1 and C2) exhibit spectrophotometric characteristics of both intra- and extracellular hemoglobins. However, their amino acid content is very different from other known hemoglobins and is characterized by a high proportion of alanine and glycine (up to 40% cumulated in C1). C1 and C2 differ markedly by their cysteine content (0.8% and 13% respectively). The coelomic fluid exhibits a strong buffer capacity due to the high hemoglobin content (3 mM heme). In vitro, CO2 accumulation (up to 10-12 mM CO2 for PCO2 = 7.5 Torr) occurs with limited pH changes and is only partly accounted for by carbamino-Hb formation. The two hemoglobins exhibit high oxygen-affinities (P50 0.4 Torr for C1 and 0.9 Torr for C2, at 10 degrees C, pH 8) and a normal Bohr effect (phi values ranging from -0.54 and -0.37 at 10 degrees C, to -0.24 and -0.28 at 30 degrees C, for C1 and C2, respectively). Cooperativity values range from 0.8 to 1.9 for C1 and from 0.8 to 1.7 for C2. The temperature sensitivity of O2 affinity reflect deltaH values that decrease from -30 to -60 kJ x mol(-1) with increasing pH. C2 exhibits a slight specific effect of CO2 on oxygenation properties.

Oxygen binding and its allosteric control in hemoglobin of the pulmonate snail, Biomphalaria glabrata

Pulmonate snails that experience extreme variations in gas tensions and temperatures possess extracellular, high-molecular mass ( approximately 1.7 x 10(6) Da) hemoglobins (Hbs) that are little known as regards oxygenation and allosteric characteristics. Biomphalaria glabrata hemolymph exhibits a high O2 affinity (half-saturation O2 tension = 6.1 mmHg; pH 7.7, 25 degreesC), pronounced Bohr effect (Bohr factor = -0.5), and pH-dependent cooperativity (Hill's cooperativity coefficient at half-saturation = 1.1-2.0). Divalent cations increase O2 affinity, Ca2+ exerting greater effect than Mg2+. Analyses in terms of the Monod-Wyman-Changeux model indicate novel O2 affinity control mechanisms. In contrast to vertebrate Hb, where organic phosphates and protons lower affinity via decreased O2 association equilibrium constant of Hb in low-affinity state (KT), and to extracellular annelid Hbs, where protons and cations primarily modulate O2 association equilibrium constant of Hb in high-affinity state (KR), in B. glabrata Hb, the Bohr effect is mediated predominantly via KR and the cation effect via KT, reflecting preferential, oxygenation-linked proton binding to oxygenated Hb and cation binding to deoxygenated Hb. CO2 has no specific (pH independent) effect. Nonlinear van't Hoff plots show temperature dependence of the overall heats of oxygenation, indicating oxy-deoxy heat capacity differences. The findings are related to possible physiological significance in pond habitats.

Interaction between haemoglobin and synthetic peptides of the N-terminal cytoplasmic fragment of trout band 3 (AE1) protein

Two acidic peptides corresponding to the first 10 and 20 amino acid residues of the N-terminal, cytoplasmic fragment of rainbow trout band 3 (AE1) protein were synthesised in order to study their interaction with trout and human haemoglobin (Hb). The peptides did not influence the oxygen affinity of the main anodic trout Hb component (Hb IV) when tested at surplus peptide concentration ([peptide]/[Hb4]=16), at high and low ionic strength and at pH values ranging from 6.5 to 7.6. With human Hb, however, the 20-mer peptide markedly decreased the oxygen affinity and increased the Bohr effect. These data suggest that the trout band 3 peptide binds preferentially to the deoxy (T) conformation of human Hb, probably at the organic phosphate binding site in the central cavity between the beta-chains, which is known to be the binding site for the acidic N terminus of human band 3. In trout Hb IV, the presence of negatively charged Asp at position NA2 of the beta-chains (in contrast to positive or neutral residues in mammalian Hb) may weaken any interaction with the highly negatively charged peptides.

