In vitro studies on sterically stabilized liposomes (SL) as enzyme carriers in organophosphorus (OP) antagonism

Recombinant Organophosphorus acid anhydrolase (OPAA) enzyme-carbon quantum dot (CQDs)-immobilized thin film biosensors for the specific detection of Ethyl Paraoxon and Methyl Parathion in water resources

Organophosphates pesticide (OP) toxicity through water resources is a large concern globally among all the emerging pollutants. Detection of OPs is a challenge which needs to be addressed considering the hazardous effects on the health of human beings. In the current research thin film biosensors of recombinant, Organophosphorus acid anhydrolase (OPAA) enzyme along with carbon quantum dots (CQDs) immobilized in thin films were developed. OPAA-CQDs thin film biosensors were used for the specific detection of two OPs Ethyl Paraoxon (EP) and Methyl Parathion (MP) in river water and household water supply. Recombinant OPAA enzyme was expressed in E. Coli, purified and immobilized on the CQD containing chitosan thin films. The CQDs used for this purpose were developed by a one-pot hydrothermal method from phthalic acid and Tri ethylene diamine. The properties of CQDs, OPAA and thin films were characterized using techniques like XPS, TEM, XRD, enzyme activity and CLSM measurements. Biosensing studies of EP and MP were performed by taking fluorescence measurements using a fiber optic spectrometer. The analytical parameters of biosensing were compared against an estimation carried out using the HPLC method. The biosensing performance indicates that the OPAA-CQDs thin film-based biosensors were able to detect both EP and MP in a range of 0-100 μM having a detection limit of 0.18 ppm/0.69 ppm for EP/MP, respectively with a response time of 5 min. The accuracy of estimation of EP/MP when spiked in water resources lie in the range of ∼100-102% which clearly indicates the OPAA-CQD based thin film biosensors can function as a point-of-use method for the detection of OP pesticides in complex water resources.

Insights in detection and analysis of organophosphates using organophosphorus acid anhydrolases (OPAA) enzyme-based biosensors

The human population is dependent on agriculture for its food requirements and survival. Several insecticides and pesticides have found their use for improvements in agricultural yields. Organophosphates (OP) are one of the many compounds used as insecticides and pesticides. OPs have also been used to develop G and V-series chemicals which act as highly toxic nerve agents that can severely influence the normal function of the nervous system in all living beings. Thus, OP compounds utilized as insecticides/pesticides and nerve agents are hazardous to the environment, lethal for humans and other non-target animals. To avoid their toxicity, approaches to detect and neutralize them have become essential. A variety of analytical procedures such as electrochemical processes and chromatography methods, namely liquid and gas chromatography, have been employed to detect OPs. Though these techniques are sensitive and highly accurate they suffer from drawbacks, for instance: their bulky nature and expensive instrumentation, the difficulty of operation, long detection times, and they can yield unpredictable results with variable sample complexities. With the advent of several types of biosensors, the assay of OP compounds has become simpler, faster, cost-effective with improved sensitivity, and provides the capability for onsite detection. OP biosensor assays typically utilize several enzymes with the capability to hydrolyze/degrade OP compounds, such as organophosphate hydrolase (OPH) and organophosphate acid hydrolase (OPAA). This review focuses on discussing various aspects of OPAA as biological recognition unit in terms of its: structure, properties, activity enhancement methods, and utilization for developing OPAA-based biosensing technologies for insecticides, pesticides, and nerve agents.

Organophosphorus poisoning in animals and enzymatic antidotes

Organophosphorus compounds (OPs) are neurotoxic molecules developed as pesticides and chemical warfare nerve agents (CWNAs). Most of them are covalent inhibitors of acetylcholinesterase (AChE), a key enzyme in nervous systems, and are therefore responsible for numerous poisonings around the world. Many animal models have been studied over the years in order to decipher the toxicity of OPs and to provide insights for therapeutic and decontamination purposes. Environmental impact on wild animal species has been analyzed to understand the consequences of OP uses in agriculture. In complement, various laboratory models, from invertebrates to aquatic organisms, rodents and primates, have been chosen to study chronic and acute toxicity as well as neurobehavioral impact, immune response, developmental disruption, and other pathological signs. Several decontamination approaches were developed to counteract the poisoning effects of OPs. Among these, enzyme-based strategies are particularly attractive as they allow efficient external decontamination without toxicity or environmental impact and may be of interest for treatment. Approaches using bioscavengers for prophylaxis, treatment, and external decontamination are emphasized and their potential is discussed in the light of toxicological observations from various animal models. The relevance of animal models, regarding their cholinergic system and the abundance of naturally protecting enzymes, is also discussed for better extrapolation of results to human.

