Identifying the conformational state of bi-liganded haemoglobin

While most researchers agree on the global features of cooperative ligand binding to haemoglobin (Hb), the internal mechanisms remain open to debate. This is not due to inaccurate measurements, but is rather a consequence of the cooperative ligand binding that decreases the equilibrium populations of the partially liganded states and makes observation of the transitions between these substates more difficult. For example, the equilibrium population of the doubly liganded tetramers is typically less than 5% of the total Hb. As a result many models with widely varying mechanisms may fit the oxygen equilibrium curve, but may not be consistent with observations of other parameters, such as ligand-binding kinetics or subunit association equilibria. The wide range of methods and models has led to divergent conclusions about the properties of specific substates. One notable debate concerns the properties of the doubly liganded forms. The simple two-state model predicts a shift in the allosteric equilibrium based on the number of ligands bound, but not on their distribution within the tetramer. From studies of dimer-tetramer equilibria of various pure and hybrid forms, it was concluded that a tetramer with two ligands bound on the same α β dimer (species 21, an asymmetric hybrid) shows an enhanced tetramer stability, similar to singly liganded Hb, relative to the other three types of doubly liganded tetramers which resemble the triply liganded forms [Ackers et al. (1992), Science 255: 54–63]. The implications of this model and the relevant experiments will be reviewed here. Received 27 April 1998; received after revision 17 July 1998; accepted 10 August 1998     

不同溶氧水平对褐牙鲆幼鱼呼吸行为和血液指标的影响

通过控制每个水族箱的水流供应,将溶解氧(DO,Mean±SE)分别控制在(2.24±0.24) mg/L、(3.14±0.24) mg/L、(4.27±0.18)mg/L、(5.38±0.25)mg/L、(6.94±0.14) mg/L,研究其对养殖40d的褐牙鲆(Paralichth ys olivaceus)幼鱼呼吸行为和血液指标的影响.实验结束时,幼鱼的鳃重和体重比值未出现明显差异,只在最高溶解氧含量处理组略微低于其余处理组.平均呼吸频率随溶解氧含量的下降明显提高,每次呼吸鳃盖的伸展幅度随溶解氧含量的下降明显加大,头部上抬的鱼的比例随溶解氧含量的降低而增加.血液中血红细胞含量、血红蛋白含量、单个红细胞血红蛋白含量均呈现与溶解氧含量负相关趋势.单个红细胞体积未表现出与溶解氧的明显关系,只有溶解氧为3.14 mg/L的处理体积明显大于其他处理.4 mg/L左右的溶解氧含量对褐牙鲆幼鱼而言可能是溶解氧正常水平与缺氧的临界范围,而呼吸频率、鳃盖伸展幅度和抬头比例的大幅提高可以作为水体缺氧的行为判断依据.褐牙鲆幼鱼为适应溶解氧含量下降,血液摄取和携带氧气的能力提高.