The immunochemical effects of metal substitution in the prosthetic group of hemoglobin

Nanoparticles derived from naturally occurring metal chelators for theranostic applications

Metals are indispensable for the activities of all living things, from single-celled organisms to higher organisms, including humans. Beyond their intrinsic quality as metal ions, metals help creatures to maintain requisite biological processes by forming coordination complexes with endogenous ligands that are broadly distributed in nature. These types of naturally occurring chelating reactions are found through the kingdoms of life, including bacteria, plants and animals. Mimicking these naturally occurring coordination complexes with intrinsic biocompatibility may offer an opportunity to develop nanomedicine toward clinical applications. Herein, we introduce representative examples of naturally occurring coordination complexes in a selection of model organisms and highlight such bio-inspired metal-chelating nanomaterials for theranostic applications.

Quantification of Active Apohemoglobin Heme-Binding Sites via Dicyanohemin Incorporation

Apohemoglobin (apoHb) is produced by removing heme from hemoglobin (Hb). However, preparations of apoHb may contain damaged globins, which render total protein assays inaccurate for active apoHb quantification. Fortunately, apoHb heme-binding sites react with heme via the proximal histidine-F8 (His-F8) residue, which can be monitored spectrophotometrically. The bond between the His-F8 residue of apoHb and heme is vital for maintenance of fully functional and cooperative Hb. Additionally, most apoHb drug delivery applications facilitate hydrophobic drug incorporation inside the apoHb hydrophobic heme-binding pocket in which the His-F8 residue resides. This makes the His-F8 residue a proper target for apoHb activity quantification. In this work, dicyanohemin (DCNh), a stable monomeric porphyrin species, was used as a probe molecule to quantify active apoHb through monocyanohemin-His-F8 bond formation. ApoHb activity was quantified via the analysis of the 420 nm equilibrium absorbance of DCNh and apoHb mixtures. His-F8 saturation was determined by the presence of an inflection point from a plot of the 420 nm absorbance of a fixed concentration of apoHb against an increasing DCNh concentration. Various concentrations of a stock apoHb solution were tested to demonstrate the precision of the assay. The accuracy of the assay was assessed via spectral deconvolution, confirming His-F8 saturation at the inflection point. The effect of the heme-binding protein bovine serum albumin and precipitated apoHb on assay sensitivity was not significant. An analysis of the biophysical properties of reconstituted Hb confirmed heme-binding pocket activity. Taken together, this assay provides a simple and reliable method for determination of apoHb activity.

Preparation and Properties of Nickel Hemoglobin

Hemoglobin A reconstituted with nickel protoporphyrin IX (NiHbA) has been prepared and characterized. Kinetics of its reaction with p-mercuribenzoate and with haptoglobin, absorption and circular dichroism spectra, and x-ray crystallographic properties have been investigated as probes of its structural conformation. The results suggest that NiHbA exists in a structure that is similar to the deoxy, or T-state of HbA. It is proposed that NiHbA and its derivatives may serve as a useful model for future studies of hemoglobin allosteric changes.

Conformational changes monitored by fluorescence study on reconstituted hemoglobins

Intrinsic steady state fluorescence measurements were performed on a series of reconstituted metal ion and hybrid hemoglobins (Hbs). At 296 nm excitation, the spectrum exhibits a broad and asymmetric feature in the case of copper and nickel reconstituted hemoglobins. Deconvolution of the fluorescence bands clearly reveals the existence of two definite peaks. A similar trend was also observed for hybrid hemoglobins (CuNi, NiCu, CuFe-CO, and NiFe-CO). A guassian fit of the fluorescence bands in these proteins again yields two prominent peaks, which are assigned as due to two different tryptophan (Trp) environments. A relative ratio of the amplitudes of these peaks indicates the percentage of T-character in these molecules. This is in support to our previous findings by other spectroscopic studies on the same molecules. These studies therefore, suggest the presence of two different environments of a tryptophan thereby revealing structural heterogeneity among the subunits.

Resonance Raman spectroscopic studies in copper reconstituted and hybrid hemoglobins: probe into subunit heterogeneity

Copper reconstituted hemoglobin (CuHb), copper containing T-state hybrid hemoglobins like alpha2(Ni)beta2(Cu), and alpha2(Cu)beta2(Ni), and intermediate R-state hybrids like alpha2(CO-Fe)beta2(Cu) and alpha2(Cu)beta2(Fe-CO) are studied using resonance Raman (RR) spectroscopy at two different excitation wavelengths. The high frequency RR region in CuHb indicates the presence of both 4- and 5-coordinate forms of Cu(II). In hybrid Hbs, the presence of two distinct metal ion environments within one particular subunit is evident. This is also consistent with previous findings using EPR spectroscopy and sulfydryl reactivity studies on these hybrid Hbs. The low frequency RR region on these copper derivatives of HbA further suggests the existence of two different heme moieties within the subunit.

The solubility of hemoglobins A and S reconstituted with various metalloporphyrins

The solubilities of hemoglobins A and S, reconstituted with various metalloporphyrins, in concentrated phosphate solutions are reported. In HbA the metallo-substituted derivatives have solubilities identical to Fe(II)HbAO2, except for PorHbA, which is less soluble. In HbS the central metal influences the solubility in the order Fe(II)HbSO2 greater than Cu(II)HbS greater than Ni(II)HbS greater than Zn(II)HbS approximately of porphyrinHbS greater than Fe(II)HbS. The changes in solubility properties of metallo-substituted HbS is probably related to conformational changes which alter interactions among hemoglobin molecules.