EPR studies on the effects of complexation of heme by hemopexin upon its reactions with organic peroxides

Binding of hemin, hematoporphyrin, and protoporphyrin with erythroid spectrin: fluorescence and molecular docking studies

Free heme has toxic effects, for example lipid peroxidation, DNA damage, and protein aggregation. In severe hemolysis, which occurs during pathological states, for example sickle cell disease, ischemia reperfusion, and malaria, levels of free heme increase inside erythrocytes. The purpose of this study was to investigate whether spectrin, the major erythroid cytoskeleton protein, is involved as an acceptor of free heme. We compared the interactions of three heme derivatives, hemin chloride, hematoporphyrin, and protoporphyrin-IX, with dimeric and tetrameric spectrin. The dissociation constants (K d) for binding to spectrin dimer and tetramer were 0.57 and 1.16 µM respectively. Thermodynamic data associated with this binding revealed the binding to be favored by a positive change in entropy. Although molecular docking studies identified the SH3 domain as the unique binding site of these heme derivatives to erythroid spectrin, experimental results indicated a binding stoichiometry of 1 heme attached to both dimeric and tetrameric spectrin, indicating the common self-associating domain to be the unique binding site. We also noticed heme-induced structural changes in the membrane skeletal protein. Erythroid spectrin could thus act as a potential acceptor of heme, particularly relevant under disease conditions.

Mechanisms of neuroprotection by hemopexin: modeling the control of heme and iron homeostasis in brain neurons in inflammatory states

Hemopexin provides neuroprotection in mouse models of stroke and intracerebral hemorrhage and protects neurons in vitro against heme or reactive oxygen species (ROS) toxicity via heme oxygenase-1 (HO1) activity. To model human brain neurons experiencing hemorrhages and inflammation, we used human neuroblastoma cells, heme-hemopexin complexes, and physiologically relevant ROS, for example, H(2)O(2) and HOCl, to provide novel insights into the underlying mechanism whereby hemopexin safely maintains heme and iron homeostasis. Human amyloid precursor protein (hAPP), needed for iron export from neurons, is induced ~twofold after heme-hemopexin endocytosis by iron from heme catabolism via the iron-regulatory element of hAPP mRNA. Heme-hemopexin is relatively resistant to damage by ROS and retains its ability to induce the cytoprotective HO1 after exposure to tert-butylhydroperoxide, although induction is impaired, but not eliminated, by exposure to high concentrations of H(2)O(2) in vitro. Apo-hemopexin, which predominates in non-hemolytic states, resists damage by H(2)O(2) and HOCl, except for the highest concentrations likely in vivo. Heme-albumin and albumin are preferential targets for ROS; thus, albumin protects hemopexin in biological fluids like CSF and plasma where it is abundant. These observations provide strong evidence that hemopexin will be neuroprotective after traumatic brain injury, with heme release in the CNS, and during the ensuing inflammation. Hemopexin sequesters heme, thus preventing unregulated heme uptake that leads to toxicity; it safely delivers heme to neuronal cells; and it activates the induction of proteins including HO1 and hAPP that keep heme and iron at safe levels in neurons.

HeLp, a heme-transporting lipoprotein with an antioxidant role

Plasma lipoproteins involved in lipid transport are target for free radical-evoked pathological conditions in several mammalian models. The main hemolymphatic protein of Boophilus microplus is a heme-binding lipoprotein (HeLp, for Heme LipoProtein) that carries dietary heme produced from degradation of vertebrate hemoglobin to tissues of the tick. Addition of heme to phospholipid liposomes resulted in intense lipid peroxidation, which was inhibited by addition of HeLp. HeLp prevented lysis of red blood cells by heme. HeLp also inhibited reactions of heme with tert-butyl hydroperoxide (t-BOOH) or hydrogen peroxide. HeLp, quite differently from other lipoproteins, presents a protective intrinsic mechanism to counteract heme toxicity, while preserving the heme molecule to be reused by the tick. This is the first report of a lipoprotein acting as an antioxidant particle against heme-induced radical damage.

A DNA oligonucleotide-hemin complex cleaves t-butyl hydroperoxide through a homolytic mechanism

Both electron paramagnetic resonance (EPR) and electronic absorption spectroscopy have been employed to investigate the reaction of a guanine-rich DNA nucleotide-hemin complex (PS2.M-hemin complex) and organic peroxide (t-Bu-OOH). Incubation of the PS2.M-hemin complex with t-Bu-OOH resulted in the time-dependent decrease in the heme Soret with concomitant changes to the visible bands of the electronic absorbance spectrum for the PS2.M-hemin complex. Parallel EPR studies using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) combined with spectral simulation demonstrated the presence of tert-butyloxyl, carbon-centered methyl, and methyl peroxyl radicals as well as a simple nitroxide (triplet) signal. Experiments, performed by maintaining a constant ratio of t-Bu-OOH/PS2.M-hemin complex ( approximately 35 mol/mol) while varying DMPO concentration, indicated that the relative contributions of each radical adduct to the composite EPR spectrum were significantly influenced by the DMPO concentration. For example, at DMPO/PS2.M-hemin of 10-50 mol/mol, a complex mixture of radicals was consistently detected, whereas at high trapping efficiency (i.e., DMPO/PS2.M-hemin of approximately 250 mol/mol) the tert-butyloxyl-DMPO adduct was predominant. In contrast, at relatively low DMPO/PS2.M-hemin complex ratios of < or =5 mol/mol, a simple nitroxide three-line EPR signal was detected largely in the absence of all other radicals. Together, these data indicate that tert-butyloxyl radical is the primary radical likely formed from the homolytic cleavage of the O-O peroxy bond of t-Bu-OOH, while methyl and methyl peroxyl radicals result from beta-scission of the primary tert-butyloxyl radical product.

The effects of heme-binding proteins on the peroxidative and catalatic activities of hemin

The plasma proteins hemopexin (Hx) and albumin (Alb) are known to bind heme with high and medium affinity, respectively. To study how this binding modifies heme catalytic reactivity, the effects of Hx, human serum Alb (HSA), and bovine serum Alb (BSA) on the peroxidase- and catalaselike activities of hemin were investigated. These hemin activities were found to be inhibited by 50 to 60% with either HSA or BSA, and by 80 to 90% with Hx. The heme complexes with Hx or Alb (1:1 = protein:heme) therefore had a much lower reactivity toward H2O2 and Cum-OOH than the nonprotein heme. A kinetic analysis suggested that binding to Hx or Alb inhibited the primary activation of heme by H2O2, the step common for both peroxidase- and catalaselike activities of hemin. It is thought that by complexing heme, the Hx and Alb can prevent the toxic effects of extracellular heme in blood plasma.