Electrochemical Probing of Human Liver Subcellular S9 Fractions for Drug Metabolite Synthesis

Human liver subcellular fractions, including liver microsomes (HLM), liver cytosol fractions, and S9 fractions, are extensively utilized in in vitro assays to predict liver metabolism. The S9 fractions are supernatants of human liver homogenates that contain both microsomes and cytosol, which include most cytochrome P450 (CYP) enzymes and soluble phase II enzymes such as glucuronosyltransferases and sulfotransferases. This study reports on the direct electrochemistry and biocatalytic features of redox-active enzymes in S9 fractions for the first time. We investigated the electrochemical properties of S9 films by immobilizing them onto a high-purity graphite (HPG) electrode and performing cyclic voltammetry under anaerobic (Ar-saturated) and aerobic (O2-saturated) conditions. The heterogeneous electron transfer rate between the S9 film and the HPG electrode was found to be 14 ± 3 s-1, with a formal potential of -0.451 V vs. Ag/AgCl reference electrode, which confirmed the electrochemical activation of the FAD/FMN cofactor containing CYP450-reductase (CPR) as the electron receiver from the electrode. The S9 films have also demonstrated catalytic oxygen reduction under aerobic conditions, identical to HLM films attached to similar electrodes. Additionally, we investigated CYP activity in the S9 biofilm for phase I metabolism using diclofenac hydroxylation as a probe reaction and identified metabolic products using liquid chromatography-mass spectrometry (LC-MS). Investigating the feasibility of utilizing liver S9 fractions in such electrochemical assays offers significant advantages for pharmacological and toxicological evaluations of new drugs in development while providing valuable insights for the development of efficient biosensor and bioreactor platforms.

Multifaceted personality and roles of heme enzymes in industrial biotechnology

Heme enzymes are the most prominent category of iron-containing metalloenzymes with the capability of catalyzing an astonishingly wide range of reactions like epoxidation, hydroxylation, demethylation, desaturation, reduction, sulfoxidation, and decarboxylation. Various enzymes in this category are P450s, heme peroxidases, catalases, myoglobin, cytochrome C, and others. Besides this, the natural promiscuity and amenability of these enzymes to protein engineering and evolution have also added several non-native reactions such as C-H, N-H, S-H insertions, cyclopropanation, and other industrially important reactions to their capabilities. Surprisingly, all of these reactions and their wide substrate scopes are attributed to changes in the active site scaffold of different heme enzymes as the center of all enzymes is constituted by a porphyrin ring containing iron. Multiple prominent research groups across the world, including 2018, Nobel Laureate Frances Arnold's group, have shown keen interest in engineering and evolving these enzymes for utilizing their industrial potential. Besides engineering the active site, researchers have also explored the possibility of these enzymes catalyzing non-native reactions by replacing the center porphyrin ring with other cofactors or by changing the iron in the porphyrin ring with other metal ions along with engineering the active site and thereby creating novel artificial metalloenzymes. Thus, in this mini-review from our group, for the first time, we are trying to catalog various activities catalyzed by heme enzymes and their engineered variants and their active usage in various industries along with shedding light on their potential for use in various applications in the future.

Solar-powered P450 catalysis: Engineering electron transfer pathways from photosynthesis to P450s

With the increasing focus on green chemistry, biocatalysis is becoming more widely used in the pharmaceutical and other chemical industries for sustainable production of high value and structurally complex chemicals. Cytochrome P450 monooxygenases (P450s) are attractive biocatalysts for industrial application due to their ability to transform a huge range of substrates in a stereo- and regiospecific manner. However, despite their appeal, the industrial application of P450s is limited by their dependence on costly reduced nicotinamide adenine dinucleotide phosphate (NADPH) and one or more auxiliary redox partner proteins. Coupling P450s to the photosynthetic machinery of a plant allows photosynthetically-generated electrons to be used to drive catalysis, overcoming this cofactor dependency. Thus, photosynthetic organisms could serve as photobioreactors with the capability to produce value-added chemicals using only light, water, CO2 and an appropriate chemical as substrate for the reaction/s of choice, yielding new opportunities for producing commodity and high-value chemicals in a carbon-negative and sustainable manner. This review will discuss recent progress in using photosynthesis for light-driven P450 biocatalysis and explore the potential for further development of such systems.

