Zoophysiology, Department of Biological Sciences, Aarhus University, Bygning 1131, C. F. Møllers Allé 3, DK 8000 Aarhus, Denmark.
J Exp Biol. 2010 May;213(Pt 9):1579-85. doi: 10.1242/jeb.040543.
The inverse relationship between temperature and hemoglobin-O(2) affinity resulting from the exothermic nature of heme oxygenation favors O(2) unloading from blood to warm, metabolically active tissues. However, this temperature sensitivity is maladaptive, and commonly countered in regional heterotherms, where it may hamper unloading (e.g. in cold extremities of arctic mammals) or increase the diffusive arterio-venous short-circuiting of O(2) (e.g. in counter-current heat exchangers of warm swimming muscles of tuna). We hypothesized analogous blood specializations in heterothermic billfish, whose warm eyes and brains increase the temporal resolution of vision, and measured hemoglobin-O(2) binding properties in three species over a wide pH range, at two temperatures, and in the absence and presence of the major red cell effector, ATP, permitting detailed assessment of overall oxygenation enthalpies (DeltaH') and contributions from oxygenation-linked proton and ATP dissociation. Billfish express multiple isohemoglobins with similar O(2) affinities and pronounced sensitivities to pH and ATP. Compared with the moderate effects associated with proton dissociation upon oxygenation, dissociation of ATP and coupled extra Bohr protons virtually obliterates the temperature sensitivities. At pH 7.4, where this effect is maximal, ATP changes DeltaH' values of blue marlin, striped marlin and shortbill spearfish hemoglobins from -39, -49 and -44 kJ mol(-1) O(2), respectively, to +26, +4 and -7 kJ mol(-1). Thus in addition to allosterically modulating hemoglobin-O(2) affinity, ATP diminishes its temperature sensitivity, reducing deleterious arterio-venous short-circuiting of oxygen in the cranial billfish heat exchangers. The mechanism underlying this reduction in oxygenation enthalpy differs fundamentally from that in tuna, supporting independent evolution of this trait in these scombroid lineages.
血红素氧合的放热性质导致温度与血红蛋白-O2 亲和力呈反比关系，有利于 O2 从血液中释放到温暖、代谢活跃的组织中。然而，这种温度敏感性是适应不良的，在区域异温动物中通常会受到阻碍，这可能会阻碍 O2 的释放（例如在北极哺乳动物的寒冷肢体中），或者增加 O2 的扩散动静脉短路（例如在金枪鱼温暖游泳肌肉的逆流热交换器中）。我们假设在异温性箭鱼中存在类似的血液特化现象，其温暖的眼睛和大脑提高了视觉的时间分辨率，并在很宽的 pH 范围内、在两个温度下以及在没有和存在主要的红细胞效应物 ATP 的情况下测量了三种箭鱼的血红蛋白-O2 结合特性，允许对整体氧合焓（ΔH'）和氧合相关质子和 ATP 解离的贡献进行详细评估。箭鱼表达具有相似 O2 亲和力和对 pH 和 ATP 显著敏感性的多种同工血红蛋白。与氧合时质子解离相关的中度影响相比，ATP 的解离和耦合额外的 Bohr 质子几乎消除了温度敏感性。在 pH 7.4 下，这种效应最大，ATP 将蓝马林、条纹马林和短嘴矛鱼血红蛋白的ΔH'值分别从 -39、-49 和 -44 kJ mol(-1) O2 改变为+26、+4 和-7 kJ mol(-1)。因此，除了变构调节血红蛋白-O2 亲和力之外，ATP 还降低了其温度敏感性，减少了颅箭鱼热交换器中氧气的有害动静脉短路。这种氧合焓降低的机制与金枪鱼中的机制根本不同，支持了这种特征在这些鲭鱼谱系中的独立进化。