Hydrogen ion titrations of the anodic and cathodic haemoglobin components of the European eel Anguilla anguilla

H+ titrations were conducted on the separated haemoglobin components of eel Anguilla anguilla in both the oxygenated and deoxygenated states. In anodic haemoglobin, the addition of GTP, and to a lesser extent C1-, increased the magnitude of the Haldane effect and shifted its maximum value into the in vivo pH range. Of the 22 histidine residues in the anodic component, only approximately seven were titratable, presumably the beta-chain residues at positions 41, 97, 109 and 146 (helical positions C7, FG4, G11 and HC3, respectively). In cathodic haemoglobin, a small negative Haldane effect was observed at pH values between 6.8 and 8.5 which disappeared in the presence of GTP (molar ratio 3:1 GTP:haemoglobin tetramer). GTP had virtually no effect on the buffer value at fixed oxygenation status, and the lowest buffer value was observed at in vivo pH values. No titratable histidine residues were observed in the cathodic component, indicating that all 14 histidines in this component are buried. We conclude that the anodic component, which constitutes two-thirds of the haemoglobin in the eel, plays the predominant role in CO2 transport and pH homeostasis in vivo.

Characterization of the myoglobin and its coding gene of the mollusc Biomphalaria glabrata

A cDNA clone isolated from a Biomphalaria glabrata (Mollusca, Gastropoda) neural cDNA library was identified as encoding a myoglobin-like protein of 148 amino acids with a single domain and a calculated mass of 16,049.29. Alignment with globin sequences with known tertiary structure confirms its overall globin nature. The expressed myoglobin was identified in the radular muscle and isolated. Oxygen equilibrium measurements on the protein reveal a high oxygen affinity. Val-B10 and Gln-E7, important residues for the determination of the oxygen affinity, are strikingly different from the standard molluscan pattern (Conti, E., Moser, C., Rizzi, M., Mattevi, A., Lionetti, C., Coda, A., Ascenzi, P., Brunori, M., Bolognesi, M. (1993) J. Mol. Biol. 233, 498-508). The single gene encoding the globin chain is interrupted by three introns at positions A3.2, B12.2, and G7.0. Comparison with other nonvertebrate globin genes reveals on the one hand conservation (B12.2 and G7.0) and on the other hand variability of the insertion positions (A3.2). The Biomphalaria myoglobin sequence was used together with all other molluscan globin sequences available to assess the origin and phylogeny of the phylum. Our results confirm the doubts raised about monophyletic origin of the Mollusca, which was first observed using SSU rRNA as a molecular marker.

Primary structures of Arenicola marina isomyoglobins: molecular basis for functional heterogeneity

The primary structures of isomyoglobins MbI and MbII from the body wall musculature of the polychaete Arenicola marina were investigated, aiming to trace the molecular basis for their functional differentiation. Unexpectedly, five chains, MbIa, MbIb, MbIIa, MbIIb and MbIIc, each consisting of 145 amino-acid residues and occurring in a ratio of = 33:17:25:12.5:12.5 were found. All substitutions can be explained by one-point mutations. With the exception of the 41(C6)Asn-->Asp(MbI/MbII) exchange that appears to be the basis for the electrophoretic separation of MbI and MbII, the substitutions do not involve drastic changes in the character of the side-chains. Pairwise comparison of MbIa and MbIIa with other invertebrate globin chains indicate the following sequence of decreasing identities: Aplysia (mollusc) Mb, Chironomus (insect) CTT III hemoglobin, whale Mb and Ascaris (nematode) Mb. The marked difference in O2 affinities between MbI and MbII appears attributable to 62Pro which distorts the E helix around E6 and occurs in all MbII chains, but in only 33% of the MbI chains (MbIb).

A two-state analysis of co-operative oxygen binding in the three human embryonic haemoglobins

The binding of oxygen to the three human embryonic haemoglobins, at pH 7.4, has been shown to occur as a co-operative process. Analysis of oxygen-binding curves obtained in the absence of organic phosphate allosteric effectors shows that the process can be described quite accurately by the two-state model of allosteric action. In the presence of organic phosphates, the binding affinity for oxygen to the T-state of the alpha 2 epsilon 2 and zeta 2 epsilon 2 haemoglobins is significantly lowered. The values of the best-fit two-state parameters determined for each of the embryonic haemoglobins together with the temperature-dependence of the overall equilibrium binding process are discussed in terms of oxygen transfer from the maternal blood supply. Fast-reaction studies have been used to determine the rate constants of the oxygen association and dissociation processes occurring in the R-state and the rate of the allosteric R > T conformational transition. Analysis of these data suggests a likely reason for the high affinity and low co-operativity of the embryonic proteins and identifies the origins of the inability of equilibrium measurements to identify chain non-equivalence in the R-state.

