Nitrosylation of rabbit ferrous heme-hemopexin

Unraveling the versatility of human serum albumin - A comprehensive review of its biological significance and therapeutic potential

Human serum albumin (HSA) is a multi-domain macromolecule with diverse ligand binding capability because of its ability to allow allosteric modulation despite being a monomeric protein. Physiologically, HSA act as the primary carrier for various exogenous and endogenous compounds and fatty acids, and alter the pharmacokinetic properties of several drugs. It has antioxidant properties and is utilized therapeutically to improve the drug delivery of pharmacological agents for the treatment of several disorders. The flexibility of albumin in holding various types of drugs coupled with a variety of modifications makes this protein a versatile drug carrier with incalculable potential in therapeutics. This review provides a brief outline of the different structural properties of HSA, and its various binding sites, moreover, an overview of the genetic, biomedical, and allosteric modulation of drugs and drug delivery aspects of HSA is also included, which may be helpful in guiding advanced clinical applications and further research on the therapeutic potential of this extraordinary protein.

Ligand-Based Regulation of Dynamics and Reactivity of Hemoproteins

Hemoproteins include several heme-binding proteins with distinct structure and function. The presence of the heme group confers specific reactivity and spectroscopic properties to hemoproteins. In this review, we provide an overview of five families of hemoproteins in terms of dynamics and reactivity. First, we describe how ligands modulate cooperativity and reactivity in globins, such as myoglobin and hemoglobin. Second, we move on to another family of hemoproteins devoted to electron transport, such as cytochromes. Later, we consider heme-based reactivity in hemopexin, the main heme-scavenging protein. Then, we focus on heme-albumin, a chronosteric hemoprotein with peculiar spectroscopic and enzymatic properties. Eventually, we analyze the reactivity and dynamics of the most recently discovered family of hemoproteins, i.e., nitrobindins.

Reductive nitrosylation of ferric microperoxidase-11

Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c, which is considered as a heme-protein model. Here, the reductive nitrosylation of ferric MP11 (MP11(III)) under anaerobic conditions has been investigated between pH 7.4 and 9.2, at T = 20.0 °C. At pH ≤ 7.7, NO binds reversibly to MP11(III) leading to the formation of the MP11(III)-NO complex. However, between pH 8.2 and 9.2, the addition of NO to MP11(III) leads to the formation of ferrous nitrosylated MP11(II) (MP11(II)-NO). In fact, the transient MP11{FeNO}⁶ species is converted to ferrous deoxygenated MP11 (MP11(II)) by OH^-- and H₂O-based catalysis, which represents the rate-limiting step of the whole reaction. Then, MP11(II) binds NO very rapidly leading to MP11(II)-NO formation. Over the whole pH range explored, the apparent values of k_on, k_off, and K (= k_off/k_on) for MP11(III)(-NO) (de)nitrosylation are essentially pH independent, ranging between 5.8 × 10⁵ M^-1 s^-1 and 1.6 × 10⁶ M^-1 s^-1, between 1.9 s^-1 and 3.7 s^-1, and between 1.4 × 10^-6 M and 4.6 × 10^-6 M, respectively. Values of the apparent pseudo-first-order rate constant for the MP11{FeNO}⁶ conversion to MP11(II) (i.e., h) increase linearly with pH; the apparent values [Formula: see text] and [Formula: see text] are 7.2 × 10² M^-1 s^-1 and 2.5 × 10^-4 s^-1, respectively. Present data confirm that MP11 is a useful molecular model to highlight the role of the protein matrix on the heme-based reactivity.

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.

