Interactions of adenosine triphosphate with snake hemoglobins. Studies in Liophis miliaris, Boa constrictor and Bothrops alternatus

Convergent High O2 Affinity but Distinct ATP-Mediated Allosteric Regulation of Hemoglobins in Oviparous and Viviparous Eremias Lizards from the Qinghai-Tibet Plateau

The functional adaptation and underlying molecular mechanisms of hemoglobins (Hbs) have primarily concentrated on mammals and birds, with few reports on reptiles. This study aimed to investigate the convergent and species-specific high-altitude adaptation mechanisms of Hbs in two Eremias lizards from the Qinghai-Tibet Plateau. The Hbs of high-altitude E. argus and E. multiocellata were characterized by significantly high overall and intrinsic Hb-O2 affinity compared to their low-altitude populations. Despite the similarly low Cl- sensitivities, the Hbs of high-altitude E. argus exhibited higher ATP sensitivity and ATP-dependent Bohr effects than that of E. multiocellata, which could facilitate O2 unloading in respiring tissues. Eremias lizards Hbs exhibited similarly low temperature sensitivities and relatively high Bohr effects at lower temperatures, which could help to stably deliver and release O2 to cold extremities at low temperatures. The oxygenation properties of Hbs in high-altitude populations might be attributed to varying ratios of β2/β1 globin and substitutions on the β2-type globin. Notably, the Asn12Ala in lowland E. argus could cause localized destabilization of the E-helix in the tetrameric Hb by elimination of hydrogen bonds, thereby resulting in its lowest O2 affinity. This study provides a valuable reference for the high-altitude adaptation mechanisms of hemoglobins in reptiles.

Effects of different levels of hypoxia and hypercarbia on ventilation and gas exchange in Boa constrictor amaralis and Crotalus durissus (Squamata: Serpentes)

Ventilation and gas exchange have been studied in relatively few species of snakes, especially regarding their response to environmental hypoxia or hypercarbia. We exposed Crotalus durissus (N = 6) and Boa constrictor (N = 6) to decreasing levels of oxygen (12, 9, 6, 3 % O₂) and increasing levels of carbon dioxide (1.5, 3.0, 4.5, 6.0 % CO₂) and analyzed the effect of the different gas mixtures on ventilation and gas exchange using open-flow respirometry. Neither hypoxia nor hypercarbia significantly altered the duration of expiration or inspiration, nor their proportions. Both hypoxia and hypercarbia increased minute ventilation, but the decrease in oxygen had a less pronounced effect on ventilation. Gas exchange under normoxic conditions was low and was not significantly affected by hypoxia, but hypercarbia decreased gas exchange significantly in both species. While B. constrictor maintained its respiratory exchange ratio (RER) under hypercarbia between 0.5 and 1.0, C. durissus showed a RER above 1.0 during hypercarbia, due to a significantly greater CO₂ excretion. The overall responses of both species to hypercarbia and especially to hypoxia were very similar, which could be associated to similar lifestyles as ambush hunting sit-and-wait predators that are able to ingest large prey items. The observed differences in gas exchange could be related to respiratory systems with macroscopically different structures, possessing only a tracheal lung in C. durissus, but two functional lungs in B. constrictor.

Oxygenation properties and isoform diversity of snake hemoglobins

Available data suggest that snake hemoglobins (Hbs) are characterized by a combination of unusual structural and functional properties relative to the Hbs of other amniote vertebrates, including oxygenation-linked tetramer-dimer dissociation. However, standardized comparative data are lacking for snake Hbs, and the Hb isoform composition of snake red blood cells has not been systematically characterized. Here we present the results of an integrated analysis of snake Hbs and the underlying α- and β-type globin genes to characterize 1) Hb isoform composition of definitive erythrocytes, and 2) the oxygenation properties of isolated isoforms as well as composite hemolysates. We used species from three families as subjects for experimental studies of Hb function: South American rattlesnake, Crotalus durissus (Viperidae); Indian python, Python molurus (Pythonidae); and yellow-bellied sea snake, Pelamis platura (Elapidae). We analyzed allosteric properties of snake Hbs in terms of the Monod-Wyman-Changeux model and Adair four-step thermodynamic model. Hbs from each of the three species exhibited high intrinsic O2 affinities, low cooperativities, small Bohr factors in the absence of phosphates, and high sensitivities to ATP. Oxygenation properties of the snake Hbs could be explained entirely by allosteric transitions in the quaternary structure of intact tetramers, suggesting that ligation-dependent dissociation of Hb tetramers into αβ-dimers is not a universal feature of snake Hbs. Surprisingly, the major Hb isoform of the South American rattlesnake is homologous to the minor HbD of other amniotes and, contrary to the pattern of Hb isoform differentiation in birds and turtles, exhibits a lower O2 affinity than the HbA isoform.