The anodic hemoglobin of Anguilla anguilla. Molecular basis for allosteric effects in a root-effect hemoglobin

The functional and structural basis for the Root effect has been investigated in the anodic hemoglobin of the European eel, Anguilla anguilla. This hemoglobin exhibits a large Bohr effect, which is accounted for by oxygen-linked binding of seven to eight protons in the presence of GTP at pH 7.5. Oxygen equilibrium curves show nonlinear lower asymptote of Hill plots, indicating the occurrence of heme-heme interactions within the T state. Analysis of the curves according to the co-operon model (Brunori, M., Coletta, M., and Di Cera, E. (1986) Biophys. Chem. 23, 215-222) reveals that T state cooperativity is positive at high pH and in the stripped hemoglobin (where the T --> R allosteric transition is operative) and negative at low pH and in the presence of organic phosphate (where the molecule is locked in the low affinity structure), indicating site heterogeneity. The complete amino acid sequence of eel anodic hemoglobin has been established and compared with that of other fish hemoglobins. The presence of the Root effect correlates with a specific configuration of the alpha1beta2 switch interface, which at low pH would stabilize subunit ligation in the T state without changing the quaternary structure. We propose that the major groups involved in the binding of oxygen-linked protons in eel anodic hemoglobin are located on the beta chain and comprise His-HC3 at the C terminus, His-FG4 at the switch interface, and Lys-EF6 and the N terminus at the phosphate-binding site.

Oxygen binding and aggregation of hemoglobin from the common European frog, Rana temporaria

The hemoglobin from the European frog, Rana temporaria, consists of one major and three minor components. The tetramers aggregate upon deoxygenation notably at pH 7:3. Aggregation due to formation of disulphide bridges, as occurs in related species, was observed only in polyacrylamide gels. The hemolysate showed a pronounced Bohr effect. Oxygen affinity decreased with increasing hemoglobin concentration, indicating that aggregation affects the functional properties of the hemolysate. Oxygen binding equilibria of unfractionated hemolysate are insensitive to chloride and show low sensitivity to ATP. Analysis of oxygen equilibrium curves in terms of the two-state allosteric model (MWC) showed that pH change exerted a greater effect on the association constant of the oxygenated state (KR) than that of the deoxy state (KT). The number of interacting binding sites (q) increased with hemoglobin concentration. Cooperativity of oxygen binding, evaluated as Hill coefficient n, never exceeded the value of 3.0. Earlier studies on hemoglobin and blood from this and related species, report significantly higher n values at high O2 saturation. Molecular adaptive aspects are discussed.

Molecular shape, dissociation, and oxygen binding of the dodecamer subunit of Lumbricus terrestris hemoglobin

Small angle x-ray scattering of the 213-kDa dodecamer of Lumbricus terrestris Hb yielded radius of gyration = 3.74 +/- 0.01 nm, maximum diameter = 10.59 +/- 0.01 nm, and volume = 255 +/- 10 nm3, with no difference between the oxy and deoxy states. Sedimentation velocity studies indicate the dodecamer to have a spherical shape and concentration- and Ca2+-dependent equilibria with its constituent subunits, the disulfide-bonded trimer of chains a-c and chain d. Equilibrium sedimentation data were fitted best with a trimer-dodecamer model, ln K4 = 7 (association K in liters3/g3) at 1 degrees C and 4 at 25 degrees C, providing DeltaH = -20 kcal/mol and DeltaS = 4.4 eu/mol. Oxydodecamer dissociation at pH 8.0, in urea, GdmCl, heteropolytungstate K8[SiW11O39] and of metdodecamer at pH 7, was followed by gel filtration. Elution profiles were fitted with exponentially modified gaussians to represent the three peaks. Two exponentials were necessary to fit all the dissociations except in [SiW11O39]-8. Equilibrium oxygen binding measurements at pH 6.5-8. 5, provided P50 = 8.5, 11.5-11.9 and 11.9-13.5 torr, and n50 = 5.2-9. 5, 3.2-4.9, and 1.8-2.7 for blood, Hb, and dodecamer, respectively, at pH 7.5, 25 degrees C. P50 was decreased 3- and 2-fold in approximately 100 mM Ca2+ and Mg2+, respectively, with concomitant but smaller increases in cooperativity.

The crystal structures of trout Hb I in the deoxy and carbonmonoxy forms

We have determined the X-ray crystallographic structure of trout Hb I in both the deoxy and carbonmonoxy forms to resolution limits of 2.3 angstroms and 2.5 angstroms, respectively. The overall fold of the molecule is highly similar to that of human HbA despite the low level of sequence identity between these proteins. Trout Hb I is unusual in displaying almost no pH dependence of oxygen binding affinity, and (at most) very weak interactions with heterotropic effector ligands such as organic phosphates. Comparison of the two quaternary states of the protein indicates how such effects are minimised and how the low-affinity T state of the protein is stabilised in the absence of heterotropic interactions.

