Development of a Direct and Continuous Phospholipase D Assay Based on the Chelation-Enhanced Fluorescence Property of 8-Hydroxyquinoline

A reversible CHEF-based NIR fluorescent probe for sensing Hg2+ and its multiple application in environmental media and biological systems

Hg²⁺ poses a great threat to human health and the environment due to its bioaccumulation and permanent damage. Herein, a reversible CHEF-based near-infrared fluorescent probe 2-(3-((E)-4-((E)-4-(diethylamino)-2- hydroxybenzylidene)amino)styryl)-5,5-dimethylcyclohex-2-en-1-ylidene)propanedinitrile (DHEY) capable of specifically recognizing Hg²⁺ was constructed. DHEY exhibits advantages of large Stokes shift (157 nm), excellent selectivity, high sensitivity (LOD = 3.2 μg/L), and fast response efficiency (<3 min). Interestingly, DHEY can also realize rapid and effective detection of Hg²⁺ after being recycled 7 times. The successful recovery of trace Hg²⁺ in different environmental water samples fully demonstrates the potential of DHEY for actual applications. In particular, DHEY enables real-time observation of the distribution and translocation pattern of exogenous Hg²⁺ in HeLa cells and zebrafish. This work provides important theoretical support for investigating the fate of heavy metal ions in the environment using fluorescence techniques.

Boron carbon oxynitride quantum dots-based ratio fluorescent nanoprobe assisted with smartphone for visualization detection of phosphate

A ratio fluorescence nanoprobe was constructed by simple mixing BCNO QDs with 8-hydroxyquinoline-5-sulfonic acid (HQSA), which had an obvious fluorescence peak at 420 nm and a weak fluorescence peak at 500 nm, corresponding to the BCNO QDs and HQSA, respectively. This fluorescence probe takes stable fluorescence of BCNO QDs as an internal standard, based on HQSA chelating enhanced fluorescence and specificity of phosphate in the presence of Mg²⁺, which realizes a rapid and sensitive detection of phosphate with good linearity in the range 0.3-50 μM and 50-100 μM and a detection limit of 0.073 μM. The recovery is between 94.1 and 111% and the relative standard deviations (RSDs) below 10%. At the same time, we took color photos of the reaction solution under 310-nm UV lamp with smartphones for visual detection through RGB data image analysis, which make the detection easier and faster. The proposed method provides a new strategy for the intelligent online detection of other targets in complex environment samples.

Phospholipids (PLs) know-how: exploring and exploiting phospholipase D for its industrial dissemination

Owing to their numerous nutritional and bioactive functions, phospholipids (PLs), which are major components of biological membranes in all living organisms, have been widely applied as nutraceuticals, food supplements, and cosmetic ingredients. To date, PLs are extracted solely from soybean or egg yolk, despite the diverse market demands and high cost, owing to a tedious and inefficient manufacturing process. A microbial-based manufacturing process, specifically phospholipase D (PLD)-based biocatalysis and biotransformation process for PLs, has the potential to address several challenges associated with the soybean- or egg yolk-based supply chain. However, poor enzyme properties and inefficient microbial expression systems for PLD limit their wide industrial dissemination. Therefore, sourcing new enzyme variants with improved properties and developing advanced PLD expression systems are important. In the present review, we systematically summarize recent achievements and trends in the discovery, their structural properties, catalytic mechanisms, expression strategies for enhancing PLD production, and its multiple applications in the context of PLs. This review is expected to assist researchers to understand current advances in this field and provide insights for further molecular engineering efforts toward PLD-mediated bioprocessing.

8-Hydroxyquinoline Fluorophore for Sensing of Metal Ions and Anions

Among various known hydroxyquinolines, 8-hydroxyquinoline (8-HQ) is the most prevalent moiety due to excellent property for the formation of the complex with different metal ions and anions, and utilized in a wide variety of applications in pharmacological and medicinal fields. 8-Hydroxyquinoline moiety and its analogues acts as fluorophoric ligands on complex formation with alkali and alkaline as well as transition metal ions and anions, thus, considered as an ideal building block in metallo-supramolecular chemistry for recognition, separation, and quantitative investigation of cations. 8-Hydroxyquinoline moiety is also used in various applications for the advancement of novel fluorescent chemosensors in a wide variety of areas viz., material chemistry, bioorganic chemistry, molecular imaging, analytical chemistry, molecular recognition, medical and biological science communities. The present review emphasises on the progress of sensing properties of 8-HQ centred small-molecule fluorescent chemosensors towards several metal ions viz., Fe³⁺ , Al³⁺ , Ag⁺ , Hg²⁺ , Cu²⁺ , Pd²⁺ , Zn²⁺ , Cr³⁺ , Cd²⁺ , Mn²⁺ , Ca²⁺ , and K⁺ and anions such as F^- , CN^- and PPi, from 2008 to 2020, because of their sensitivity and selectivity in terms of diverse colour changes for different species. This critical and comprehensive review might facilitate the improvement of more prevailing chemosensors for future exciting and broad applications.