[Organophosphorus poisoning: Towards enzymatic treatments]

Organophosphorus compounds (OP) are toxic molecules developed as insecticides and chemical warfare nerve agents (CWNAs). Most OP are neurotoxic and act as nervous system disruptors by blocking cholinergic transmission. They are therefore responsible for many poisonings worldwide. OP toxicity may result either from acute or chronic exposure, and their poisoning effect were evaluated using several animal models. These latter were also used for evaluating the efficacy of antidotes. Strategies based on enzymes that can trap (stoichiometric bioscavengers) or degrade (catalytic bioscavengers) OP, were particularly studied since they allow effective decontamination, without toxicity or environmental impact. This review summarizes the results obtained in vivo with enzymes through three levels: prophylaxis, treatment and external decontamination. The efficiency of enzymatic treatments in different animal models is presented and the relevance of these models is also discussed for a better extrapolation to humans.

Enzyme therapeutics for systemic detoxification

Life relies on numerous biochemical processes working synergistically and correctly. Certain substances disrupt these processes, inducing living organism into an abnormal state termed intoxication. Managing intoxication usually requires interventions, which is referred as detoxification. Decades of development on detoxification reveals the potential of enzymes as ideal therapeutics and antidotes, because their high substrate specificity and catalytic efficiency are essential for clearing intoxicating substances without adverse effects. However, intrinsic shortcomings of enzymes including low stability and high immunogenicity are major hurdles, which could be overcome by delivering enzymes with specially designed nanocarriers. Extensive investigations on protein delivery indicate three types of enzyme-nanocarrier architectures that show more promise than others for systemic detoxification, including liposome-wrapped enzymes, polymer-enzyme conjugates, and polymer-encapsulated enzymes. This review highlights recent advances in these nano-architectures and discusses their applications in systemic detoxifications. Therapeutic potential of various enzymes as well as associated challenges in achieving effective delivery of therapeutic enzymes will also be discussed.

Selection of a human butyrylcholinesterase-like antibody single-chain variable fragment resistant to AChE inhibitors from a phage library expressed in E. coli

Organophosphates are potent poisoning agents that cause severe cholinergic toxicity. Current treatment has been reported to be unsatisfactory and novel antidotes are needed. In this study, we used a single-chain variable fragment (scFv) library to select a recombinant antibody fragment (WZ1-14.2.1) with butyrylcholinesterase-like catalytic activity by using an innovative method integrating genetic selection and the bait-and-switch strategy. Ellman assay demonstrated that WZ1-14.2.1 has Michaelis-Menten kinetics in the hydrolysis of all the three substrates used, acetylthiocholine, propionylthiocholine and butyrylthiocholine. Notably, the catalytic activity was resistant to the following acetylcholinesterase inhibitors: neostigmine, iso-OMPA, chlorpyrifos oxon, dichlorvos, and paraoxon ethyl. Otherwise, the enzymatic activity of WZ1-14.2.1 was inhibited by the selective butyrylcholinesterase inhibitor, ethopropazine, and by the Ser-blocking agent phenylmethanesuphonyl fluoride. A hypothetical 3D structure of the WZ1-14.2.1 catalytic site, compatible with functional results, is proposed on the basis of a molecular modeling analysis.

Rate of solute incorporation to liposomes evaluated from encapsulated enzymes activities

There are numerous studies on systems comprising an enzyme encapsulated in unilamellar liposomes and its substrate initially present in the external aqueous media. Most of these studies are focused on enzyme stability and activity in a restricted media. However, the rate of the process is also determined by the capacity of the substrate to permeate towards the liposome inner pool. In spite of this, there are few studies aimed at a quantitative evaluation of the substrate permeation rate and its lifetime inside the liposome pool. In the present work, we describe, in terms of a very simple mechanism, the permeation of glucose and hydrogen peroxide in DPPC unilamellar liposomes. To this aim, we evaluated the rate of the process employing encapsulated glucose oxidase and catalase in the kinetic diffusion controlled limit. Under this condition, the rate of the process becomes zero order in the enzyme and allows a direct evaluation of the rate constant for the permeation process and the lifetime of a substrate molecule incorporated into the liposome inner pool.

