Studies on avian erythrocyte metabolism. Inositol tetrakisphosphate: the major phosphate compound in the erythrocytes of the ostrich (Struthio camelus camelus)

Structural studies of hemoglobin from two flightless birds, ostrich and turkey: insights into their differing oxygen-binding properties

Crystal structures of hemoglobin (Hb) from two flightless birds, ostrich (Struthio camelus) and turkey (Meleagris gallopova), were determined. The ostrich Hb structure was solved to a resolution of 2.22 Å, whereas two forms of turkey Hb were solved to resolutions of 1.66 Å (turkey monoclinic structure; TMS) and 1.39 Å (turkey orthorhombic structure; TOS). Comparison of the amino-acid sequences of ostrich and turkey Hb with those from other avian species revealed no difference in the number of charged residues, but variations were observed in the numbers of hydrophobic and polar residues. Amino-acid-composition-based computation of various physical parameters, in particular their lower inverse transition temperatures and higher average hydrophobicities, indicated that the structures of ostrich and turkey Hb are likely to be highly ordered when compared with other avian Hbs. From the crystal structure analysis, the liganded state of ostrich Hb was confirmed by the presence of an oxygen molecule between the Fe atom and the proximal histidine residue in all four heme regions. In turkey Hb (both TMS and TOS), a water molecule was bound instead of an oxygen molecule in all four heme regions, thus confirming that they assumed the aqua-met form. Analysis of tertiary- and quaternary-structural features led to the conclusion that ostrich oxy Hb and turkey aqua-met Hb adopt the R-/R_H-state conformation.

Parakeet Hemoglobin - Its Crystal Structure and Oxygen Affinity in Relation to Some Avian Hemoglobins

BACKGROUND

"Avians" often show efficient oxygen management to meet the demands of their metabolism. Hemoglobin, a transporter protein consists of four non-covalently linked subunits contain haem binding hydrophobic pocket serves as a site of allosteric cooperativity. The physiology and anatomy of both mammals and avian are functionally different, in birds, the respiratory system formed by small air sacs that serve as tidal ventilation for the lungs and have no significant exchange across their cells. Parakeet (Psittacula krameri) a tropical and non-migrating species and it is easily adapted to living in disturbed habitat. The sequence analysis reveals that α and β chain of parakeet hemoglobin highly similar grey lag goose and bar headed goose hemoglobin respectively. Thus it has been tempted us to study in to analyzing the sequence and structural comparison of this hemoglobin to find out the physiological capabilities of parakeet hemoglobin.

OBJECTIVE

The structure determination studies of parakeet hemoglobin by X-ray diffraction. The sequence and structure are compared with goose, chicken and human Hb, emphasizing the role of amino acids in the subunit contacts that facilitate survival by low oxygen demand.

METHODS

The Hb was purified and crystallized by hanging drop vapor diffusion method using poly ethylene glycol (PEG) 3350 and sodium phosphate buffer. X-ray diffracted data set was collected at 3Å resolution, the data was processed in Automar and molecular replacement, refinements, model building was carried out in CCP4i program package. The final refined model was deposited in protein data bank with accession id 2zfb.

RESULTS

The tertiary structure of Parakeet Hb is compared with the met form of BHG Hb (1c40) and oxy form of GLG (1faw) and oxy form of human Hbs (1hho). Superimposing parakeet Hb α1β1 subunit with 'R' state human Hb shows an r.m.s.d of 0.98 Å and for BHG and GLG Hb, the r.m.s.d shows 0.72 and 0.61 Å. The replacement of α115Asp in parakeet Hb as against the α115Glu in human Hb results in the movement of GH corners. The amino acid proline at α50 present only in Parakeet Hb and Chicken HbD and not present in any other avian family which includes human Hb. The residue α78Thr located in EF corner loop region, which slightly diverge when superimposing with human and BHG Hb and also replacement of α113Asn present only in Parakeet Hb placed near the FG helix corner.