Phospholipase D inhibitors screening: Probing and evaluation of ancient and novel molecules

Phospholipase D (PLD) is a ubiquitous enzyme that cleaves the distal phosphoester bond of phospholipids generating phosphatidic acid (PA). In plants, PA is involved in numerous cell responses triggered by stress. Similarly, in mammals, PA is also a second messenger involved in tumorigenesis. PLD is nowadays considered as a therapeutic target and blocking its activity with specific inhibitors constitutes a promising strategy to treat cancers. Starting from already described PLD inhibitors, this study aims to investigate the effect of their structural modifications on the enzyme's activity, as well as identifying new potent inhibitors of eukaryotic PLDs. Being able to purify the plant PLD from Vigna unguiculata (VuPLD), we obtained a SAXS model of its structure. We then used a fluorescence-based test suitable for high-throughput screening to review the effect of eukaryotic PLD inhibitors described in the literature. In this regard, we found that only few molecules were in fact able to inhibit VuPLD and we confirmed that vanadate is the most potent of all with an IC₅₀ around 58 μM. Moreover, the small-scale screening of a chemical library of 3120 compounds allowed us to optimize the different screening's steps and paved the way towards the discovery of new potent inhibitors.

Assay of Phospholipase D Activity by an Amperometric Choline Oxidase Biosensor

A novel electrochemical method to assay phospholipase D (PLD) activity is proposed based on the employment of a choline biosensor realized by immobilizing choline oxidase through co-crosslinking on an overoxidized polypyrrole film previously deposited on a platinum electrode. To perform the assay, an aliquot of a PLD standard solution is typically added to borate buffer containing phosphatidylcholine at a certain concentration and the oxidation current of hydrogen peroxide is then measured at the rotating modified electrode by applying a detection potential of +0.7 V vs. SCE. Various experimental parameters influencing the assay were studied and optimized. The employment of 0.75% (v/v) Triton X-100, 0.2 mM calcium chloride, 5 mM phosphatidylcholine, and borate buffer at pH 8.0, ionic strength (I) 0.05 M allowed to achieve considerable current responses. In order to assure a controlled mass transport and, at the same time, high sensitivity, an electrode rotation rate of 200 rpm was selected. The proposed method showed a sensitivity of 24 (nA/s)⋅(IU/mL)⁻¹, a wide linear range up to 0.33 IU/mL, fast response time and appreciable long-term stability. The limit of detection, evaluated from the linear calibration curve, was 0.005 IU/mL (S/N = 3). Finally, due to the presence of overoxidized polypyrrole film characterized by notable rejection properties towards electroactive compounds, a practical application to real sample analysis can be envisaged.

An efficient water-soluble fluorescent chemosensor based on furan Schiff base functionalized PEG for the sensitive detection of Al3+ in pure aqueous solution

An efficient reversible fluorescent chemosensor, PEGFB, based on polyethylene glycol bearing a furan Schiff base unit has been successfully developed to sensitively detect Al³⁺ in pure aqueous solution.

Fluorogenic probes for disease-relevant enzymes

Traditional biochemical methods for enzyme detection are mainly based on antibody-based immunoassays, which lack the ability to monitor the spatiotemporal distribution and, in particular, the in situ activity of enzymes in live cells and in vivo. In this review, we comprehensively summarize recent progress that has been made in the development of small-molecule as well as material-based fluorogenic probes for sensitive detection of the activities of enzymes that are related to a number of human diseases. The principles utilized to design these probes as well as their applications are reviewed. Specific attention is given to fluorogenic probes that have been developed for analysis of the activities of enzymes including oxidases and reductases, those that act on biomacromolecules including DNAs, proteins/peptides/amino acids, carbohydrates and lipids, and those that are responsible for translational modifications. We envision that this review will serve as an ideal reference for practitioners as well as beginners in relevant research fields.

Direct and Continuous Measurement of Phospholipase D Activities Using the Chelation-Enhanced Fluorescence Property of 8-Hydroxyquinoline

Phospholipase D (PLD) hydrolyzes phospholipids to form phosphatidic acid (PA) and the corresponding headgroup. To date, PLD has been linked to several pathologies, such as cancer, making this enzyme an important therapeutic target. However, most PLD assays developed so far are either discontinuous or based on the indirect determination of choline released upon phosphatidylcholine (PC) hydrolysis. Therefore, we designed a PLD assay that is based on the chelation-enhanced fluorescence property of 8-hydroxyquinoline. This assay exhibits a strong fluorescence signal upon Ca²⁺ complexation with the PLD-generated PA and is not limited to PC as the substrate but allows the use of natural phospholipids with various headgroups. Besides, this easy-to-handle assay allows to monitor prokaryotic and eukaryotic PLD activities in a continuous way and on a microplate scale.

