The primary structure of the hemoglobin of the dogfish shark (Squalus acanthias). Antagonistic effects of ATP and urea on oxygen affinity of an elasmobranch hemoglobin

Regulation of erythrocyte function: Multiple evolutionary solutions for respiratory gas transport and its regulation in fish

Gas transport concepts in vertebrates have naturally been formulated based on human blood. However, the first vertebrates were aquatic, and fish and tetrapods diverged hundreds of millions years ago. Water-breathing vertebrates live in an environment with low and variable O₂ levels, making environmental O₂ an important evolutionary selection pressure in fishes, and various features of their gas transport differ from humans. Erythrocyte function in fish is of current interest, because current environmental changes affect gas transport, and because especially zebrafish is used as a model in biomedical studies, making it important to understand the differences in gas transport between fish and mammals to be able to carry out meaningful studies. Of the close to thirty thousand fish species, teleosts are the most species-numerous group. However, two additional radiations are discussed: agnathans and elasmobranchs. The gas transport by elasmobranchs may be closest to the ancestors of tetrapods. The major difference in their haemoglobin (Hb) function to humans is their high urea tolerance. Agnathans differ from other vertebrates by having Hbs, where cooperativity is achieved by monomer-oligomer equilibria. Their erythrocytes also lack the anion exchange pathway with profound effects on CO₂ transport. Teleosts are characterized by highly pH sensitive Hbs, which can fail to become fully O₂ -saturated at low pH. An adrenergically stimulated Na⁺ /H⁺ exchanger has evolved in their erythrocyte membrane, and plasma-accessible carbonic anhydrase can be differentially distributed among their tissues. Together, and differing from other vertebrates, these features can maximize O₂ unloading in muscle while ensuring O₂ loading in gills.

The Greenland shark Somniosus microcephalus-Hemoglobins and ligand-binding properties

A large amount of data is currently available on the adaptive mechanisms of polar bony fish hemoglobins, but structural information on those of cartilaginous species is scarce. This study presents the first characterisation of the hemoglobin system of one of the longest-living vertebrate species (392 ± 120 years), the Arctic shark Somniosus microcephalus. Three major hemoglobins are found in its red blood cells and are made of two copies of the same α globin combined with two copies of three very similar β subunits. The three hemoglobins show very similar oxygenation and carbonylation properties, which are unaffected by urea, a very important compound in marine elasmobranch physiology. They display identical electronic absorption and resonance Raman spectra, indicating that their heme-pocket structures are identical or highly similar. The quaternary transition equilibrium between the relaxed (R) and the tense (T) states is more dependent on physiological allosteric effectors than in human hemoglobin, as also demonstrated in polar teleost hemoglobins. Similar to other cartilaginous fishes, we found no evidence for functional differentiation among the three isoforms. The very similar ligand-binding properties suggest that regulatory control of O₂ transport may be at the cellular level and that it may involve changes in the cellular concentrations of allosteric effectors and/or variations of other systemic factors. The hemoglobins of this polar shark have evolved adaptive decreases in O₂ affinity in comparison to temperate sharks.

Osmoregulatory bicarbonate secretion exploits H(+)-sensitive haemoglobins to autoregulate intestinal O2 delivery in euryhaline teleosts

Marine teleost fish secrete bicarbonate (HCO3 (-)) into the intestine to aid osmoregulation and limit Ca(2+) uptake by carbonate precipitation. Intestinal HCO3 (-) secretion is associated with an equimolar transport of protons (H(+)) into the blood, both being proportional to environmental salinity. We hypothesized that the H(+)-sensitive haemoglobin (Hb) system of seawater teleosts could be exploited via the Bohr and/or Root effects (reduced Hb-O2 affinity and/or capacity with decreasing pH) to improve O2 delivery to intestinal cells during high metabolic demand associated with osmoregulation. To test this, we characterized H(+) equilibria and gas exchange properties of European flounder (Platichthys flesus) haemoglobin and constructed a model incorporating these values, intestinal blood flow rates and arterial-venous acidification at three different environmental salinities (33, 60 and 90). The model suggested red blood cell pH (pHi) during passage through intestinal capillaries could be reduced by 0.14-0.33 units (depending on external salinity) which is sufficient to activate the Bohr effect (Bohr coefficient of -0.63), and perhaps even the Root effect, and enhance tissue O2 delivery by up to 42 % without changing blood flow. In vivo measurements of intestinal venous blood pH were not possible in flounder but were in seawater-acclimated rainbow trout which confirmed a blood acidification of no less than 0.2 units (equivalent to -0.12 for pHi). When using trout-specific values for the model variables, predicted values were consistent with measured in vivo values, further supporting the model. Thus this system is an elegant example of autoregulation: as the need for costly osmoregulatory processes (including HCO3 (-) secretion) increases at higher environmental salinity, so does the enhancement of O2 delivery to the intestine via a localized acidosis and the Bohr (and possibly Root) effect.

