Kinetics of NO and O2 binding to a maleimide poly(ethylene glycol)conjugated human haemoglobin

The hypertensive effect observed with most cell-free haemoglobins has been proposed to result from NO scavenging. However, a newly developed PEG [poly(ethylene glycol)]-conjugated haemoglobin, MalPEG-Hb [maleimide-activated PEG-conjugated haemoglobin], is non-hypertensive with unique physicochemical properties: high O-2 affinity, low co-operativity and large molecular radius. It is therefore of interest to compare the ligand-binding properties of MalPEG-Hb with unmodified cell-free HbA (stroma-free human haemoglobin). NO association rates for deoxy and oxyMalPEG-Hb and HbA were found to be identical. These results confirm the lack of correlation between hypertension and NO for a similar modified haemoglobin with high molecular radius and low p50 (pO(2) at which haemoglobin is half-saturated with O-2) [Rohlfs, Bruner, Chiu, Gonzales, Gonzales, Magde, Magde, Vandegriff and Winslow (1998) J. Biol. Chem. 273, 12128-12134]. The R-state 0, association kinetic constants were also the same for the two haemoglobins. However, even though the p50 of MaIPEG-Hb is approx. half of that of HbA, the biphasic O-2 dissociation rates measured at relatively high pO(2) (150 Torr) were 2-fold higher, giving rise to a 2-fold lower R-state equilibrium association constant for MalPEG-Hb compared with HbA. Thus the O-2 affinity of MalPEG-Hb is higher only at pO(2) values lower than the intersection point of the O-2 equilibrium curves for MalPEG-Hb and HbA. In summary, the present studies found similar rates of NO binding to HbA and MaIPEG-Hb, eliminating the possibility that the lack of vasoactivity of MaIPEG-Hb is simply the result of reduced molecular reactivity with NO. Alternatively, the unique O-2-binding characteristics with low p50 and co-operativity suggest that the 'R-state' conformation of MaIPEG-Hb is in a more T-state configuration and restricted from confonnational change.