OXYGEN-BINDING TO FALLOW-DEER (DAMA-DAMA) HEMOGLOBIN - STEPWISE ENTHALPIES AT PH-7.4

High-precision thin-layer gas-solution microcalorimetry has been used to study the oxygen binding properties of fallow-deer (Damn damn) hemo globin under physiological conditions. This method measures directly t he enthalpy of macromolecular ligand binding by changing the ligand ac tivity in a manner analogous to that of the Gill thin-layer optical ap paratus ([1], D. Dolman and S.J. Gill, Anal. Biochem., 87 (1978) 127-1 34). By logarithmically lowering the partial pressure of oxygen we hav e generated differential heat binding curves of oxygen binding to fall ow-deer hemoglobin in phosphate buffer at pH 7.4. In order to enlarge the data field, the temperature dependence of the oxygen affinity was examined by generating binding curves at a number of different tempera tures allowing for separation of enthalpy and free energy parameters. This type of experimental analysis makes no assumption of optical line arity between the various heme groups and reveals initially that overa ll oxygen binding to fallow-deer hemoglobin is less exothermic and of lower affinity than for human hemoglobin A(0), In addition, previous o ptical work on the ancestrally related reindeer hemoglobin (Rangifer t arandus; [2], B. Giardina, O. Brix, M. Nuutinen, S. Sherbini, A, Bardg ard, G. Lazzarino and S, Condo, FEES Lett., 247 (1989) 135) has indica ted that the enthalpy associated with its final two oxygen binding ste ps is minimal. Our calorimetric determination with fallow-deer hemoglo bin also reveals this tendency, Presumably, this adaptation would make it easier for these animals to maintain a consistent hemoglobin oxyge n saturation level under environmental conditions where the temperatur e fluctuates.