Electrochemical transformations catalyzed by cytochrome P450s and peroxidases

Cytochrome P450s (Cyt P450s) and peroxidases are enzymes featuring iron heme cofactors that have wide applicability as biocatalysts in chemical syntheses. Cyt P450s are a family of monooxygenases that oxidize fatty acids, steroids, and xenobiotics, synthesize hormones, and convert drugs and other chemicals to metabolites. Peroxidases are involved in breaking down hydrogen peroxide and can oxidize organic compounds during this process. Both heme-containing enzymes utilize active Fe^IVO intermediates to oxidize reactants. By incorporating these enzymes in stable thin films on electrodes, Cyt P450s and peroxidases can accept electrons from an electrode, albeit by different mechanisms, and catalyze organic transformations in a feasible and cost-effective way. This is an advantageous approach, often called bioelectrocatalysis, compared to their biological pathways in solution that require expensive biochemical reductants such as NADPH or additional enzymes to recycle NADPH for Cyt P450s. Bioelectrocatalysis also serves as an ex situ platform to investigate metabolism of drugs and bio-relevant chemicals. In this paper we review biocatalytic electrochemical reactions using Cyt P450s including C-H activation, S-oxidation, epoxidation, N-hydroxylation, and oxidative N-, and O-dealkylation; as well as reactions catalyzed by peroxidases including synthetically important oxidations of organic compounds. Design aspects of these bioelectrocatalytic reactions are presented and discussed, including enzyme film formation on electrodes, temperature, pH, solvents, and activation of the enzymes. Finally, we discuss challenges and future perspective of these two important bioelectrocatalytic systems.

Electrochemistry-coupled to liquid chromatography-mass spectrometry-density functional theory as a new tool to mimic the environmental degradation of selected phenylurea herbicides

In vitro and in vivo experimental models, mainly based on cell cultures, animals, healthy humans and clinical trials, are useful approaches for identifying the main metabolic pathways. However, time, cost, and matrix complexity often hinder the success of these methods. In this study, we propose an alternative non-enzymatic method, using electrochemistry (EC) coupled to liquid chromatography (LC) - high resolution mass spectrometry (HRMS) - DFT theoretical calculations (EC/LC-MS/DFT) for the mimicry/simulation of the environmental degradation of phenylurea herbicides, and for the mechanism elucidation of this class of herbicides. Fenuron, monuron, isoproturon, linuron, monolinuron, metoxuron and chlortoluron were selected as relevant model compounds. The intended compounds are oxidized by EC, separated by LC and detected using electrospray ionization HRMS. The main oxidation products were hydroxylated compounds obtained by substitution and addition reactions. Unstable quinone imines/methines, rarely observed by conventional methods, have been identified during the oxidative degradation of phenylurea herbicides for the first time in this study. Some were directly observed and the others were trapped by glutathione GSH. Reactions such as hydrolytic substitutions (-Cl/+OH and -C₃H₇/+OH and -CH₃/+OH and -OCH₃/+OH), aromatic hydroxylation, alkyl carbon hydroxylation, dehydrochlorination/dehydromethylation/dehydromethoxylation and conjugation have been successfully mimicked. The obtained results, supported by theoretical calculations, are useful for simulating/understanding and predicting the oxidative degradation pathways of pesticides in the environment.

