Metal ions and electrolytes regulate the dissociation of heme from human hemopexin at physiological pH

Abstract

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 NH4HCO 3 buffer (pH 7.4, 20mM, 25°C) containing Zn2+ (100 μM), 14% of the heme dissociates from this complex (4.5 μM) 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 KHCO 3 solution, and is minimal in NaHCO3. In NH 4HCO3 buffer, dissociation of the Hx-heme complex is accelerated by addition of divalent metals with decreasing efficiency in the order Zn2+ > Cu2+ ≫ Ni2+ > Co 2+≫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°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. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.