Oxygen equilibria of cathodic eel hemoglobin analysed in terms of the MWC model and Adair's successive oxygenation theory

Allosteric effects of erythrocytic NTP and proton concentrations on cathodic eel Hb were investigated by precise measurement of Hb-O2 equilibria (including extreme saturation values) and analysis in terms of the MWC two-state model and the Adair four-step oxygenation theory. Stripped cathodic Hb shows a reverse Bohr effect and high sensitivities to ATP and GTP that extend to high pH values (> 8.5). A decrease in pH raises KT and lowers the allosteric constant L, compared to opposite effects in 'normal' Bohr effect Hbs. Phosphates even at low concentrations (GTP/Hb = 0.5) annihilate the reverse Bohr effect. GTP exerts a greater effect than ATP due to greater changes in KT and L, and NTP slightly reduces KR. In the absence of NTP, about 1.1 protons are released on deoxygenation at pH 8.15 (where most protons are released), indicating a pK value of the reverse Bohr group of approximately 8.2 (higher in oxy-Hb and lower in deoxy-Hb). The pH and NTP dependence of the Adair association constants and calculated fractional populations of Hb molecules in different oxygenation stages show that NTP effectors stabilise the T structure and postpone the T-R transition, whereas protons in the absence of NTP have the opposite effect. A molecular mechanism for the reverse Bohr effect is suggested.

Spectroscopical and functional characterization of the hemoglobin of Nostoc commune (UTEX 584 (Cyanobacterial)

Structural analysis of a monomeric hemoglobin from the cyanobacterium Nostoc commune strain UTEX 584, cyanoglobin (Potts et al. (1992) Science 256, 1690-1692), is presented. Cyanoglobin binds molecular oxygen reversibly, with high oxygen affinity and non-cooperativity. There was no evidence for decreased stability of the pigment at 37 degrees C. Cyanoglobin-specific antibodies showed no cross-reactivity with two reference hemoglobins, leghemoglobin a and sperm whale myoglobin. The absorption spectral properties of cyanoglobin differ significantly from those of the two reference hemoglobins. The spectrum of oxy-cyanoglobin most closely resembles that of an oxy-hemoglobin from the protozoan Tetrahymena pyriformis, a hemoprotein that shares substantial amino-acid sequence identity with cyanoglobin. Met-cyanoglobin possesses spectral characteristics at pH 7.0-9.0 that resemble those of the alkaline met-hemoglobin (a putative hemichrome) of another protozoan, Paramecium caudatum. The spin-state character of met-cyanoglobin is pH-dependent. Met-cyanoglobin does not coordinate the strong-field ligands, cyanide and azide, at pH 7.0. The capacity of cyanoglobin to coordinate cyanide increased with decreasing pH. Far-UV CD spectra of cyanoglobin are indicative of a protein with a significant amount of alpha-helical structure. Data from Soret-region CD spectra suggest that the orientations of the heme moieties in cyanoglobin and leghemoglobin a are similar to one another.

Mass spectrometric composition, molecular mass and oxygen binding of Macrobdella decora hemoglobin and its tetramer and monomer subunits