Prolidase function in proline metabolism and its medical and biotechnological applications

Prolidase is a multifunctional enzyme that possesses the unique ability to degrade imidodipeptides in which a proline or hydroxyproline residue is located at the C-terminal end. Prolidases have been isolated from archaea and bacteria, where they are thought to participate in proline recycling. In mammalian species, prolidases are found in the cytoplasm and function primarily to liberate proline in the final stage of protein catabolism, particularly during the biosynthesis and degradation of collagen. Collagen comprises nearly one-third of the total protein in the body, and it is essential in maintaining tissue structure and integrity. Prolidase deficiency (PD), a rare autosomal recessive disorder in which mutations in the PEPD gene affect prolidase functionality, tends to have serious and sometimes life-threatening clinical symptoms. Recombinant prolidases have many applications and have been investigated not only as a possible treatment for PD, but also as a part of anti-cancer strategies, a component of biodecontamination cocktails and in the dairy industry. This review will serve to discuss the many in vivo functions of procaryotic and eucaryotic prolidases, as well as the most recent advances in therapeutic and biotechnological application of prolidases.

Nano-intercalated organophosphorus-hydrolyzing enzymes in organophosphorus antagonism

A dendritic poly(2-alkyloxazoline)-based polymer was studied as a new carrier system for the organophosphorus-hydrolyzing recombinant enzymes, organophosphorus acid anhydrolase and organophosphorus hydrolase. Paraoxon (PO) and diisopropylfluorophosphate (DFP) were used as model organophosphorus compounds. Changes in plasma cholinesterase activity were monitored. The cholinesterase activity was proportional to the concentrations of DFP or PO. Plasma cholinesterase activity was higher in animals receiving enzyme and oxime before the organophosphates than in the oxime-only pretreated groups. These studies suggest that cholinesterase activity can serve as an indicator for the in vivo protection by the nano-intercalated organophosphorus acid anhydrolase or organophosphorus hydrolase against organophosphorus intoxications. These studies represent a practical application of polymeric nano-delivery systems as enzyme carriers in drug antidotal therapy.

Nanoencapsulated and microencapsulated enzymes in drug antidotal therapy

A catalytic bioscavenger for the therapeutic and prophylactic defense against recognized chemical threat agents has been a long-standing objective of civilian and military research. Among the toxic agents, organophosphate molecules and cyanide have been widely studied. In order to overcome the limitations of traditional antidotal therapies, isolated, purified, recombinant enzymes with bacterial origin possessing fast catalytic activity were used in in vitro and in vivo experiments. However, the fast degradation, excretion and adverse immunologic reaction against enzymes limit their in vivo use. Development of biodegradable, nontoxic carrier systems, microparticles, and nanoparticles—offering advantageous pharmacokinetic parameters was suggested. Present work deals with the perspectives of carrier systems, such as resealed and annealed erythrocytes and sterically stabilized liposomes. Dendritic polymers and polymer-conjugated enzymes, being in the focus of extensive research efforts nowadays, are also discussed.

Physicochemical characterization of stealth liposomes encapsulating an organophosphate hydrolyzing enzyme

The present studies were focused on the preparation and characterization of stericaly stabilized liposomes (SLs) encapsulating a recombinant organophosphorus hydrolyzing phosphotriesterase (OPH) enzyme for the antagonism of organophosphorus intoxication. Earlier results indicate that the liposomal carrier system provides an enhanced protective effect against the organophosphorus molecule paraoxon, presenting a more effective therapy with less toxicity than the most commonly used antidotes. Physicochemical characterization of the liposomal OPH delivery system is essential in order to get information on its in vitro stability and in vivo fate. Osmolarity, pH, viscosity, and encapsulation efficiency of the SL preparation and the surface potential of the vesicles were determined. The membrane rigidity and the impact of OPH enzyme on it was studied by electron-paramagnetic resonance spectroscopy, using spin probes. The in vitro stability of the liposomal preparations, the vesicle size distribution, and its alteration during a 3-week storage were followed by dynamic light-scattering measurements. Further, the stability of encapsulated and nonencapsulated OPH was compared in puffer and plasma.

