Abacavir modulates peroxynitrite-mediated oxidation of ferrous nitrosylated human serum heme–albumin

Heme Scavenging and Delivery: The Role of Human Serum Albumin

Heme is the reactive center of several metal-based proteins that are involved in multiple biological processes. However, free heme, defined as the labile heme pool, has toxic properties that are derived from its hydrophobic nature and the Fe-atom. Therefore, the heme concentration must be tightly controlled to maintain cellular homeostasis and to avoid pathological conditions. Therefore, different systems have been developed to scavenge either Hb (i.e., haptoglobin (Hp)) or the free heme (i.e., high-density lipoproteins (HDL), low-density lipoproteins (LDL), hemopexin (Hx), and human serum albumin (HSA)). In the first seconds after heme appearance in the plasma, more than 80% of the heme binds to HDL and LDL, and only the remaining 20% binds to Hx and HSA. Then, HSA slowly removes most of the heme from HDL and LDL, and finally, heme transits to Hx, which releases it into hepatic parenchymal cells. The Hx:heme or HSA:heme complexes are internalized via endocytosis mediated by the CD91 and CD71 receptors, respectively. As heme constitutes a major iron source for pathogens, bacteria have evolved hemophores that can extract and uptake heme from host proteins, including HSA:heme. Here, the molecular mechanisms underlying heme scavenging and delivery from HSA are reviewed. Moreover, the relevance of HSA in disease states associated with increased heme plasma concentrations are discussed.

Serum Albumin: A Multifaced Enzyme

Human serum albumin (HSA) is the most abundant protein in plasma, contributing actively to oncotic pressure maintenance and fluid distribution between body compartments. HSA acts as the main carrier of fatty acids, recognizes metal ions, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays esterase, enolase, glucuronidase, and peroxidase (pseudo)-enzymatic activities. HSA-based catalysis is physiologically relevant, affecting the metabolism of endogenous and exogenous compounds including proteins, lipids, cholesterol, reactive oxygen species (ROS), and drugs. Catalytic properties of HSA are modulated by allosteric effectors, competitive inhibitors, chemical modifications, pathological conditions, and aging. HSA displays anti-oxidant properties and is critical for plasma detoxification from toxic agents and for pro-drugs activation. The enzymatic properties of HSA can be also exploited by chemical industries as a scaffold to produce libraries of catalysts with improved proficiency and stereoselectivity for water decontamination from poisonous agents and environmental contaminants, in the so called “green chemistry” field. Here, an overview of the intrinsic and metal dependent (pseudo-)enzymatic properties of HSA is reported to highlight the roles played by this multifaced protein.

Structural Basis of Drug Recognition by Human Serum Albumin

Background:

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule with at least nine binding sites for endogenous and exogenous ligands. HSA displays an extraordinary ligand binding capacity as a depot and carrier for many compounds including most acidic drugs. Consequently, HSA has the potential to influence the pharmacokinetics and pharmacodynamics of drugs.

Objective:

In this review, the structural determinants of drug binding to the multiple sites of HSA are analyzed and discussed in detail. Moreover, insight into the allosteric and competitive mechanisms underpinning drug recognition, delivery, and efficacy are analyzed and discussed.

Conclusion:

As several factors can modulate drug binding to HSA (e.g., concurrent administration of drugs competing for the same binding site, ligand binding to allosteric-coupled clefts, genetic inherited diseases, and post-translational modifications), ligand binding to HSA is relevant not only under physiological conditions, but also in the pharmacological therapy management.

Structural and functional properties of Antarctic fish cytoglobins-1: Cold-reactivity in multi-ligand reactions

While the functions of the recently discovered cytoglobin, ubiquitously expressed in vertebrate tissues, remain uncertain, Antarctic fish provide unparalleled models to study novel protein traits that may arise from cold adaptation. We report here the spectral, ligand-binding and enzymatic properties (peroxynitrite isomerization, nitrite-reductase activity) of cytoglobin-1 from two Antarctic fish, Chaenocephalus aceratus and Dissostichus mawsoni, and present the crystal structure of D. mawsoni cytoglobin-1. The Antarctic cytoglobins-1 display high O₂ affinity, scarcely compatible with an O₂-supply role, a slow rate constant for nitrite-reductase activity, and do not catalyze peroxynitrite isomerization. Compared with mesophilic orthologues, the cold-adapted cytoglobins favor binding of exogenous ligands to the hexa-coordinated bis-histidyl species, a trait related to their higher rate constant for distal-His/heme-Fe dissociation relative to human cytoglobin. At the light of a remarkable 3D-structure conservation, the observed differences in ligand-binding kinetics may reflect Antarctic fish cytoglobin-1 specific features in the dynamics of the heme distal region and of protein matrix cavities, suggesting adaptation to functional requirements posed by the cold environment. Taken together, the biochemical and biophysical data presented suggest that in Antarctic fish, as in humans, cytoglobin-1 unlikely plays a role in O₂ transport, rather it may be involved in processes such as NO detoxification.