The hexagonal bilayer hemoglobin (Hb) of the leech Macrobdella decora has an equilibrium sedimentation mass of 3544(+/- 80) kDa. Maximum entropy analysis of the electrospray ionization mass spectra of the Hb show three groups of peaks: two peaks of equal intensity at approximately 17 kDa, A (16,770.1 Da) and B (16,841.9 Da); three peaks at approximately 24 kDa, C (24,340.1 Da), D (24,398.6 Da) and E (24,420.0 Da) with relative intensities of 1:6:3, respectively; and three peaks of equal intensities at approximately 33 kDa, F (32,586.0 Da), G (32,714.5 Da) and H (32,849.9 Da). Although reduction with dithiothreitol does not affect the masses of peaks A through E, the approximately 33 kDa peaks give rise to four new peaks at approximately 16 kDa, P (16,052.2 Da), Q (16,537.3 Da), R (16,666.7 Da) and S (16,792.9 Da), indicating that F, G and H represent disulfide-bonded dimers of globin chains, P + Q, P + R and P + S, respectively. The relative intensities of the three groups of peaks are (A + B) to (C + D + E) to (F + G + H) = 0.39:0.26:0.32, and the globin to linker ratio 0.71:0.29 is in good agreement with the ratio 0.72:0.28 obtained by HPLC. The largest functional subunit obtained by dissociation at pH 7 in 4 M urea, is a subunit lacking linker chains with apparent mass 63(+/- 3) kDa. The equilibrium sedimentation profile of this subunit is fitted best as a monomer-dimer-tetramer equilibrium, with association constants K1,2 = 365 l g-1 and K1,4 = 8.1 x 10(5) l3 g-3. A model of the Hb consisting of a hexagonal bilayer of 36 tetramer and 42 linker subunits provides a total mass and globin to linker ratio closest to the experimental values. Equilibrium O2 binding measurements of the native Hb and its tetramer and monomer subunits were carried out over the pH range 6.6 to 8.0 at 10 and 25 degrees C, and in the absence and presence of Na+, Mg2+ and Ca2+. The Hb exhibits a moderately high O2 affinity, P50 = 4.4 torr at pH 7.5 and 25 degrees C, a high cooperativity (n50 approximately 3) and a substantial Bohr effect, phi = delta log P50/delta pH = -0.38. The tetramer subunit has a higher affinity, lower cooperativity and smaller Bohr effect, 1.9 torr, 1.3 to 1.5 and -0.30, respectively. The monomer subunit has a much higher affinity (P50 = 0.29 torr) and no cooperativity or Bohr effect.(ABSTRACT TRUNCATED AT 400 WORDS)

The cathodic hemoglobin of Anguilla anguilla. Amino acid sequence and oxygen equilibria of a reverse Bohr effect hemoglobin with high oxygen affinity and high phosphate sensitivity

As in other fish, the cathodic hemoglobin of the eel Anguilla anguilla is considered to play an important role in oxygen transport under hypoxic and acidotic conditions. In the absence of phosphates this hemoglobin shows a reverse Bohr effect and high oxygen affinity, which is strongly modulated over a side pH range by GTP (whose concentration in the red blood cells varies with ambient oxygen availability). GTP obliterates the reverse Bohr effects in the cathodic hemoglobin. The molecular basis for the reverse Bohr effect in fish hemoglobins has remained obscure due to the lack of structural data. We have determined the complete amino acid sequence of the alpha and beta chains of the cathodic hemoglobins of A. anguilla and relate it to the oxygen equilibrium characteristics. Several substitutions in crucial positions are observed compared with other hemoglobins, such as the replacement of the C-terminal His of the beta chain of Phe (that suppresses the alkaline Bohr effect) and of residues at the switch region between alpha and beta subunits (that may alter the allosteric equilibrium, thus causing the high intrinsic oxygen affinity and low cooperativity). The residues binding organic phosphate in the beta cleft of fish hemoglobins are conserved, which explains the strong effect of GTP on oxygen affinity and suggests that these residues contribute to the reverse Bohr effect in the absence of alkaline Bohr groups. Moreover, His beta 143 that is considered to be responsible for the reverse Bohr effect in human and tadpole Hbs is replaced by Lys.

The hemoglobin system of the hagfish Myxine glutinosa: aggregation state and functional properties

Hemoglobin (Hb) from the hagfish Myxine glutinosa is composed of six major monomeric subunits. Some of these subunits aggregate to dimers at low pH, and to tetramers when deoxygenated and at high protein concentration. The aggregation is inhibited by the presence of KCl. Oxygen equilibrium studies show the presence of a small Bohr effect which is strongly reduced by KCl, indicating that it originates from pH-dependent aggregation. ATP and DPG cause a similar decrease in the Bohr effect. O2 affinity is dependent on protein concentration, temperature and presence of CO2. Cooperativity is practically absent. O2 binding properties of the separated aggregating and non-aggregating Hbs purified at low pH cannot account for the functional properties of the composite hemolysate, suggesting the presence of other subunits interactions. The results are discussed in relation to literature data for other cyclostome Hbs and for M. glutinosa Hb, where the presence of three major monomeric Hbs and a possible CO2-dependent aggregation had been reported.