CONCLUSION

The present study describes the structure determination of parakeet hemoglobin and its structural features to understand its oxygen affinity characteristics. The crystals were obtained by buffered low-salt conditions, like those of chicken HbD, carbonmonoxy and cyanomet human Hb. The present study reveals several interesting and unique modifications in the finer aspects of the quaternary structure of parakeet Hb, which are involved in oxygen affinity characteristics and the α1β1 subunit contacts. Crystallization of parakeet Hb with allosteric effectors like Inositol pentaphosphate may bring further understanding of the influence of physiological and environmental factors on the quaternary structure.

A mathematical model for pressure-based organs behaving as biological pressure vessels

We introduce a mathematical model that describes the allometry of physical characteristics of hollow organs behaving as pressure vessels based on the physics of ideal pressure vessels. The model was validated by studying parameters such as body and organ mass, systolic and diastolic pressures, internal and external dimensions, pressurization energy and organ energy output measurements of pressure-based organs in a wide range of mammals and birds. Seven rules were derived that govern amongst others, lack of size efficiency on scaling to larger organ sizes, matching organ size in the same species, equal relative efficiency in pressurization energy across species and direct size matching between organ mass and mass of contents. The lung, heart and bladder follow these predicted theoretical relationships with a similar relative efficiency across various mammalian and avian species; an exception is cardiac output in mammals with a mass exceeding 10 kg. This may limit massive body size in mammals, breaking Cope's rule that populations evolve to increase in body size over time. Such a limit was not found in large flightless birds exceeding 100 kg, leading to speculation about unlimited dinosaur size should dinosaurs carry avian-like cardiac characteristics.

Purification, crystallization and preliminary X-ray analysis of haemoglobin from ostrich (Struthio camelus)

Haemoglobin is a tetrameric protein that carries oxygen from the lungs to tissues and carbon dioxide from tissues back to the lungs. The oxygen-binding properties of haemoglobin are regulated through the binding of allosteric effectors. The respiratory system of avian species is unique and complex in nature when compared with that of mammals. In avian species, inositol pentaphosphate (inositol-P(5)) is present in the erythrocytes of the adult and is thought to be the major factor responsible for the relatively high oxygen affinity of the whole blood. The ostrich (Struthio camelus) is a large flightless bird which contains inositol tetrakisphosphate (inositol-P(4)) in its erythrocytes and its whole blood oxygen affinity is higher. Efforts have been made to explore the structure-function relationship of ostrich haemoglobin. Ostrich haemoglobin was purified using ion-exchange chromatography. Haemoglobin crystals were grown by the hanging-drop vapour-diffusion method using PEG 3350 as the precipitant in 50 mM phosphate buffer pH 7.2. Data were collected using a MAR345 image-plate detector system. The crystals of ostrich haemoglobin diffracted to 2.2 A resolution. They belonged to the orthorhombic space group P2(1)2(1)2(1) with one whole biological molecule in the asymmetric unit; the unit-cell parameters were a = 80.93, b = 81.68, c = 102.05 A.

High-altitude adaptations in vertebrate hemoglobins

Vertebrates at high altitude are subjected to hypoxic conditions that challenge aerobic metabolism. O(2) transport from the respiratory surfaces to tissues requires matching between the O(2) loading and unloading tensions and the O(2)-affinity of blood, which is an integrated function of hemoglobin's intrinsic O(2)-affinity and its allosteric interaction with cellular effectors (organic phosphates, protons and chloride). Whereas short-term altitudinal adaptations predominantly involve adjustments in allosteric interactions, long-term, genetically-coded adaptations typically involve changes in the structure of the haemoglobin molecules. The latter commonly comprise substitutions of amino acid residues at the effector binding sites, the heme-protein contacts, or at intersubunit contacts that stabilize either the low-affinity ('Tense') or the high-affinity ('Relaxed') structures of the molecules. Molecular heterogeneity (multiple isoHbs with differentiated oxygenation properties) can further broaden the range of physico-chemical conditions where Hb functions under altitudinal hypoxia. This treatise reviews the molecular and cellular mechanisms that adapt haemoglobin-oxygen affinities in mammals, birds and ectothermic vertebrates at high altitude.