Warfarin inhibits allosterically the reductive nitrosylation of ferric human serum heme-albumin

Human serum heme-albumin (HSA-heme-Fe) displays heme-based ligand binding and (pseudo-)enzymatic properties. Here, the effect of the prototypical drug warfarin on kinetics and thermodynamics of NO binding to ferric and ferrous HSA-heme-Fe (HSA-heme-Fe(III) and HSA-heme-Fe(II), respectively) and on the NO-mediated reductive nitrosylation of the heme-Fe atom is reported; data were obtained between pH5.5 and 9.5 at 20.0°C. Since warfarin is a common drug, its effect on the reactivity of HSA-heme-Fe represents a relevant issue in the pharmacological therapy management. The inhibition of NO binding to HSA-heme-Fe(III) and HSA-heme-Fe(II) as well as of the NO-mediated reductive nitrosylation of the heme-Fe(III) atom by warfarin has been ascribed to drug binding to the fatty acid binding site 2 (FA2), shifting allosterically the penta-to-six coordination equilibrium of the heme-Fe atom toward the low reactive species showing the six-coordinated metal center by His146 and Tyr161 residues. These data: (i) support the role of HSA-heme-Fe in trapping NO, (ii) highlight the modulation of the heme-Fe-based reactivity by drugs, and (iii) could be relevant for the modulation of HSA functions by drugs in vivo.

Albumin and multiple sclerosis

Leakage of the blood-brain barrier (BBB) is a common pathological feature in multiple sclerosis (MS). Following a breach of the BBB, albumin, the most abundant protein in plasma, gains access to CNS tissue where it is exposed to an inflammatory milieu and tissue damage, e.g., demyelination. Once in the CNS, albumin can participate in protective mechanisms. For example, due to its high concentration and molecular properties, albumin becomes a target for oxidation and nitration reactions. Furthermore, albumin binds metals and heme thereby limiting their ability to produce reactive oxygen and reactive nitrogen species. Albumin also has the potential to worsen disease. Similar to pathogenic processes that occur during epilepsy, extravasated albumin could induce the expression of proinflammatory cytokines and affect the ability of astrocytes to maintain potassium homeostasis thereby possibly making neurons more vulnerable to glutamate exicitotoxicity, which is thought to be a pathogenic mechanism in MS. The albumin quotient, albumin in cerebrospinal fluid (CSF)/albumin in serum, is used as a measure of blood-CSF barrier dysfunction in MS, but it may be inaccurate since albumin levels in the CSF can be influenced by multiple factors including: 1) albumin becomes proteolytically cleaved during disease, 2) extravasated albumin is taken up by macrophages, microglia, and astrocytes, and 3) the location of BBB damage affects the entry of extravasated albumin into ventricular CSF. A discussion of the roles that albumin performs during MS is put forth.

Heme-based catalytic properties of human serum albumin

Human serum albumin (HSA): (i) controls the plasma oncotic pressure, (ii) modulates fluid distribution between the body compartments, (iii) represents the depot and carrier of endogenous and exogenous compounds, (iv) increases the apparent solubility and lifetime of hydrophobic compounds, (v) affects pharmacokinetics of many drugs, (vi) inactivates toxic compounds, (vii) induces chemical modifications of some ligands, (viii) displays antioxidant properties, and (ix) shows enzymatic properties. Under physiological and pathological conditions, HSA has a pivotal role in heme scavenging transferring the metal-macrocycle from high- and low-density lipoproteins to hemopexin, thus acquiring globin-like reactivity. Here, the heme-based catalytic properties of HSA are reviewed and the structural bases of drug-dependent allosteric regulation are highlighted.