Characterization of liposomal vesicles encapsulating rhodanese for cyanide antagonism

The major mechanism of removing cyanide from the body is its enzymatic conversion by a sulfurtransferase, e.g. rhodanese, to the less toxic thiocyanate in the presence of a sulfur donor. Earlier results demonstrated that externally administered encapsulated rhodanese significantly enhances the in vivo efficacy of the given sulfur donor. Present studies are focused on liposomal carrier systems encapsulating rhodanese. Physicochemical properties, e.g. membrane rigidity, size distribution, surface potential, osmolarity, and viscosity, were determined for various liposomal lipid compositions and hydrating buffers to establish in vitro stability and in vivo fate. Lipid composition was also optimized to achieve maximum encapsulation efficiency.

Current and Emerging Detoxification Therapies for Critical Care

Toxicity resulting from prescription drugs such as tricyclic antidepressants and cardioactive steroids, as well as drugs of abuse and exposure to environmental chemicals, represents a major need for detoxification treatments. Particles and colloids, antibody fragments (Fab), and indirect treatment methods such as macroemulsions, are currently being developed or employed as detoxification therapies. Colloids, particles, and protein fragments typically mitigate toxicity by binding to the toxin and reducing its concentration in vital organs. Indirect methods such as macroemulsions and sodium bicarbonate act directly on the affected organs, rather than the toxin. In this review, key design parameters (i.e. binding affinity, biocompatibility, pharmacokinetics) are discussed for each type of detoxification treatment. In addition, some of the latest research in each area is reviewed.

Solvent-solute interactions in hydrofluoroalkane propellants

Understanding solvation in hydrofluoroalkane (HFA) propellants is of great importance for the development of novel pressurized metered-dose inhaler (pMDI) formulations. HFA-based pMDIs are not only the most widely used inhalation therapy devices for delivering small drug molecules to the respiratory tract, but they also hold promise as vehicles for the delivery of therapeutic biomolecules to and through the lungs. In this work we use binding energy calculations to determine the degree of interaction between HFA propellants and candidate HFA-philes, including a methyl-based tail (isohexane, ISO), and fragments of poly(ethylene oxide) (EO), poly(propylene oxide) (PO), and poly(lactide) (LA). The distinct nature of solvation forces of the two HFA propellants approved by the FDA for use in pMDIs, 1,1,1,2-tetrafluoroethane (HFA134a) and 1,1,1,2,3,3,3-heptafluoropropane (HFA227), is also studied. Binding energy (Ebst) calculations demonstrated that an increase in tail polarity through the addition of oxygen atoms in the fragment backbone provides for sites capable of interacting with the HFA propellant molecules, thus enhancing the stabilization energy of the complexes. The interaction energy between HFA227 and LA (EbHFA227-LA = -24.7 kJ.mol(-1)) is significantly more favorable than that between HFA227 and its hydrocarbon analog (EbHFA227-ISO = -10.0 kJ.mol(-1)). However, it was shown that not only the fragment polarity is of relevance in stabilizing the complexes. The accessibility of the oxygen atoms in the fragments of interest is also relevant. Cluster studies indicate that although both oxygen atoms in the LA fragment are available to form H-bonds with the propellant molecules, the ether oxygen in PO is accessible to only one propellant molecule, thus decreasing significantly the stabilization energy of the cluster. The results shown here serve as a guide for the design of novel HFA-philes for HFA-based pMDIs.

[Novel method for preparing vesicles from a monodisperse emulsion aimed at controlling the size and improving the entrapment yield]

