Marcinek D J, Bonaventura J, Wittenberg J B, Block B A
Tuna Research and Conservation Center and Hopkins Marine Station, Stanford University, Pacific Grove, CA 93950, USA.
Am J Physiol Regul Integr Comp Physiol. 2001 Apr;280(4):R1123-33. doi: 10.1152/ajpregu.2001.280.4.R1123.
Myoglobin (Mb) buffers intracellular O2 and facilitates diffusion of O2 through the cell. These functions of Mb will be most effective when intracellular PO2 is near the partial pressure of oxygen at which Mb is half saturated (P50) of the molecule. We test the hypothesis that Mb oxygen affinity has evolved such that it is conserved when adjusted for body temperature among closely related animals. We measure oxygen P50s tonometrically and oxygen dissociation rate constants with stopped flow and generate amino acid sequence from cDNA of Mbs from fish with different body temperatures. P50s for the endothermic bluefin tuna, skipjack tuna, and blue marlin at 20 degrees C were 0.62 +/- 0.02, 0.59 +/- 0.01, 0.58 +/- 0.04 mmHg, respectively, and were significantly lower than those for ectothermic bonito (1.03 +/- 0.07 mmHg) and mackerel (1.39 +/- 0.03 mmHg). Because the oxygen affinity of Mb decreases with increasing temperature, the above differences in oxygen affinity between endothermic and ectothermic fish are reduced when adjusted for the in vivo muscle temperature of the animal. Oxygen dissociation rate constants at 20 degrees C for the endothermic species ranged from 34.1 to 49.3 s(-1), whereas those for mackerel and bonito were 102 and 62 s(-1), respectively. Correlated with the low oxygen affinity and fast dissociation kinetics of mackerel Mb is a substitution of alanine for proline that would likely result in a more flexible mackerel protein.
肌红蛋白（Mb）缓冲细胞内的氧气，并促进氧气在细胞内的扩散。当细胞内的氧分压接近肌红蛋白分子半饱和时的氧分压（P50）时，肌红蛋白的这些功能将最为有效。我们检验了这样一个假设：肌红蛋白的氧亲和力已经进化，使得在密切相关的动物中，经体温校正后它是保守的。我们通过测压法测量氧P50，并使用停流法测量氧解离速率常数，同时从不同体温的鱼类的肌红蛋白cDNA生成氨基酸序列。在20摄氏度时，恒温的蓝鳍金枪鱼、鲣鱼和蓝枪鱼的P50分别为0.62±0.02、0.59±0.01、0.58±0.04 mmHg，显著低于变温的鲣鱼（1.03±0.07 mmHg）和鲭鱼（1.39±0.03 mmHg）。由于肌红蛋白的氧亲和力随温度升高而降低，当根据动物体内肌肉温度进行校正时，恒温鱼和变温鱼之间上述氧亲和力的差异会减小。在20摄氏度时，恒温物种的氧解离速率常数范围为34.1至49.3 s⁻¹，而鲭鱼和鲣鱼的氧解离速率常数分别为102和62 s⁻¹。鲭鱼肌红蛋白的低氧亲和力和快速解离动力学与丙氨酸取代脯氨酸有关，这可能会使鲭鱼蛋白更具柔韧性。