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

Human serum albumin (SA) is best known for its extraordinary ligand-binding capacity. Here, kinetics of peroxynitrite-mediated oxidation of SA-heme(II)-NO is reported. Peroxynitrite reacts with SA-heme(II)-NO leading to SA-heme(III) and ()NO by way of the transient SA-heme(III)-NO species. Abacavir facilitates peroxynitrite-mediated oxidation of SA-heme(II)-NO, in the absence and presence of CO2. Values of the second order rate constant for peroxynitrite-mediated oxidation of SA-heme(II)-NO are (6.5+/-0.9) x 10(3) M(-1) s(-1) in the absence of CO2 and abacavir, (1.3+/-0.2) x 10(5) M(-1) s(-1) in the presence of CO2, (2.2+/-0.2) x 10(4) M(-1) s(-1) in the presence of abacavir, and (3.6+/-0.3) x 10(5) M(-1) s(-1) in the presence of both CO2 and abacavir. The value of the first-order rate constant for *NO dissociation from the SA-heme(III)-NO complex (=(1.8+/-0.3) x 10(-1) s(-1)) is CO2- and abacavir-independent, representing the rate-limiting step. Present data represent the first evidence for the allosteric modulation of SA-heme reactivity by heterotropic interaction(s).

Reductive nitrosylation and peroxynitrite-mediated oxidation of heme-hemopexin

Hemopexin (HPX), which serves as a scavenger and transporter of toxic plasma heme, has been postulated to play a key role in the homeostasis of NO. In fact, HPX-heme(II) reversibly binds NO and facilitates NO scavenging by O(2). HPX-heme is formed by two four-bladed beta-propeller domains. The heme is bound between the two beta-propeller domains, residues His213 and His266 coordinate the heme iron atom. HPX-heme displays structural features of heme-proteins endowed with (pseudo-)enzymatic activities. In this study, the kinetics of rabbit HPX-heme(III) reductive nitrosylation and peroxynitrite-mediated oxidation of HPX-heme(II)-NO are reported. In the presence of excess NO, HPX-heme(III) is converted to HPX-heme(II)-NO by reductive nitrosylation. The second-order rate constant for HPX-heme(III) reductive nitrosylation is (1.3 +/- 0.1) x 10(1) m(-1).s(-1), at pH 7.0 and 10.0 degrees C. NO binding to HPX-heme(III) is rate limiting. In the absence and presence of CO2 (1.2 x 10(-3) m), excess peroxynitrite reacts with HPX-heme(II)-NO (2.6 x 10(-6) m) leading to HPX-heme(III) and NO, via the transient HPX-heme(III)-NO species. Values of the second-order rate constant for HPX-heme(III)-NO formation are (8.6 +/- 0.8) x 10(4) and (1.2 +/- 0.2) x 10(6) m(-1).s(-1) in the absence and presence of CO2, respectively, at pH 7.0 and 10.0 degrees C. The CO2-independent value of the first-order rate constant for HPX-heme(III)-NO denitrosylation is (4.3 +/- 0.4) x 10(-1) s(-1), at pH 7.0 and 10.0 degrees C. HPX-heme(III)-NO denitrosylation is rate limiting. HPX-heme(II)-NO appears to act as an efficient scavenger of peroxynitrite and of strong oxidants and nitrating species following the reaction of peroxynitrite with CO2 (e.g. ONOOC(O)O-, CO3-, and NO2).