Warfarin modulates the nitrite reductase activity of ferrous human serum heme-albumin

Human serum heme-albumin (HSA-heme-Fe) displays reactivity and spectroscopic properties similar to those of heme proteins. Here, the nitrite reductase activity of ferrous HSA-heme-Fe [HSA-heme-Fe(II)] is reported. The value of the second-order rate constant for the reduction of [Formula: see text] to NO and the concomitant formation of nitrosylated HSA-heme-Fe(II) (i.e., k on) is 1.3 M(-1) s(-1) at pH 7.4 and 20 °C. Values of k on increase by about one order of magnitude for each pH unit decrease between pH 6.5 to 8.2, indicating that the reaction requires one proton. Warfarin inhibits the HSA-heme-Fe(II) reductase activity, highlighting the allosteric linkage between the heme binding site [also named the fatty acid (FA) binding site 1; FA1] and the drug-binding cleft FA2. The dissociation equilibrium constant for warfarin binding to HSA-heme-Fe(II) is (3.1 ± 0.4) × 10(-4) M at pH 7.4 and 20 °C. These results: (1) represent the first evidence for the [Formula: see text] reductase activity of HSA-heme-Fe(II), (2) highlight the role of drugs (e.g., warfarin) in modulating HSA(-heme-Fe) functions, and (3) strongly support the view that HSA acts not only as a heme carrier but also displays transient heme-based reactivity.

Reciprocal allosteric modulation of carbon monoxide and warfarin binding to ferrous human serum heme-albumin

Human serum albumin (HSA), the most abundant protein in human plasma, could be considered as a prototypic monomeric allosteric protein, since the ligand-dependent conformational adaptability of HSA spreads beyond the immediate proximity of the binding site(s). As a matter of fact, HSA is a major transport protein in the bloodstream and the regulation of the functional allosteric interrelationships between the different binding sites represents a fundamental information for the knowledge of its transport function. Here, kinetics and thermodynamics of the allosteric modulation: (i) of carbon monoxide (CO) binding to ferrous human serum heme-albumin (HSA-heme-Fe(II)) by warfarin (WF), and (ii) of WF binding to HSA-heme-Fe(II) by CO are reported. All data were obtained at pH 7.0 and 25°C. Kinetics of CO and WF binding to the FA1 and FA7 sites of HSA-heme-Fe(II), respectively, follows a multi-exponential behavior (with the same relative percentage for the two ligands). This can be accounted for by the existence of multiple conformations and/or heme-protein axial coordination forms of HSA-heme-Fe(II). The HSA-heme-Fe(II) populations have been characterized by resonance Raman spectroscopy, indicating the coexistence of different species characterized by four-, five- and six-coordination of the heme-Fe atom. As a whole, these results suggest that: (i) upon CO binding a conformational change of HSA-heme-Fe(II) takes place (likely reflecting the displacement of an endogenous ligand by CO), and (ii) CO and/or WF binding brings about a ligand-dependent variation of the HSA-heme-Fe(II) population distribution of the various coordinating species. The detailed thermodynamic and kinetic analysis here reported allows a quantitative description of the mutual allosteric effect of CO and WF binding to HSA-heme-Fe(II).

Human serum albumin: from bench to bedside

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule, representing the main determinant of plasma oncotic pressure and the main modulator of fluid distribution between body compartments. HSA displays an extraordinary ligand binding capacity, providing a depot and carrier for many endogenous and exogenous compounds. Indeed, HSA represents the main carrier for fatty acids, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays (pseudo-)enzymatic properties. HSA is a valuable biomarker of many diseases, including cancer, rheumatoid arthritis, ischemia, post-menopausal obesity, severe acute graft-versus-host disease, and diseases that need monitoring of the glycemic control. Moreover, HSA is widely used clinically to treat several diseases, including hypovolemia, shock, burns, surgical blood loss, trauma, hemorrhage, cardiopulmonary bypass, acute respiratory distress syndrome, hemodialysis, acute liver failure, chronic liver disease, nutrition support, resuscitation, and hypoalbuminemia. Recently, biotechnological applications of HSA, including implantable biomaterials, surgical adhesives and sealants, biochromatography, ligand trapping, and fusion proteins, have been reported. Here, genetic, biochemical, biomedical, and biotechnological aspects of HSA are reviewed.

