Metal Ions and Electrolytes Regulate the Dissociation of Heme from Human Hemopexin at Physiological pH

The stability of the hemopexin-heme (Hx-heme) complex to dissociation of the heme prosthetic group has been examined in bicarbonate buffers in the presence and absence of various divalent metal ions. In NH4HCO3 buffer (pH 7.4, 20 mM, 25 degrees C) containing Zn2+ (100 mu M), 14% of the heme dissociates from this complex (4.5 mu M) within 10 min, and 50% dissociates within 2 h. In the absence of metal ions, the rate of dissociation of this complex is far lower, is decreased further in KHCO3 solution, and is minimal in NaHCO3. In NH4HCO3 buffer, dissociation of the Hx-heme complex is accelerated by addition of divalent metals with decreasing efficiency in the order Zn2+ > Cu2+ >> Ni2+ > Co2+ >> Mn2+. Addition of Ca2+ prior to addition of Zn2+ stabilizes the Hx-heme complex to dissociation of the heme group, and addition of Ca2+ after Zn2+-induced dissociation of the Hx-heme complex results in re-formation of the Hx-heme complex. These effects are greatly accelerated at 37 degrees C and diminished in other buffers. Overall, the solution conditions that promote formation of the Hx-heme complex are similar to those found in blood plasma, and conditions that promote release of heme are similar to those that the Hx-heme complex should encounter in endosomes following endocytosis of the complex formed with its hepatic receptor.