Chromatin supraorganization, DNA fragmentation, and cell death in snake erythrocytes

In nucleate erythrocytes of several vertebrate groups, the frequency and intensity of DNA fragmentation associated with programed cell death vary considerably. Although hemoglobin efficiency may be related to erythrocyte life span, and hemoglobin types and erythrocyte life spans are assumed to vary in reptiles, no data on DNA fragmentation and chromatin organization as related to cell death exist for snakes. In the present study, chromatin supraorganization, DNA fragmentation, and cell death were investigated in four snake species (Crotalus durissus terrificus, Bothrops jararaca, Bothrops alternatus, and Bothrops neuwiedii), which differ in their geographical distribution and habitats, by using image analysis of Feulgen hydrolysis kinetics, the TUNEL assay, single-cell gel electrophoresis, and transmission electron microscopy. Relatively few circulating erythrocytes were found to be simultaneously committed to cell death, although there was some variation among the snake species. Conspicuous nuclear and cytoplasmic organelles suggestive of metabolic activity were seen ultrastructurally in most snake erythrocytes. The DNA of the snake erythrocyte chromatin was much more resistant to Feulgen acid hydrolysis (DNA depurination and breakdown) than that of young adult bullfrog erythrocytes, which had a high frequency and intensity of DNA fragmentation. Of the species studied, B. neuwiedii and C. d. terrificus showed the greatest resistance to Feulgen acid hydrolysis and to the DNA fragmentation, revealed by the TUNEL assay. Although B. neuwiedii also showed the lowest frequency of cells with more damaged DNA in the single-cell gel electrophoresis assay, C. d. terrificus had the highest frequency of damaged cells, possibly because of the abundance of alkaline-sensitive DNA sites. The results for DNA fragmentation and cell death in erythrocytes of B. jararaca and B. alternatus generally differed from those for C. d. terrificus and B. neuwiedii and may reflect differences in the biology of these species selected under different geographical habitats. The differences in erythrocyte cell biology reported here may be related to hemoglobin variants selected in the mentioned snake species and that would lead the cells to different resistances to unfavorable environmental conditions.Key words: erythrocytes, snakes, DNA fragmentation, chromatin supraorganization, cell death.

Allosteric water and phosphate effects in Hoplosternum littorale hemoglobins

This paper reports the results obtained using the osmotic stress method applied to the purified cathodic and anodic hemoglobins (Hbs) from the catfish Hoplosternum littorale, a species that displays facultative accessorial air oxygenation. We demonstrate that water potential affects the oxygen affinity of H. littorale Hbs in the presence of an inert solute (sucrose). Oxygen affinity increases when water activity increases, indicating that water molecules stabilize the high-affinity state of the Hb. This effect is the same as that observed in tetrameric vertebrate Hbs. We show that both anodic and cathodic Hbs show conformational substrates similar to other vertebrate Hbs. For both Hbs, addition of anionic effectors, especially chloride, strongly increases the number of water molecules bound, although anodic Hb did not exhibit sensitivity to saturating levels of ATP. Accordingly, for both Hbs, we propose that the deoxy conformations coexist in at least two anion-dependent allosteric states, T(o) and T(x), as occurs for human Hb. We found a single phosphate binding site for the cathodic Hb.

The primary structure of three hemoglobin chains from the indigo snake (Drymarchon corais erebennus, Serpentes): first evidence for alphaD chains and two beta chain types in snakes

The hemoglobin of the indigo snake (Drymarchon corais erebennus, Colubrinae) consists of two components, HbA and HbD, in the ratio of 1:1. They differ in both their alpha and beta chains. The amino acid sequences of both a chains (alphaA and alphaD) and one beta chain (betaI) were determined. The presence of an alphaD chain in a snake hemoglobin is described for the first time. A comparison of all snake beta chain sequences revealed the existence of two paralogous beta chain types in snakes as well, which are designated as betaI and betaII type. For the discussion of the physiological properties of Drymarchon hemoglobin, the sequences were compared with those of the human alpha and beta chains and those of the closely related water snake Liophis milians where functional data are available. Among the heme contacts, the substitution alphaD58(E7)His-->Gln is unusual but most likely without any effect. The residues responsible for the main part of the Bohr effect are the same as in mammalian hemoglobins. In each of the three globin chains only two residues at positions involved in the alpha1/beta2 interface contacts, most important for the stability and the properties of the hemoglobin molecule, are substituted with regard to human hemoglobin. On the contrary, nine, eleven, and six alpha1/beta1 contact residues are replaced in the alphaA, alphaD, betaI chains, respectively.