Hemoglobins from Plasmodium-infected rat erythrocytes: functional and molecular characteristics

Aiming to evaluate the mechanisms responsible for altered O2-transporting properties in blood of Plasmodium-infected animals, stripped (cofactor-free) hemoglobin (Hb) solutions were prepared from infected erythrocytes (IE) and noninfected erythrocytes (NIE) of rats inoculated with Plasmodium berghei bergei for functional and structural characterization. At normal intraerythrocytic pH (+/- 7.2), Hb from IE showed a higher affinity, a larger Bohr effect, and lower sensitivities to 2,3-diphosphoglycerate (DPG) and to temperature than did NIE Hb. Moreover, as judged from electrophoresis, isoelectric focusing, and gel filtration experiments, Hb from IE show changes in charge and molecular assembly. The results indicate that the higher O2 affinity and greater Bohr factors observed in IE compared with those for NIE are attributable to chemical modification of the Hb that increases its intrinsic O2 affinity and decreases its sensitivity to DPG as well as to changes in the intracellular physicochemical milieu, including reduced DPG levels.

Purification and characterization of recombinant polymeric hemoglobin P1 of Glycera dibranchiata

The apoprotein of component P1 of the polymeric fraction of the intracellular hemoglobin of the marine polychaete Glycera dibranchiata has been expressed at a high level in Escherichia coli. The expressed globin was reconstituted with heme and purified. The N-terminal sequence of the recombinant P1 is identical to the cDNA-derived sequence of cloned P1 (Zafar et al., Biochem. Biophys. Acta, 1041, 117-123, 1990). Gel filtration, SDS-PAGE, optical spectra over the range 200-650 nm, and circular dichroism over the range 200-250 nm of the purified recombinant P1 were very similar to the polymeric fraction of native Glycera hemoglobin. The molar ellipticity at 222 nm provided an estimate of 77% for the alpha-helical content of the recombinant P1, in excellent agreement with that calculated from the crystal structure of Glycera monomeric component M-II. Although the oxygen binding affinity of the recombinant P1 is higher than that of the polymeric fraction of Glycera hemoglobin (3-4 torr vs 7-13 torr), which consists of at least six different single-chain hemoglobins, the Hill coefficient is lower (1.0-1.2 vs 1.2-1.4).

Mutant hemoglobins (alpha 119-Ala and beta 55-Ser): functions related to high-altitude respiration in geese

The unusually high blood-O2 affinity in the bar-headed and Andean geese is a necessary adaptation for migration across high mountain ranges. The amino acid residues alpha-119 and beta-55, which form an alpha 1 beta 1 contact in human hemoglobin (Hb), are altered in bar-headed and Andean geese, respectively, which suggests that loss of this contact increases O2 affinity. Two mutant human Hbs with equivalent mutations at these sites prepared by site-directed mutagenesis show the same increase in O2 affinity compared with Hb A, which indicates that these mutations are responsible for the changes in the protein. The intrinsic affinity difference compared with native Hb A is amplified by organic phosphates. Whereas the recombinant and native Hbs displayed similar sensitivities to pH, chloride, and 2,3-diphosphoglycerate, the oxygenation heat of the alpha-chain mutant decreased in the presence of 2,3-diphosphoglycerate. O2 association constants for the deoxygenated state of the alpha-mutant were about three times those for Hb A. The mutant Hb analogously exhibited higher affinity constants for binding the first three O2 molecules. Calculated heme-heme interaction energies indicated that loss of a single contact, resulting in destabilization of the deoxy (tense) structure, underlies the increased O2 affinity. Adaptations securing Hb-O2 binding at extreme altitude are discussed.

Arterial blood gas tensions during breath-hold diving in the Korean ama

Korean female unassisted divers (cachido ama) breath-hold dive > 100 times to depths of 3-7 m during a work day. We sought to determine the extent of arterial hypoxemia during normal working dives and reasonable time limits for breath-hold diving by measuring radial artery blood gas tensions and pH in five cachido ama who dove to a fixed depth of 4-5 m and then continued to breath hold for various times after their return to the surface. Eighty-two blood samples were withdrawn from indwelling radial artery catheters during 37 ocean dives. We measured compression hyperoxia [arterial PO2 = 141 +/- 24 (SD) Torr] and hypercapnia (arterial PCO2 = 46.6 +/- 2.4 Torr) at depth. Mean arterial PO2 near the end of breath-hold dives lasting 32-95 s (62 +/- 14 s) was decreased (62.6 +/- 13.5 Torr). Mean arterial PCO2 reached 49.9 +/- 5.4 Torr. Complete return of these values to their baseline did not occur until 15-20 s after breathing was resumed. In dives of usual working duration (< 30 s), blood gas tensions remained within normal ranges. Detailed analysis of hemoglobin components and intrinsic oxygenation properties revealed no evidence for adaptive changes that could increase the tolerance of the ama to hypoxic or hypothermic conditions associated with repetitive diving.