A vesicle is a compartment composed of lipid bilayer of amphiphilic molecules. The vesicle is applied to carriers of drugs, cosmetics and functional food ingredients in industries. Vesicles are also applied as a model for artificial cell membrane and expected as micro- and nano-reactors. They are generally prepared by the hydration of dry lipid film, but there is no method to prepare vesicles of a controlled size and high entrapment yield of hydrophilic materials inside them. In this article, a microchannel (MC) emulsification method was applied to prepare vesicles aimed at controlling the size and improving the entrapment yield. Firstly, monodisperse water-in-oil (W/O) emulsions were prepared by the MC emulsification method. In this process, hydrophilic materials to be entrapped were contained inside the water droplets of the emulsions. Keeping the water droplets frozen, the emulsifier was replaced by a bilayer-forming lipid mixture, and then the oil phase was evaporated. After hydration of lipid layers surrounding the water droplets, vesicles were formed. We call this preparation "lipid-coated ice droplet hydration method". The final sizes of the prepared vesicles were comparable to the original emulsion droplet sizes. This means that the size of vesicles can be controlled by controlling the size of original water droplets of the W/O emulsions. Furthermore, calcein as a hydrophilic fluorescent marker and biopolymers, such as enzyme and polysaccharide, were entrapped into the internal water phases of vesicles. The method proposed in this study enables the formation of vesicles with a controlled size and high entrapment yields, potentially useful for expanding the application fields of vesicles as biocompatible carriers and micro- and nano-reactors for biochemical reactions.

Liposomes : support à la formation de capsules plus stables à base de polyéléctrolytes réticulés ou de silice

Uses of enzymes for therapeutic purpose or for biosensing require a well-controlled nanoenvironnement to avoid degradation by proteolytic agents, pH variations or dilution effects. A solution is encapsulation under undenaturating conditions into a nanometer sized and stable capsule. The nanometer scall decreases recognition by the reticulo-endothelial system recognition and subsequent immune reaction. Liposomes are the method of choice since they allow protein encapsulation under mild conditions. However they lack in stability. In contrast, other type of capsules exhibit strong stability but with conditions required for formation that are incompatible with enzyme integrity. Here we combine different capsule formation techniques and use liposomes as templates for further stabilization. Here we demonstrate two types of multicomposite capsules. The first type is to coat the liposome surface with polyelectrolytes followed by secondary covalent crosslinking of the polyelectrolytes multilayer. In the second type of capsules we used silica to build an inorganic shell around liposome. Both techniques allow the formation of detergent stable nanocapsules which exhibits properties protective against acetylcholinesterase protein degradation, an enzyme of much interest for pesticide detection.

Microbial degradation of organophosphorus compounds

Synthetic organophosphorus compounds are used as pesticides, plasticizers, air fuel ingredients and chemical warfare agents. Organophosphorus compounds are the most widely used insecticides, accounting for an estimated 34% of world-wide insecticide sales. Contamination of soil from pesticides as a result of their bulk handling at the farmyard or following application in the field or accidental release may lead occasionally to contamination of surface and ground water. Several reports suggest that a wide range of water and terrestrial ecosystems may be contaminated with organophosphorus compounds. These compounds possess high mammalian toxicity and it is therefore essential to remove them from the environments. In addition, about 200,000 metric tons of nerve (chemical warfare) agents have to be destroyed world-wide under Chemical Weapons Convention (1993). Bioremediation can offer an efficient and cheap option for decontamination of polluted ecosystems and destruction of nerve agents. The first micro-organism that could degrade organophosphorus compounds was isolated in 1973 and identified as Flavobacterium sp. Since then several bacterial and a few fungal species have been isolated which can degrade a wide range of organophosphorus compounds in liquid cultures and soil systems. The biochemistry of organophosphorus compound degradation by most of the bacteria seems to be identical, in which a structurally similar enzyme called organophosphate hydrolase or phosphotriesterase catalyzes the first step of the degradation. organophosphate hydrolase encoding gene opd (organophosphate degrading) gene has been isolated from geographically different regions and taxonomically different species. This gene has been sequenced, cloned in different organisms, and altered for better activity and stability. Recently, genes with similar function but different sequences have also been isolated and characterized. Engineered microorganisms have been tested for their ability to degrade different organophosphorus pollutants, including nerve agents. In this article, we review and propose pathways for degradation of some organophosphorus compounds by microorganisms. Isolation, characterization, utilization and manipulation of the major detoxifying enzymes and the molecular basis of degradation are discussed. The major achievements and technological advancements towards bioremediation of organophosphorus compounds, limitations of available technologies and future challenge are also discussed.