O2-mediated oxidation of hemopexin-heme(II)-NO

Hemopexin (HPX), serving as scavenger and transporter of toxic plasma heme, has been postulated to play a key role in the homeostasis of NO. Here, kinetics of HPX-heme(II) nitrosylation and O2-mediated oxidation of HPX-heme(II)-NO are reported. NO reacts reversibly with HPX-heme(II) yielding HPX-heme(II)-NO, according to the minimum reaction scheme: HPX-heme(II)+NO kon<-->koff HPX-heme(II)-NO values of kon, koff, and K (=kon/koff) are (6.3+/-0.3)x10(3)M-1s-1, (9.1+/-0.4)x10(-4)s-1, and (6.9+/-0.6)x10(6)M-1, respectively, at pH 7.0 and 10.0 degrees C. O2 reacts with HPX-heme(II)-NO yielding HPX-heme(III) and NO3-, by means of the ferric heme-bound peroxynitrite intermediate (HPX-heme(III)-N(O)OO), according to the minimum reaction scheme: HPX-heme(II)-NO+O2 hon<--> HPX-heme(III)-N(O)OO l-->HPX-heme(III)+NO3- the backward reaction rate is negligible. Values of hon and l are (2.4+/-0.3)x10(1)M-1s-1 and (1.4+/-0.2)x10(-3)s-1, respectively, at pH 7.0 and 10.0 degrees C. The decay of HPX-heme(III)-N(O)OO (i.e., l) is rate limiting. The HPX-heme(III)-N(O)OO intermediate has been characterized by optical absorption spectroscopy in the Soret region (lambdamax=409 nm and epsilon409=1.51x10(5)M-1cm-1). These results, representing the first kinetic evidence for HPX-heme(II) nitrosylation and O2-mediated oxidation of HPX-heme(II)-NO, might be predictive of transient (pseudo-enzymatic) function(s) of heme carriers.

Hemoglobin and heme scavenging

Release of hemoglobin into plasma is a physiological phenomenon associated with intravascular hemolysis. In plasma, stable haptoglobin-hemoglobin complexes are formed and these are subsequently delivered to the reticulo-endothelial system by CD163 receptor-mediated endocytosis. Heme arising from the degradation of hemoglobin, myoglobin, and of enzymes with heme prosthetic groups could be delivered in plasma. Albumin, haptoglobin, hemopexin, and high and low density lipoproteins cooperate to trap the plasma heme, thereby ensuring its complete clearance. Then hemopexin releases the heme into hepatic parenchymal cells only after internalization of the hemopexin-heme complex by CD91 receptor-mediated endocytosis. Moreover, alpha1-microglobulin contributes to heme degradation by a still unknown mechanism, with the concomitant formation of heterogeneous yellow-brown kynurenine-derived chromophores which are very tightly bound to amino acid residues close to the rim of the lipocalin pocket. During hemoglobin synthesis, the erythroid alpha-chain hemoglobin-stabilizing protein specifically binds free alpha-hemoglobin subunits limiting the free protein toxicity. Although highly toxic because capable of catalyzing free radical formation, heme is also a major and readily available source of iron for pathogenic organisms. Gram-negative bacteria pick up the heme-bound iron through the secretion of a hemophore that takes up either free heme or heme bound to heme-proteins and transports it to a specific receptor, which, in turn, releases the heme and hence iron into the bacterium. Here, hemoglobin and heme trapping mechanisms are summarized.

Proton-linked subunit heterogeneity in ferrous nitrosylated human adult hemoglobin: an EPR study

The effect of pH on the X-band electron paramagnetic resonance (EPR) spectrum of ferrous nitrosylated human adult tetrameric hemoglobin (HbNO) as well as of ferrous nitrosylated monomeric alpha- and beta-chains has been investigated, at -163 degrees C. At pH 7.3, the X-band EPR spectrum of tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains displays a rhombic shape. Lowering the pH from 7.3 to 3.0, tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains undergo a transition towards a species characterized by a X-band EPR spectrum with a three-line splitting centered at 334mT. These pH-dependent spectroscopic changes may be taken as indicative of the cleavage, or the severe weakening, of the proximal HisF8-Fe bond. In tetrameric HbNO, the pH-dependent spectroscopic changes depend on the acid-base equilibrium of two apparent ionizing groups with pK(a) values of 5.8 and 3.8. By contrast, the pH-dependent spectroscopic changes occurring in ferrous nitrosylated monomeric alpha- and beta-chains depend on the acid-base equilibrium of one apparent ionizing group with pK(a) values of 4.8 and 4.7, respectively. The different pK(a) values for the proton-linked spectroscopic transition(s) of tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains suggest that the quaternary assembly drastically affects the strength of the proximal HisF8-Fe bond in both subunits. This probably reflects a 'quaternary effect', i.e., structural changes in both subunits upon tetrameric assembly, which is associated to a relevant variation of functional properties (i.e., proton affinity).