Hybrid Carbon Film Electrodes for Electroanalysis

Carbon materials have been widely used for electrochemical analysis and include carbon nanotubes, graphene, and boron-doped diamond electrodes in addition to conventional carbon electrodes, such as those made of glassy carbon and graphite. Of the carbon-based electrodes, carbon film has advantages because it can be fabricated reproducibly and micro- or nanofabricated into electrodes with a wide range of shapes and sizes. Here, we report two categories of hybrid-type carbon film electrodes for mainly electroanalytical applications. The first category consists of carbon films doped or surface terminated with other atoms such as nitrogen, oxygen and fluorine, which can control surface hydrophilicity and lipophilicity or electrocatalytic performance, and are used to detect various electroactive biochemicals. The second category comprises metal nanoparticles embedded in carbon film electrodes fabricated by co-sputtering, which exhibits high electrocatalytic activity for environmental and biological samples including toxic heavy metal ions and clinical sugar markers, which are difficult to detect at pure carbon-based electrodes.

Recent Advances in Electrochemical Biosensors Based on Enzyme Inhibition for Clinical and Pharmaceutical Applications

A large number of enzyme inhibitors are used as drugs to treat several diseases such as gout, diabetes, AIDS, depression, Parkinson's and Alzheimer's diseases. Electrochemical biosensors based on enzyme inhibition are useful devices for an easy, fast and environment friendly monitoring of inhibitors like drugs. In the last decades, electrochemical biosensors have shown great potentials in the detection of different drugs like neostigmine, ketoconazole, donepezil, allopurinol and many others. They attracted increasing attention due to the advantage of being high sensitive and accurate analytical tools, able to reach low detection limits and the possibility to be performed on real samples. This review will spotlight the research conducted in the past 10 years (2007-2017) on inhibition based enzymatic electrochemical biosensors for the analysis of different drugs. New assays based on novel bio-devices will be debated. Moreover, the exploration of the recent graphical approach in diagnosis of reversible and irreversible inhibition mechanism will be discussed. The accurate and the fast diagnosis of inhibition type will help researchers in further drug design improvements and the identification of new molecules that will serve as new enzyme targets.

A Review on the Respiratory System Toxicity of Carbon Nanoparticles

The respiratory system represents the main gateway for nanoparticles’ entry into the human body. Although there is a myriad of engineered nanoparticles, carbon nanoparticles/nanotubes (CNPs/CNTs) have received much attention mainly due to their light weight, very high surface area, durability, and their diverse applications. Since their discovery and manufacture over two decades ago, much has been learned about nanoparticles’ interactions with diverse biological system models. In particular, the respiratory system has been of great interest because various natural and man-made fibrous particles are known to be responsible for chronic and debilitating lung diseases. In this review, we present up-to-date the literature regarding the effects of CNTs or carbon nanofibers (CNFs) on the human respiratory system with respect to respiratory toxicity pathways and associated pathologies. This article is intended to emphasize the potentially dangerous effects to the human respiratory system if inadequate measures are used in the manufacture, handling, and preparation and applications of CNP or CNP-based products.

Nanocomposites of graphene and cytochrome P450 2D6 isozyme for electrochemical-driven tramadol metabolism

Cytochrome P450 enzymes (cyt P450s) with an active center of iron protoheme are involved in most clinical drugs metabolism process. Herein, an electrochemical platform for the investigation of drug metabolism in vitro was constructed by immobilizing cytochrome P450 2D6 (CYP2D6) with cyt P450 reductase (CPR) on graphene modified glass carbon electrode. Direct and reversible electron transfer of the immobilized CYP2D6 with the direct electron transfer constant of 0.47 s(-1) and midpoint potential of -0.483 V was obtained. In the presence of substrate tramadol, the electrochemical-driven CYP2D6 mediated catalytic behavior toward the conversion of tramadol to o-demethyl-tramadol was confirmed. The Michaelis-Menten constant (Km(app)) and heterogeneous reaction rate constant during the metabolism of tramadol were calculated to be 23.85 μM and 1.96 cm s(-1), respectively. The inhibition effect of quinidine on CYP2D6 catalyze-cycle was also investigated. Furthermore, this system was applied to studying the metabolism of other drugs.