A simple Schiff base as dual-responsive fluorescent sensor for bioimaging recognition of Zn2+ and Al3+ in living cells

A simple Schiff base fluorescent sensor (BDNOL) was synthesized from the reaction of picolinohydrazide and 4-(diethylamino)salicylaldehyde, and developed for selective detection of Al³⁺ and Zn²⁺. This non-fluorescent sensor displayed obvious fluorescence enhancement after binding to Al³⁺/Zn²⁺ ions with high sensitivity and selectivity, accompanied by obvious fluorescence emission enhancement (504 nm for Al³⁺ and 575 nm for Zn²⁺). The detection limits were found to be 8.30 × 10^-8 M for Al³⁺ and 1.24 × 10^-7 M for Zn²⁺. The binding mechanisms between BDNOL and Al³⁺/Zn²⁺ ions were supported by ¹H NMR and HR-MS analysis, and a density functional theory (DFT) study. The sensing behavior was also studied with molecular logic functions of OR, AND, and NOT gates. Furthermore, the fluorescent sensor was successfully used to recognize Al³⁺ and Zn²⁺ in living cells, suggesting that this simple biosensor has great potential in biological imaging applications.

Phospholipases: An Overview

Phospholipases are lipolytic enzymes that hydrolyze phospholipid substrates at specific ester bonds. Phospholipases are widespread in nature and play very diverse roles from aggression in snake venom to signal transduction, lipid mediator production, and metabolite digestion in humans. Phospholipases vary considerably in structure, function, regulation, and mode of action. Tremendous advances in understanding the structure and function of phospholipases have occurred in the last decades. This introductory chapter is aimed at providing a general framework of the current understanding of phospholipases and a discussion of their mechanisms of action and emerging biological functions.

Long-Chain 4-Aminoquinolines as Quorum Sensing Inhibitors in Serratia marcescens and Pseudomonas aeruginosa

Antibiotic resistance has become a serious global threat to public health; therefore, improved strategies and structurally novel antimicrobials are urgently needed to combat infectious diseases. Here we report a new type of highly potent 4-aminoquinoline derivatives as quorum sensing inhibitors in Serratia marcescens and Pseudomonas aeruginosa, exhibiting weak bactericidal activities (minimum inhibitory concentration (MIC) > 400 μM). Through detailed structure-activity study, we have identified 7-Cl and 7-CF₃ substituted N-dodecylamino-4-aminoquinolines (5 and 10) as biofilm formation inhibitors with 50% biofilm inhibition at 69 μM and 63 μM in S. marcescens and P. aeruginosa, respectively. These two compounds, 5 and 10, are the first quinoline derivatives with anti-biofilm formation activity reported in S. marcescens. Quantitative structure-activity relationship (QSAR) analysis identified structural descriptors such as Wiener indices, hyper-distance-path index (HDPI), mean topological charge (MTC), topological charge index (TCI), and log D(o/w)_exp as the most influential in biofilm inhibition in this bacterial species. Derivative 10 is one of the most potent quinoline type inhibitors of pyocyanin production described so far (IC₅₀ = 2.5 μM). While we have demonstrated that 5 and 10 act as Pseudomonas quinolone system (PQS) antagonists, the mechanism of inhibition of S. marcescens biofilm formation with these compounds remains open since signaling similar to P. aeruginosa PQS system has not yet been described in Serratia and activity of these compounds on acylhomoserine lactone (AHL) signaling has not been detected. Our data show that 7-Cl and 7-CF₃ substituted N-dodecylamino-4-aminoquinolines present the promising scaffolds for developing antivirulence and anti-biofilm formation agents against multidrug-resistant bacterial species.

Functional Characterization of the N-Terminal C2 Domain from Arabidopsis thaliana Phospholipase Dα and Dβ

Most of plant phospholipases D (PLD) exhibit a C2-lipid binding domain of around 130 amino acid residues at their N-terminal region, involved in their Ca²⁺-dependent membrane binding. In this study, we expressed and partially purified catalytically active PLDα from Arabidopsis thaliana (AtPLDα) in the yeast Pichia pastoris. The N-terminal amino acid sequence of the recombinant AtPLDα was found to be NVEETIGV and thus to lack the first 35 amino acid belonging to the C2 domain, as found in other recombinant or plant purified PLDs. To investigate the impact of such a cleavage on the functionality of C2 domains, we expressed, in E. coli, purified, and refolded the mature-like form of the C2 domain of the AtPLDα along with its equivalent C2 domain of the AtPLDβ, for the sake of comparison. Using Förster Resonance Energy Transfer and dot-blot assays, both C2 domains were shown to bind phosphatidylglycerol in a Ca²⁺-independent manner while phosphatidic acid and phosphatidylserine binding were found to be enhanced in the presence of Ca²⁺. Amino acid sequence alignment and molecular modeling of both C2 domains with known C2 domain structures revealed the presence of a novel Ca²⁺-binding site within the C2 domain of AtPLDα.

Target-controlled gating liposome “off–on” cascade amplification for sensitive and accurate detection of phospholipase D in breast cancer cells with a low-background signal

Here we developed a simple, sensitive and accurate PLD detection method based on a target-controlled gating liposome (TCGL) “off–on” cascade amplified strategy and personal glucose meters (PGMs).