Identification of differentially expressed proteins in ruminal epithelium in response to a concentrate-supplemented diet

Ruminal epithelium adapts to dietary change with well-coordinated alterations in metabolism, proliferation, and permeability. To further understand the molecular events controlling diet effects, the aim of this study was to evaluate protein expression patterns of ruminal epithelium in response to various feeding regimes. Sheep were fed with a concentrate-supplemented diet for up to 6 wk. The control group received hay only. Proteome analysis with differential in gel electrophoresis technology revealed that, after 2 days, 60 proteins were significantly modulated in ruminal epithelium in a comparison between hay-fed and concentrate-fed sheep (P < 0.05). Forty proteins were upregulated and 20 proteins were downregulated in response to concentrate diet. After 6 wk of this diet, only 14 proteins were differentially expressed. Among these, 11 proteins were upregulated and 3 downregulated. To identify proteins that were modulated by dietary change, two-dimensional electrophoresis was coupled with liquid chromatography electrospray ionization mass spectrometry. The differential expression of selected proteins, such as esterase D, annexin 5, peroxiredoxin 6, carbonic anhydrase I, and actin-related protein 3, was verified by immunoblotting and/or mRNA analysis. The identified proteins were mainly associated with functions related to cellular stress, metabolism, and differentiation. These results suggest new candidate proteins that may contribute to a better understanding of the signaling pathways and mechanisms that mediate rumen epithelial adaptation to high-concentrate diet.

Peroxynitrite scavenging by ferryl sperm whale myoglobin and human hemoglobin

Globins protect from the oxidative and nitrosative cell damage. Here, kinetics of peroxynitrite scavenging by ferryl sperm whale myoglobin (MbFe(IV)O) and human hemoglobin (HbFe(IV)O), between pH 5.8 and 8.3 at 20.0 degrees C, are reported. In the absence of CO(2), values of the second-order rate constant for peroxynitrite scavenging by MbFe(IV)O and HbFe(IV)O (i.e., for MbFe(III) and HbFe(III) formation; k(on)) are 4.6 x 10(4)M(-1)s(-1) and 3.3 x 10(4)M(-1)s(-1), respectively, at pH 7.1. Values of k(on) increase on decreasing pH with pK(a) values of 6.9 and 6.7, this suggests that the ONOOH species reacts preferentially with MbFe(IV)O and HbFe(IV)O. In the presence of CO(2) (=1.2 x 10(-3)M), values of k(on) for peroxynitrite scavenging by MbFe(IV)O and HbFe(IV)O are essentially pH-independent, the average k(on) values are 7.1 x 10(4)M(-1)s(-1) and 1.2 x 10(5)M(-1)s(-1), respectively. As a whole, MbFe(IV)O and HbFe(IV)O, obtained by treatment with H(2)O(2), undertake within the same cycle H(2)O(2) and peroxynitrite detoxification.

Ibuprofen impairs allosterically peroxynitrite isomerization by ferric human serum heme-albumin

Human serum albumin (HSA) participates in heme scavenging; in turn, heme endows HSA with myoglobin-like reactivity and spectroscopic properties. Here, the allosteric effect of ibuprofen on peroxynitrite isomerization to NO(3)(-) catalyzed by ferric human serum heme-albumin (HSA-heme-Fe(III)) is reported. Data were obtained at 22.0 degrees C. HSA-heme-Fe(III) catalyzes peroxynitrite isomerization in the absence and presence of CO(2); the values of the second order catalytic rate constant (k(on)) are 4.1 x 10(5) and 4.5 x 10(5) m(-1) s(-1), respectively. Moreover, HSA-heme-Fe(III) prevents peroxynitrite-mediated nitration of free added l-tyrosine. The pH dependence of k(on) (pK(a) = 6.9) suggests that peroxynitrous acid reacts preferentially with the heme-Fe(III) atom, in the absence and presence of CO(2). The HSA-heme-Fe(III)-catalyzed isomerization of peroxynitrite has been ascribed to the reactive pentacoordinated heme-Fe(III) atom. In the absence and presence of CO(2), ibuprofen impairs dose-dependently peroxynitrite isomerization by HSA-heme-Fe(III) and facilitates the nitration of free added l-tyrosine; the value of the dissociation equilibrium constant for ibuprofen binding to HSA-heme-Fe(III) (L) ranges between 7.7 x 10(-4) and 9.7 x 10(-4) m. Under conditions where [ibuprofen] is >>L, the kinetics of HSA-heme-Fe(III)-catalyzed isomerization of peroxynitrite is superimposable to that obtained in the absence of HSA-heme-Fe(III) or in the presence of non-catalytic HSA-heme-Fe(III)-cyanide complex and HSA. Ibuprofen binding impairs allosterically peroxynitrite isomerization by HSA-heme-Fe(III), inducing the hexacoordination of the heme-Fe(III) atom. These results represent the first evidence for peroxynitrite isomerization by HSA-heme-Fe(III), highlighting the allosteric modulation of HSA-heme-Fe(III) reactivity by heterotropic interaction(s), and outlining the role of drugs in modulating HSA functions. The present results could be relevant for the drug-dependent protective role of HSA-heme-Fe(III) in vivo.