Functional characterization of the single hemoglobin of the migratory bird Ciconia ciconia

Hemolysate from white stork displayed a single hemoglobin component, thus resulting into two bands and two globin peaks in dissociating PAGE and reversed phase-HPLC, respectively. Stripped hemoglobin showed an oxygen affinity higher than that of human HbA, a small Bohr effect, and a cooperative oxygen binding. A small decrease of oxygen affinity, of the same extent in all the pH range examined, was observed by addition of chloride, thus indicating an unusual chloride-independent Bohr effect (DeltalogP50/Deltalog pH=-0.24). Saturating amounts of inositol hexakisphosphate, largely decreased hemoglobin-oxygen affinity (DeltalogP(50)=1.17 at pH 7.0), and increased the extent of its Bohr effect (DeltalogP50/DeltalogpH=-0.45). The phosphate binding curve allowed to measure a very high overall binding constant (K=1.18 x 10(5) M(-1)). The effect of temperature on the oxygen affinity was measured, and the enthalpy change of oxygenation resulted almost independent on pH. Structural-functional relationships are discussed by considering some amino acid residues situated at alpha1/beta1 and alpha1/beta2 interfaces, such as alpha38 and alpha89 positions. The presence of only one hemoglobin component, a rare event among birds, and its functional properties have been related to the physiological oxygen requirements of this soaring migrant bird and to its technique of flight during migration.

The Bohr effect of haemoglobin in vertebrates: an example of molecular adaptation to different physiological requirements

The Bohr effect, i.e. the pH dependence of the oxygen affinity of haemoglobins (Hbs) from a variety of vertebrates, and its modulation by temperature and other heterotropic effectors has been reviewed. Haemoglobins from vertebrates were not reviewed following the usual classification (i.e. mammals, birds, etc.); instead we have selected several key examples of animals, which are confronted with a similar environmental situation therefore displaying a similar life style. Hence, the paper starts from a description of the general concepts at the basis of the Bohr effect as exemplified by human HbA and goes towards the analysis of the modulation mechanisms which have been observed in different animals in response to the needs induced by: (i) life in cold environments; (ii) diving behaviour; (iii) flight; and (iv) aquatic life. The emerging picture indicates a complex organization of the information contained in the Hb molecule, the oxygen-binding properties of which depend both on the intrinsic characteristics of the protein and on its heterotropic interactions with ligands such as protons (Bohr effect), small anions like chloride and organic phosphates. In addition, each one of the functional effects induced by binding of a given effector appears to be under the strict control of temperature that enhances or decreases its relative weight with respect to all the others. It is just by this sophisticated network of interactions that the Hb molecule is able to satisfy the physiological requirements of a multitude of organisms without changing dramatically its quaternary structure.

Inositol monophosphatase is a highly conserved enzyme having localized structural similarity to both glycerol 3-phosphate dehydrogenase and haemoglobin

The cDNA coding for an inositol monophosphatase in the oocytes of the African clawed frog, Xenopus laevis, has been isolated and sequenced. The predicted primary structure of this enzyme is markedly conserved when it is compared with its mammalian functional homologues; up to 84% of the amino acid residues are identical, and conservative substitutions increase the similarity to 95%, suggesting that this sequence represents the most parsimonious primary structure for the protein to maintain not only catalytic activity but also perhaps the facility to interact with other macromolecules. Two regions of the protein, each of about 11 residues and separated by about 90 residues, have been identified as a consensus found also in glycerol 3-phosphate dehydrogenase (EC 1.1.1.8). One of these regions is also found to be particularly conserved in the alpha-globin of birds and reptiles; birds and some turtles are known to modulate the oxygen affinity of their haemoglobin with inositol polyphosphate in the same way as with 2,3-bisphosphoglycerate in other species. This region is also conserved in the beta-globin of most species, beginning with lysine-82, which is known to participate in the binding of organic phosphates. These regions of the inositol monophosphatase may represent motifs for the binding of its substrate.