Organisation of the Hb 1 genes of the Antarctic skate Bathyraja eatonii: new insights into the evolution of globin genes

An extensive investigation of the organisation of globin genes has greatly contributed to the understanding of universal mechanisms of gene evolution and expression. Cartilaginous fish are the first organisms that have evolved the tetrameric form of hemoglobin (Hb). So far, there has been absolute lack of data about globin genes in chondrichthyans. Bathyraja is the dominant rajid south of 60 degrees S. In the framework of the investigations on globin genes of Antarctic red-blooded and Hb-less fish we obtained the cloning of the alpha- and beta-globin cDNAs of the main Hb (Hb 1) of the skate Bathyraja eatonii. Then, a genomic fragment of 6.2 kb was isolated where the Hb 1 alpha and beta genes are linked in a tail-to-head (3' to 5') orientation. The beta-globin gene promoter region and the chromosomal organisation of the Hb 1 genes of B. eatonii have been compared to their homologues in other vertebrates. The finding of a tail-to-head linkage of the Hb 1 alpha- and beta-globin genes in B. eatonii is the first characterisation of the organisation of globin genes in chondrichthyes; such finding offers a novel contribution to the understanding of the evolution of this class of genes. Moreover, the characterisation of chondrichthyan genes is very important for gaining insight into the ancestral state of vertebrate genomes.

Structure, function and molecular adaptations of haemoglobins of the polar cartilaginous fish Bathyraja eatonii and Raja hyperborea

Cartilaginous fish are very ancient organisms. In the Antarctic sea, the modern chondrichthyan genera are poorly represented, with only three species of sharks and eight species of skates; the paucity of chondrichthyans is probably an ecological consequence of unusual trophic or habitat conditions in the Southern Ocean. In the Arctic, there are 26 species belonging to the class Chondrichthyes. Fish in the two polar regions have been subjected to different regional histories that have influenced the development of diversity: Antarctic marine organisms are highly stenothermal, in response to stable water temperatures, whereas the Arctic communities are exposed to seasonal temperature variations. The structure and function of the oxygen-transport haem protein from the Antarctic skate Bathyraja eatonii and from the Arctic skate Raja hyperborea (both of the subclass Elasmobranchii, order Rajiformes, family Rajidae) is reported in the present paper. These species have a single major haemoglobin (Hb 1; over 80% of the total). The Bohr-proton and the organophosphate-binding sites are absent. Thus the haemoglobins of northern and southern polar skates appear functionally similar, whereas differences were observed with several temperate elasmobranchs. Such evidence suggests that, in temperate and polar habitats, physiological adaptations have evolved along distinct pathways, whereas, in this case, the effect of the differences characterizing the two polar environments is negligible.

Asymmetric hemoglobins, their thiol content, and blood glutathione of the scalloped hammerhead shark, Sphyrna lewini

Starch gel electrophoresis pH 8.6, or PAGE pH 8.9, of the scalloped hammerhead shark hemolysates showed three hemoglobins (Hb). An additional Hb between the two most mobile electrophoretic components was seen in starch gel electrophoresis, pH 8.1, and also in highly loaded PAGE gels. The relative concentration of these Hbs was variable among individuals, when accessed at pH 8.1. Dilution of hemolysates led to a redistribution of the Hb tetramer subunits. Under denaturing conditions, the unfractionated hemolysate was resolved in 3 Hb subunits. Isolated Hbs, named SL I-SL IV, showed unusual subunit compositions: SL I, the least mobile, is "b3c"; SL II is "a2bc"; SL III and SL IV are composed only by "a" subunits. Hemoglobins in the whole hemolysate have an average of two reactive cysteines per tetramer, which were not easily S-thiolated by glutathione, as is the case for related species. After hemoglobin denaturation, six additional -SH groups were titrated by Ellman's reagent. Methemoglobin content was low in the erythrocytes of nine examined specimens, 1.13 +/- 1.90%. High values for total erythrocyte glutathione (GSH) were found: 4.5 +/- 0.7 mM; n = 7. The ratio of 1.4 +/- 0.4 GSH/Hb is higher than usually reported for mammalians.

