Quaternary structure regulates hemin dissociation from human hemoglobin.

Chiancone and co-workers (1) published studies of hemin loss from immobilized αβ dimers and isolated α and β subunits of human hemoglobin that we failed to read and to take into account. Their conclusions were the same as ours but were reported 1 year earlier: “... in human hemoglobin the heme-globin linkage in the α chains is stabilized by interactions between unlike chains at the α₁β₁ interface, whereas heme binding to the β chains is stabilized by interactions at the α₁β₂interface” (1). As they noted, the rate constants for hemin dissociation from proteins attached to Sepharose 4B are 3 to 10 times greater than those for hemin loss in solution. Part of this difference is due to alterations produced by the cross-linking reaction, and part may be due to the difficulty of fitting for the rate constants when human serum albumin is used to extract the hemin. However, the ratios of the monomeric, dimeric, and tetrameric rate constants for hemin loss determined by Gattoni et al. (1) are almost identical to ours, which were determined in solution using H64Y/V68F apoMb as an irreversible heme scavenging reagent. Thus, their original conclusion is confirmed by our work.