The quantitative spectrum of inositol phosphate metabolites in avian erythrocytes, analysed by proton n.m.r. and h.p.l.c. with direct isomer detection

The spectrum of inositol phosphate isomers present in avian erythrocytes was investigated in qualitative and quantitative terms. Inositol phosphates were isolated in micromolar quantities from turkey blood by anion-exchange chromatography on Q-Sepharose and subjected to proton n.m.r. and h.p.l.c. analysis. We employed a h.p.l.c. technique with a novel, recently described complexometric post-column detection system, called 'metal-dye detection' [Mayr (1988) Biochem. J. 254, 585-591], which enabled us to identify non-radioactively labelled inositol phosphate isomers and to determine their masses. The results indicate that avian erythrocytes contain the same inositol phosphate isomers as mammalian cells. Denoted by the 'lowest-locant rule' [NC-IUB Recommendations (1988) Biochem. J. 258, 1-2] irrespective of true enantiomerism, these are Ins(1,4)P2, Ins(1,6)P2, Ins(1,3,4)P3, Ins(1,4,5)P3, Ins(1,3,4,5)P4, Ins(1,3,4,6)P4, Ins(1,4,5,6)P4, Ins(1,3,4,5,6)P5, and InsP6. Furthermore, we identified two inositol trisphosphate isomers hitherto not described for mammalian cells, namely Ins(1,5,6)P3 and Ins(2,4,5)P3. The possible position of these two isomers in inositol phosphate metabolism and implications resulting from absolute abundances of inositol phosphates are discussed.

Studies on avian erythrocyte metabolism. XVII. Kinetics and transport properties of myo-inositol in chicken reticulocytes

The uptake of myo-inositol was determined in a reticulocyte-enriched fraction prepared from chicken blood and compared with uptake in mature erythrocytes. While reticulocytes accumulated inositol at levels more than threefold that of the plasma concentration, erythrocyte levels were only slightly higher than that of the plasma concentration. The rate of uptake in reticulocytes was approximately 66 mumol/ml rbc/h compared to 5 mumol/ml rbc/h in mature erythrocytes when measured at an inositol medium concentration of 250 microM. The kinetic analysis of inositol influx by reticulocytes reveals a two component system: saturable and nonsaturable. The saturable component, which has a Km for inositol of approximately 222 microM, is Na-dependent. This Na-dependent saturable component, which presumably reflects active transport of inositol, accounts for 30-35% of the transport process. The saturable component is completely inhibited by amiloride but to a lesser extent by ouabain and bumetanide. Moreover, in the course of reticulocyte maturation, the saturable component is lost concomitantly with the completion of the synthesis of myo-inositol pentakisphosphate and the drastic decrease in the membrane permeability to inositol. In addition, phloretin and cytochalasin B, which bind to hexose carriers and inhibit hexose sugar transport, also inhibited inositol transport. The uptake of inositol was not affected by excesses of 3-O-methylglucose (100 mM) or by physiological concentrations of D-glucose. Thus, the transport mechanism of myo-inositol appears distinct from that of D-glucose.

Inositol phosphates: proliferation, metabolism and function

After the initial discovery of receptor-linked generation of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) it was generally assumed that Ins(1,4,5)P3 and its proposed breakdown products inositol(1,4)bisphosphate (Ins(1,4)P2) and Ins1P, along with cyclic inositol monophosphate, were the only inositol phosphates found in significant amounts in animal cells. Since then, three levels of complexity have been introduced. Firstly, Ins(1,4,5)P3 can be phosphorylated to Ins(1,3,4,5)P4, and the subsequent metabolism of these two compounds has been found to be intricate and probably different between various tissues. The functions of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 are almost certainly to regulate cytosolic Ca2+ concentrations, but the reasons for the labyrinth of the metabolic pathways after their deactivation by a specific 5-phosphatase remain obscure. Secondly, inositol pentakis- and hexakisphosphates have been found in many animal cells other than avian erythrocytes. It has been shown that their synthesis pathway is entirely separate from the inositol phosphates discussed above, both in terms of many of the isomers involved and probably in the subcellular localization; some possible functions of InsP5 and InsP6 are discussed here. Thirdly, cyclic inositol polyphosphates have been reported in stimulated tissues; the evidence for their occurrence in vivo and their possible physiological significance are also discussed.

