Calorimetry of the transfer of Fe(3) from nitrilotriacetate to apotransferrin in the presence and in the absence of bicarbonate.

The enthalpy change for saturation of half the Fe(II1) binding sites was compared with the enthalpy change for saturation of the hall-saturated protein. In the presence of excess HCOBthese values were -10.90 f 0.19 Cal per mole of bound Fe(II1) and -10.43 f 0.76 Cal per mole of bound Fe(III), respectively. In the absence of HCOI-, corresponding values were -4.64 + 0.05 Cal per mole of bound Fe(II1) and -4.52 + 0.08 Cal per mole of bound Fe(III), respectively. These facts give support to the model of independent and equivalent Fe(II1) binding sites in apotransferrin but do not prove it. No calorimetric evidence was found for [Fe(III)NTA], dimer formation under conditions favorable to its presence. Comparison of time-dependent calorimetric curves indicates that the apotransferrin + Fe(III)NTA + HCO,reaction occurs in two steps. An instantaneous reaction occurs, followed by a slower reaction.