Future applications of phosphotriesterases in the prophylaxis and treatment of organophosporus insecticide and nerve agent poisonings

Organophosphorus compounds (OPs) are being used as insecticides and warfare agents. OP insecticides represent an important problem of public health, causing around 200,000 deaths annually. The World Health Organization has pointed to the necessity to introduce new medical practices that improve the results of classical treatments. Many studies have shown that the administration of phosphotriesterases (enzymes that detoxify OPs through hydrolysis) is a promising treatment of persons poisoned with OPs. Such an enzyme-based treatment might introduce important improvements in the treatment of patients having ingested large amounts of OPs. Phosphotriesterases might also be suitable for prophylactic treatment of persons at risk to be severely exposed. The new experimental treatments do not exhibit the intrinsic neurotoxicity of the classical prophylaxis based on carbamates and antimuscarinic drugs. Experimental data suggest that might be time to initiate clinical trials in order to study the efficacy of phosphotriesterases in the therapy and prophylaxis of OP intoxication.

Langmuir and Langmuir-Blodgett films of organophosphorus acid anhydrolase

In this paper, we describe the preparation and characterization of Langmuir and Langmuir-Blodgett (LB) monolayers of the enzyme organophosphorus acid anhydrolase (OPAA). Langmuir films of OPAA were characterized on different subphases, such as phosphate, ammonium carbonate, and bis-tris-propane buffers. Monolayers at the air-water interface were characterized by measuring the surface pressure and surface potential-area isotherms. In situ UV-vis absorption spectra were also recorded from the Langmuir monolayers. The enzyme activity at the air-water interface was tested by the addition of diisopropylfluorophosphate (DFP) to the subphase. LB films of OPAA were transferred to mica substrates to be studied by atomic force microscopy. Finally, a one-layer LB film of OPAA labeled with a fluorescent probe, fluorescein isothiocyanate (FITC), was deposited onto a quartz slide to be tested as sensor for DFP. The clear, pronounced response and the stability of the LB film as a DFP sensor show the potential of this system as a biosensor.

Enzymes inside lipid vesicles: preparation, reactivity and applications

There are a number of methods that can be used for the preparation of enzyme-containing lipid vesicles (liposomes) which are lipid dispersions that contain water-soluble enzymes in the trapped aqueous space. This has been shown by many investigations carried out with a variety of enzymes. A review of these studies is given and some of the main results are summarized. With respect to the vesicle-forming amphiphiles used, most preparations are based on phosphatidylcholine, either the natural mixtures obtained from soybean or egg yolk, or chemically defined compounds, such as DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) or POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). Charged enzyme-containing lipid vesicles are often prepared by adding a certain amount of a negatively charged amphiphile (typically dicetylphosphate) or a positively charged lipid (usually stearylamine). The presence of charges in the vesicle membrane may lead to an adsorption of the enzyme onto the interior or exterior site of the vesicle bilayers. If (i) the high enzyme encapsulation efficiencies; (ii) avoidance of the use of organic solvents during the entrapment procedure; (iii) relatively monodisperse spherical vesicles of about 100 nm diameter; and (iv) a high degree of unilamellarity are required, then the use of the so-called 'dehydration-rehydration method', followed by the 'extrusion technique' has shown to be superior over other procedures. In addition to many investigations in the field of cheese production--there are several studies on the (potential) medical and biomedical applications of enzyme-containing lipid vesicles (e.g. in the enzyme-replacement therapy or for immunoassays)--including a few in vivo studies. In many cases, the enzyme molecules are expected to be released from the vesicles at the target site, and the vesicles in these cases serve as the carrier system. For (potential) medical applications as enzyme carriers in the blood circulation, the preparation of sterically stabilized lipid vesicles has proven to be advantageous. Regarding the use of enzyme-containing vesicles as submicrometer-sized nanoreactors, substrates are added to the bulk phase. Upon permeation across the vesicle bilayer(s), the trapped enzymes inside the vesicles catalyze the conversion of the substrate molecules into products. Using physical (e.g. microwave irradiation) or chemical methods (e.g. addition of micelle-forming amphiphiles at sublytic concentration), the bilayer permeability can be controlled to a certain extent. A detailed molecular understanding of these (usually) submicrometer-sized bioreactor systems is still not there. There are only a few approaches towards a deeper understanding and modeling of the catalytic activity of the entrapped enzyme molecules upon externally added substrates. Using micrometer-sized vesicles (so-called 'giant vesicles') as simple models for the lipidic matrix of biological cells, enzyme molecules can be microinjected inside individual target vesicles, and the corresponding enzymatic reaction can be monitored by fluorescence microscopy using appropriate fluorogenic substrate molecules.