Rightshifted dextran-hemoglobin as blood substitute

Covalent modification of hemoglobin and dextran-hemoglobin by reductive alkylation under aerobic conditions with periodated inositol tetrakisphosphate brought about a rightshifting of their oxygen dissociation curves. The rightshifted dextran-hemoglobin conjugate provided adequate oxygen delivery in macaques exchange-transfused to 90% with respect to their erythrocyte content. The conjugate did not exhibit the nephrotoxicity of free hemoglobin. It was also more resistant to precipitation, and therefore more amenable to viral sterilisation, by ethanol.

Studies on avian erythrocyte metabolism. XVI. Accumulation of 2,3-bisphosphoglycerate with shifts in oxygen affinity of chicken erythrocytes

The ability of the chicken erythrocyte to accumulate 2,3-bisphosphoglycerate (2,3-P2-glycerate) and its effect upon the oxygen affinity (P50) of the cell suspensions have been determined. Erythrocytes from chick embryos, which contain 4-6 mM 2,3-P2-glycerate, and from chickens at various ages, which contain 3-4 mM inositol pentakisphosphate but no 2,3-P2-glycerate, were incubated with inosine, pyruvate, and inorganic phosphate. Red blood cells from 20-day chick embryos incubated in Krebs-Ringer, pH 7.45, containing 20 mM inosine and 20 mM pyruvate had an increase in 2,3-P2-glycerate from 4.3 to 11.9 mM after 4 h. Importantly, as 2,3-P2-glycerate concentration increased there was a corresponding increase in P50 of the cell suspension. Further, erythrocytes from 9- and 11-week, and 7-, 14-, 24-, and 28-month-old chickens when incubated similarly with inosine and pyruvate accumulated 2,3-P2-glycerate with corresponding increases in P50 of the cell suspensions. The ability of the red cell to accumulate this compound under the incubation conditions used apparently decreases with age of the bird (e.g., 11.9 mM in the 20-day embryo to 1.1 mM in the 28-month-old chicken after 4 h incubation). Despite the presence of significant amounts of inositol-P5, the accumulation of 2,3-P2-glycerate markedly decreases oxygen affinity of the cell suspensions. The delta P50/mumol increase in 2,3-P2-glycerate in the red cells of the 20-day chick embryo after 4 h incubation is 1.5 Torr; conversely, the delta P50/mumol decrease in 2,3-P2-glycerate in the red cells of the 17-day embryo after 6 h incubation in the presence of sodium bisulfite is 2.8 Torr. The demonstrated ability of the chicken erythrocyte to accumulate 2,3-P2-glycerate in response to certain substrates suggests that regulation of concentration of this compound could contribute significantly to regulation of blood oxygen affinity in birds.

Perturbation of the human T-cell antigen receptor-T3 complex leads to the production of inositol tetrakisphosphate: evidence for conversion from inositol trisphosphate

Antibodies directed against the T-cell antigen receptor-T3 complex mimic antigen and lead to cellular changes consistent with activation. When cells of the human T-cell line Jurkat were stimulated with a monoclonal antibody directed against T3, inositol phosphates were produced. In addition to inositol trisphosphate, which is the product of phosphatidylinositol bisphosphate cleavage, a second inositol polyphosphate was formed. This compound was more polar than inositol trisphosphate but less polar than inositol pentakisphosphate. It cochromatographed with inositol tetrakisphosphate from ostrich erythrocytes. In permeabilized Jurkat cells, this compound was shown to be formed from inositol 1,4,5-trisphosphate, but only in the presence of ATP, and 32P was incorporated into it from [gamma-32P]ATP. There also was coincident formation of inositol 1,3,4-trisphosphate. We conclude that the more polar compound is inositol tetrakisphosphate, which is formed by phosphorylation of inositol 1,4,5-trisphosphate and may be the precursor of inositol 1,3,4-trisphosphate.

Erythrocytic organic phosphates: diel and seasonal cycles in the frog, Rana berlandieri

Hemoglobin (Hb) concentration, hematocrit (Hct), total organic phosphates (Ptot), nucleoside triphosphates (NTP), 2,3-diphosphoglycerate (DPG), and "inositol polyphosphates" (IPP) were measured in the erythrocytes of adult frogs at different times of day in winter and summer after acclimatization to 15 degrees C and an LD 12:12 photoperiod. The same measurements were also made on animals acclimated to LD 16:8 in summer. All of the measured parameters varied significantly at different times of the day and between seasons in animals acclimated to an LD 12:12 photoperiod. Summer animals acclimated to LD 16:8 showed significant diel cycles only in Hct and Hb.

