Two Pathways of Carbon Dioxide Catalyzed Oxidative Coupling of Phenol by Peroxynitrite

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.

Carbon dioxide-catalyzed peroxynitrite reactivity - The resilience of the radical mechanism after two decades of research

Peroxynitrite, ONOO^-, formed in tissues that are simultaneously generating NO^• and O₂^•-, is widely regarded as a major contributor to oxidative stress. Many of the reactions involved are catalyzed by CO₂ via formation of an unstable adduct, ONOOC(O)O^-, that undergoes O-O bond homolysis to produce NO₂^• and CO₃^•- radicals, whose yields are equal at about 0.33 with respect to the ONOO^- reactant. Since its inception two decades ago, this radical-based mechanism has been frequently but unsuccessfully challenged. The most recent among these [Serrano-Luginbuehl et al. Chem. Res. Toxicol.31:721-730; 2018] claims that ONOOC(O)O^- is stable, predicts a yield of NO₂^•/CO₃^•- of less than 0.01 under physiological conditions and, contrary to widely accepted viewpoints, suggests that radical generation is inconsequential to peroxynitrite-induced oxidative damage. Here we review the experimental and theoretical evidence that support the radical model and show this recently proposed alternative mechanism to be incorrect.

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.

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.

Peroxynitrite-An ugly biofactor?

Cellular damage occurring under oxidative conditions has been ascribed mainly to the formation of peroxynitrite (ONOOH/ONOO(-)) that originates from the reaction of NO(*) with O(2) (*-). The detrimental effects of peroxynitrite are exacerbated by the reaction with CO(2) that leads to ONOOC(O)O(-), which further decays to the strong oxidant radicals NO(2) (*) and CO(3) (*-). The reaction with CO(2), however, may redirect peroxynitrite specificity. An excessive formation of peroxynitrite represents an important mechanism contributing to the DNA damage, the inactivation of metabolic enzymes, ionic pumps, and structural proteins, and the disruption of cell membranes. Because of its ability to oxidize biomolecules, peroxynitrite is implicated in an increasing list of diseases, including neurodegenerative and cardiovascular disorders, inflammation, pain, autoimmunity, cancer, and aging. However, peroxynitrite displays also protective activities: (i) at high concentrations, it shows anti-viral, anti-microbial, and anti-parasitic actions; and (ii) at low concentrations, it stimulates protective mechanisms in the cardiovascular, nervous, and respiratory systems. The detrimental effects of peroxynitrite and related reactive species are impaired by (pseudo-) enzymatic systems, mainly represented by heme-proteins (e.g., hemoglobin and myoglobin). Here, we report biochemical aspects of peroxynitrite actions being at the root of its biomedical effects.

Nitration processes of acetaminophen in nitrifying activated sludge

This work is an attempt to elucidate the quantitative significance of acetaminophen (APAP) nitration in nitrifying activated sludge and to propose a reaction mechanism for this process. The link between nitrification and nitration of APAP was investigated at different scales. Results from field studies showed the occurrence of 3-nitro-APAP and to a lesser extent 3-chloro-5-nitro-APAP at concentration levels in the 50-300 ng/L range in effluents of a full scale wastewater treatment plant (WWTP) operated with nitrogen removal, whereas 3-hydroxy-APAP was eliminated after the nitrification step. Batch experiments with nitrifying activated sludge confirmed APAP transformation by nitration and suggested that nitrifying bacteria may play a role in this transformation process through the release of reactive nitrogen species. In vitro assays provided evidence that nitration through the production of nitrous acid is a very unlikely pathway. In contrast, nitric oxide (*NO) produced by nitrifying bacteria is probably involved in APAP nitration through the formation of peroxynitrite in presence of superoxide anion. The production of 3-nitro-APAP would only account for a few percents of the total transformation rate of APAP in WWTPs. The production of nitrated derivatives is highly relevant because of the potential ecotoxicological risks of these compounds.

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).