Faster heme loss from hemoglobin E than HbS, in acidic pH: Effect of aminophospholipids

Abstract

We report studies on loss of heme at or below pH 3.0 from two clinically important hemoglobin variants, HbE and HbS, in the presence and absence of phopholipid membranes. The kinetics of heme loss has been studied at pH 3.0 to simulate the same at a faster rate than at physiological pH, for spectroscopic investigation. Results obtained from the study clearly establish the probable fate of the lost heme to partition into the phospholipid bilayer independent of the pH range. This is also of particular importance to membranes containing the aminophospholipid and cholesterol which are predominantly localized in the inner leaflet of erythrocytes. Absorption measurements indicated such loss of heme when the Soret peak at 415 nm blue-shifted to 380 nm at pH 3.0. The extent of this blue shift decreased from 35 nm to (approx.) 15 nm in the presence of small unilammelar vesicles of both dimyristoyl- and dioleoyl-based phosphatidylcholine and phosphatidylethanolamine, indicating partitioning of the released heme in the membrane bilayer. The kinetics of heme loss was faster from HbE than HbA and HbS, obeying first-order reaction kinetics. Released heme could be involved in the premature destruction of erythrocytes in hemoglobin disorders.

Homodimerization Counteracts the Detrimental Effect of Nitrogenous Heme Ligands on the Enzymatic Activity of Acanthamoeba castellanii CYP51

“…Thus, we propose that the heme macrocycle is no longer tightly anchored in its crevice, so that it slips partially or completely out of the protein and resides in different environments. The observed spectral features resemble those reported for acid- and temperature-induced denaturation of hemoglobin A2 and HemQ …”

Oxidative instability of hemoglobin E (β26 Glu→Lys) is increased in the presence of free α subunits and reversed by α-hemoglobin stabilizing protein (AHSP): Relevance to HbE/β-thalassemia

“…First, drug (menadione)-induced oxidation studies of HbE (and a number of Hb variants) indicated that it oxidizes slightly faster than HbA, and produces larger quantities of low-spin ferric Hb (hemichromes) [45] . Secondly, HbE loses heme to lipid monolayers as well as phospholipid bilayers more readily than other variant Hbs [46] , [47] . This unusual response to oxidation has an impact on HbE structural stability and has been suggested to be an important factor influencing the clinical course in HbE/β thalassemia patients particularly during febrile episodes [9] .…”

Significantly Enhanced Heme Retention Ability of Myoglobin Engineered to Mimic the Third Covalent Linkage by Nonaxial Histidine to Heme (Vinyl) in Synechocystis Hemoglobin

“…It is known that if a denaturant destabilizes a Hb, the characteristic Soret peak undergoes a blue shift in wavelength along with a significant concomitant decrease in the absorbance intensity (46). Complete heme dissociation from a globin results in a broad visible absorbance peak typical of free heme with Soret wavelength maximum as low as ϳ350 -370 nm and significant reduction in intensity (47). The changes in the Soret wavelength maxima of SynHb and its mutants were monitored with the change in pH (2.0 -11.0).…”