Regulation of hemoglobin function and whole blood oxygen affinity by carbon dioxide and pH in the loggerhead (Caretta caretta) and green sea turtle (Chelonia mydas mydas)

The oxygen affinity of suspensions of erythrocytes from juvenile and adult loggerhead (Caretta caretta) and green sea (Chelonia mydas mydas) turtles decreased markedly with increasing concentrations of carbon dioxide (0 to near 15%) or hydrogen ion (pH 7.6 to pH 7.2). The P50's were higher with increases in pCO2, particularly at pH near 7.4, than were the P50's with increases in hydrogen ion concentration at any given CO2 concentration. Solutions of hemoglobins from the juvenile loggerhead (8-9 mos.) and green sea (10 mos.) turtles responded to 2, 3-DPG, ATP, or inositol-P5 when added at molar ratios of phosphate to hemoglobin of 4:1 and 20:1 in 0% and 6.29% CO2 but showed no decrease in oxygen affinity at these two CO2 levels in the green turtle when the molar ratio of phosphate to hemoglobin was 0.4. These compounds had little effect on the P50 of these hemoglobins in 14.6% CO2. The P50 of the adult loggerhead turtle hemoglobin did not increase in the presence of organic phosphates beyond the effect induced by CO2 alone. The P50 of hemoglobin from the adult green sea turtle increased only slightly when the molar ratio of phosphate to hemoglobin was 20:1 and at 0 and 6% CO2 concentration; little effect was observed at 14.6% CO2. These data demonstrate that blood oxygen affinities and hemoglobin function in these two species of marine turtles are altered significantly by CO2 and to a lesser degree by pH. It is suggested that such alterations may be of significance in vivo during prolonged diving when there are dramatic rises in blood pCO2 and [H+] and profound decreases in pO2.

The nutritional significance, metabolism, and function of myo-inositol and phosphatidylinositol in health and disease

Recent advances in nutritional and biochemical research have substantiated the importance of inositol as a dietary and cellular constituent. The processes involved in the metabolism of inositol and its derivatives in mammalian tissues have been characterized both in vivo and at the enzyme level. Biochemical functions elucidated for phosphatidylinositol in biological membranes include the mediation of cellular responses to external stimuli, nerve transmission, and the regulation of enzyme activity through specific interactions with various proteins. Inositol deficiency in animals has been shown to produce an accumulation of triglyceride in liver, intestinal lipodystrophy, and other abnormalities. The metabolic mechanisms giving rise to these latter phenomena have been extensively studied as a function of dietary inositol. Altered metabolism of inositol has been documented in patients with diabetes mellitus, chronic renal failure, galactosemia, and multiple sclerosis. A moderate increase in plasma and nerve inositol levels by dietary supplementation has been suggested as a means of treating diabetic neuropathy, although excessively high levels, such as are found in uremic patients, may be neurotoxic. A thorough consideration of the biochemical functions of inositol and a further characterization of various diseases with the aid of appropriate animal models may suggest a possible role for inositol and other dietary components in their prevention and treatment

The influence of infusion rate on the acute intravenous toxicity of phytic acid, a calcium-binding agent

The intravenous toxicity of phytic acid (inositol hexakisphosphate, IHP) has recently become of interest because of the potential for IHP incorporation into red blood cells to achieve a therapeutically useful shift in the hemoglobin-oxygen dissociation curve (Gersonde, K. and Nicolau, C., Blut, 39 (1979) 1). The observed acute intravenous toxicity of IHP in rodents is consistent with its recognized capacity to bind calcium. The toxic manifestations of intravenous IHP are a function of rate of infusion as well as total dose, with some seemingly anomalous variations which may related to compensatory mechanisms. The data suggest that significant alterations of plasma calcium and the toxic potential of such alterations are not likely to result from the administration of red blood cells with IHP incorporated.