PCR amplification of cDNAs of fish hemoglobin beta chains using a consensus primer: cDNA-derived amino acid sequences of beta chains from the catfish Parasilurus asotus and the scad Decapterus maruadsi

Hemoglobin beta chains were isolated from the catfish Parasilurus asotus, the scad Decapterus maruadsi, the filefish Thamnaconus modestus, and the scorpaenoid Sebastiscus marmoratus by reverse-phase chromatography, and the N-terminal sequences were determined. To obtain the complete amino acid sequence, a 20-meric redundant consensus primer based on the N-terminal amino acid sequences of the beta chains was designed. Using this primer and oligo-dT adaptor, we amplified successfully the beta-chain cDNAs of about 600 bp from the four fishes. The amplified products from Parasilurus and Decapterus were subcloned in the SmaI site of pUC18 and cDNA-derived amino acid sequences of 147 residues were determined, of which 69 and 76 residues, respectively, were identified by the chemical amino acid sequencing of internal peptides. Thus this PCR methodology using the consensus primer should be widely applicable for amplifying hemoglobin beta chains from teleosts.

Was the loss of the D helix in alpha globin a functionally neutral mutation?

Proteins in the globin family are found in a variety of species from bacteria to man. From the many globin sequences known, evolutionary trees have been constructed showing that alpha and beta globins diverged from a common ancestor between 425 and 500 million years ago, after vertebrate species had appeared and roughly when sharks and bony vertebrates diverged. The alpha and beta globins assemble to form tetrameric haemoglobin, alpha 2 beta 2, which can switch between quaternary states having high and low oxygen affinity. This allows the protein to bind oxygen cooperatively and therefore efficiently transport oxygen from the lungs to respiring tissues. The alpha and beta globins have closely related tertiary structures, being alpha-helical proteins with similar haem-binding sites. Most globins consist of eight helices, designated A to H from the N terminus, connected by short nonhelical segments, but all known vertebrate alpha globins lack a D helix. Because the loss of this helix by alpha globin occurred shortly before tetrameric haemoglobin appeared, it might be a functionally important mutation required for a tetramer assembly or allostery. We have now tested this idea by engineering human haemoglobins containing beta subunits without a D helix and alpha subunits with a D helix. Both of these mutations have little effect on the oxygen-binding properties of the molecule. Thus it is possible that deletion of the D helix in the alpha subunit was caused by a neutral mutation.

Cooperativity and allosteric regulation in non-mammalian vertebrate haemoglobins

1. This review illustrates the vast range of molecular functions expressed in non-mammalian vertebrate haemoglobins; with particular reference to the degree of aggregation of haemoglobin subunits and their interactions with allosteric effectors. 2. In at least the broadest sense, these properties suggest that haemoglobin function in non-mammalian vertebrates can be viewed against the evolutionary hierarchy of organisms rather than from a purely adaptive perspective.

High hemoglobin mixed disulfide content in hemolysates from stressed shark

1. Hemolysate from heavily stressed smooth hammerhead shark, Sphyrna zygaena, shows three electrophoretic components, SZ I, SZ II and SZ III, whose relative concentrations are 36.4 +/- 6.8, 36.4 +/- 5.0 and 20.8 +/- 5.7%, respectively. After reduction with DTE only SZ I remained. 2. SZ I reacted with glutathione disulfide reconstitute SZ II and SZ III. 3. Non-reduced, DTE-reduced, and denatured hemoglobin were found to have 2.0 +/- 0.4, 3.7 +/- 0.6, and 9.4 +/- 0.7-SH groups, respectively. 4. Erythrocyte non-protein--SH (NPSH), including glutathione present as mixed disulfide with SZ II and SZ III, is 1.7 NPSH/Hb.

Mole rat hemoglobin: primary structure and evolutionary aspects in a second karyotype of Spalax ehrenbergi, Rodentia, (2n = 52)

The hemoglobins of the four karyotypes of Spalax ehrenbergi (2n = 52, 54, 58, 60) did not show any differences in their electrophoretic pattern and in high performance liquid chromatography. The complete amino-acid sequence of mole rat hemoglobin (Spalax ehrenbergi), chromosome species 2n = 52, is presented. It was elucidated by automatic Edman degradation of the chains, the tryptic peptides, and the C-terminal peptide obtained by acid hydrolysis of the Asp-Pro bond in beta-chains. The alpha- and beta-chains are identical with those of the chromosome species 2n = 60. A comparison of the hemoglobins of mole rat, mouse, and other rodents shows homology but no indication of adaptation to subterranean life. In all probability alpha 11(A9)Arg and alpha 120(H3)Gly, unique in mole rat among all mammalian hemoglobins, are not involved in high oxygen affinity. The construction of a phylogenetic tree by the maximum parsimony method, based on hemoglobin sequences, made it possible to show that Rodentia originated as a monophyletic clade, and to find the phylogenetic relationship of Spalacidae to other Rodentia (Mus, Rattus, Ondatra, Mesocricetus, Citellus, and Cavia). Among all rodents the slowest rate of nucleotide replacements occurred in the lineage to Spalax (20%) and the fastest in the lineage to Cavia (59%).