STABILIZATION OF THE T-STATE OF FERROUS HUMAN ADULT AND FETAL HEMOGLOBIN BY LN(III) COMPLEXES - A THERMODYNAMIC STUDY

The effect of the lanthanide(III) complexes [Gd(1,4,7,10-tetraazacyclo dodecane- N,N',N'',N'''-tetrakis(methylenephosphonate))](5-) (Gd-DOTP) and La-DOTP on the oxygen binding and spectroscopic properties of hum an adult and fetal hemoglobin (HbA and HbF, respectively) has been inv estigated. The affinity of Gd-DOTP and La-DOTP for oxygenated HbA (HbA O(2); K-HbAO2 = 2.6 x 10(-3) M) is closely similar to that observed fo r Ln(III) complexes association to nitrosylated HbA (HbANO K-HbANO = 1 .8 x 10(-3) M) and to aquo-met HbA (met-HbA; Kmet-HbA = 1.9 x 10(-3) M ), being lower than that determined for Gd-DOTP and La-DOTP binding to the deoxygenated form of the tetramer (HbAd; K-HbAd = 3.0 x 10(-4) M) . The affinity of Gd-DOTP for deoxygenated HbF (HbFd; K-HbFd = 9.5 x 1 0(-4) M) and oxygenated HbF (HbF(O)2; K-HbFO2 = 3.7 x10(-3) M) is lowe r than that observed for Ln(III) complexes association to HbAd and HbA O(2), respectively. Gd-DOTP and La-DOTP bind to HbA and HbF with a 1:1 stoichiometry per tetramer. Increasing Gd-DOTP and La-DOTP concentrat ion, oxygen affinity for HbA decreases (i.e. P-50 increases), this eff ect being minor for HbF. Upon binding of Ln(III) complexes to HbANO, t he X-band EPR spectrum and the absorption spectrum in the Soret region display the characteristics which have been attributed to the T-state of the ligated tetramer. These results represent a clear cut evidence for the specific binding of Gd-DOTP and La-DOTP to the 2,3-D-glycerat e bisphosphate (BPG) pocket (i.e. at the dyad axis, in between the P-c hains) of HbA and HbF. The effect of Ln(III) complexes on the ligand b inding and spectroscopic properties of HbA and HbF is reminiscent that of BPG, the physiological modulator of human Hb action. (C) 1998 Else vier Science Inc. All rights reserved.