Ibuprofen modulates allosterically NO dissociation from ferrous nitrosylated human serum heme-albumin by binding to three sites

Human serum albumin (HSA) is a monomeric allosteric protein. Here, the effect of ibuprofen on denitrosylation kinetics (k(off)) and spectroscopic properties of HSA-heme-Fe(II)-NO is reported. The k(off) value increases from (1.4+/-0.2)x10(-4)s(-1), in the absence of the drug, to (9.5+/-1.2)x10(-3)s(-1), in the presence of 1.0x10(-2)M ibuprofen, at pH 7.0 and 10.0 degrees C. From the dependence of k(off) on the drug concentration, values of the dissociation equilibrium constants for ibuprofen binding to HSA-heme-Fe(II)-NO (K(1)=(3.1+/-0.4)x10(-7)M, K(2)=(1.7+/-0.2)x10(-4)M, and K(3)=(2.2+/-0.2)x10(-3)M) were determined. The K(3) value corresponds to the value of the dissociation equilibrium constant for ibuprofen binding to HSA-heme-Fe(II)-NO determined by monitoring drug-dependent absorbance spectroscopic changes (H=(2.6+/-0.3)x10(-3)M). Present data indicate that ibuprofen binds to the FA3-FA4 cleft (Sudlow's site II), to the FA6 site, and possibly to the FA2 pocket, inducing the hexa-coordination of HSA-heme-Fe(II)-NO and triggering the heme-ligand dissociation kinetics.

Modulation of heme and myristate binding to human serum albumin by anti-HIV drugs. An optical and NMR spectroscopic study

Human serum albumin (HSA) has an extraordinary ligand-binding capacity, and transports Fe(III)heme and medium- and long-chain fatty acids. In human immunodeficiency virus-infected patients the administered drugs bind to HSA and act as allosteric effectors. Here, the binding of Fe(III)heme to HSA in the presence of three representative anti-HIV drugs and myristate is investigated. Values of the dissociation equilibrium constant K(d) for Fe(III)heme binding to HSA were determined at different myristate concentrations, in the absence and presence of anti-HIV drugs. Nuclear magnetic relaxation dispersion profiles of HSA-Fe(III)heme were measured, at different myristate concentrations, in the absence and presence of anti-HIV drugs. Structural bases for anti-HIV drug binding to HSA are provided by automatic docking simulation. Abacavir and nevirapine bind to HSA with K(d) values of 1 x 10(-6) and 2 x 10(-6) M, respectively. Therefore, at concentrations used in therapy (in the 1-5 x 10(-6) M range) abacavir and nevirapine bind to HSA and increase the affinity of heme for HSA. In the presence of abacavir or nevirapine, the affinity is not lowered by myristate. FA7 should therefore be intended as a secondary binding site for abacavir and nevirapine. Binding of atazanavir is limited by the large size of the drug, although preferential binding may be envisaged to a site positively coupled with FA1 and FA2, and negatively coupled to FA7. As a whole, these results provide a foundation for the comprehension of the complex network of links modulating HSA-binding properties.

Ferricyanide-mediated oxidation of ferrous nitrosylated sperm whale myoglobin involves the formation of the ferric nitrosylated intermediate

Ferricyanide-mediated oxidation of ferrous oxygenated and carbonylated myoglobin (Mb(II)-O(2) and Mb(II)-CO, respectively) is limited by O(2) and CO dissociation, respectively, then the transient deoxygenated derivative (Mb(II)) is rapidly oxidized. Here, kinetics of ferricyanide-mediated oxidation of ferrous nitrosylated sperm whale myoblobin (Mb(II)-NO) is reported. Unlike for Mb(II)-O(2) and Mb(II)-CO, ferricyanide reacts with Mb(II)-NO forming first a transient ferric nitrosylated species (Mb(III)-NO), followed by the ()NO dissociation from Mb(III)-NO. Values of the second-order rate constant for ferricyanide-mediated oxidation of Mb(II)-NO (i.e., for the formation of the transient Mb(III)-NO species) and of the first-order rate constant for ()NO dissociation from Mb(III)-NO (i.e., for Mb(III) formation) are (1.3+/-0.2)x10(6)M(-1)s(-1) and 7.6+/-1.3s(-1), respectively, at pH 8.3 and 20.0 degrees C. Since ()NO dissociation from Mb(II)-NO is very slow, and (unlike O(2) and CO) ()NO is a ligand for both Mb(II) and Mb(III), Mb(II)-NO can be oxidized without requiring ()NO dissociation.