Single-Molecule Analysis Determines Isozymes of Human Alkaline Phosphatase in Serum

Fluorescence of europium activated by molecular-like silver clusters for the detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is abnormally expressed in some cancers and promotes the growth, metastasis, and invasion of cancer cells. The detection of ALP is of great significance for both pathological study and clinical detection. In this work, a europium (Eu)-based fluorescence detection sensor was prepared in a mild reaction condition. LaF3:Eu nanoparticles was mixed with ethylene imine polymer (PEI) and Ag+ ions. PEI was used as stabilizer and reducing agent, and Ag+ ions were reduced as molecular-like silver clusters (ML-Ag NCs). The fluorescence of LaF3:Eu nanoparticles was enhanced by ML-Ag NCs through energy transfer. When ascorbic acid 2-phosphate (AAP) was hydrolyzed to ascorbic acid (AA) in the presence of ALP, AA reduced Ag+ ions to silver nanoparticles (Ag NPs) and quenched the fluorescence of LaF3:Eu/PEI/Ag. The activity of ALP was detected by measuring the fluorescence intensity of Eu3+ at 618 nm. In the concentration range from 2.0 to 16.0 U/L, the fluorescence intensity ratio ((F0-F)/F0) had a linear relationship with the logarithm of ALP concentration. The limit of detection (LOD) was 1.3 U/L. Moreover, the ALP activity was detected successfully in cancer cells by this method. The sensing platform has application potential in the detection of ALP activity in biological systems.

Serum liver enzymes and risk of stroke: Systematic review with meta-analyses and Mendelian randomization studies

BACKGROUND AND PURPOSE

Previous observational studies have identified correlations between liver enzyme levels and stroke risk. However, the strength and consistency of these associations vary. To comprehensively evaluate the relationship between liver enzymes and stroke risk, we conducted meta-analyses complemented by Mendelian randomization (MR) analyses.

METHODS

Following the PRISMA guidelines, we performed meta-analyses of prospective studies and conducted subgroup analyses stratified by sex and stroke subtype. Subsequently, adhering to the STROBE-MR guidelines, we performed two-sample bidirectional univariable MR (UVMR) and multivariable MR (MVMR) analyses using the largest genome-wide association studies summary data. Finally, the single-nucleotide polymorphisms associated with liver enzymes on sex differences underwent gene annotation, gene set enrichment, and tissue enrichment analyses.

RESULTS

In the meta-analyses of 17 prospective studies, we found the relative risks for serum γ-glutamyl transferase (GGT) and alkaline phosphatase (ALP) were 1.23 (95% CI: 1.16-1.31) and 1.3 (95% CI: 1.19-1.43), respectively. Subgroup analyses revealed sex and stroke subtype differences in liver enzyme-related stroke risk. Bidirectional UVMR analyses confirmed that elevated GGT, alanine aminotransferase, and aspartate aminotransferase levels were associated with increased stroke occurrence. The primary results from the MVMR analyses revealed that higher ALP levels significantly increased the risk of stroke and ischemic stroke. Gene set and tissue enrichment analyses supported genetic differences in liver enzymes across sexes.

CONCLUSIONS

Our study provides evidence linking liver enzyme levels to stroke risk, suggesting liver enzymes as potential biomarkers for early identification of high-risk individuals. Personalized, sex-specific interventions targeting liver enzymes could offer new strategies for stroke prevention.

Enzymatic fluorescent supramolecular hydrogel with aggregation-induced emission characteristics for sensing alkaline phosphatase

Alkaline phosphatase is an important biomarker for medical diagnosis. An enzymatic fluorescence supramolecular hydrogel with AIE properties was developed and used for sensing alkaline phosphatase in vitro and in living cells. In the presence of ALP, K(TPE)EFYp was partially converted to the hydrogelator K(TPE)EFY and self-assembled into nanofibers to form Hydrogel. With the sol-gel transition and the AIE effect, the fluorescence emission was turned on. The linear concentration range of ALP activity in vitro quantified by this method was determined as 0-3 U/L with aLODat 0.02 U/L. In addition, cell imaging and serum experiment showed that K(TPE)EFYp could also be used to detect ALP activity in living cells and biological samples.

Correlations between bone metabolism biomarkers and fluoride exposure in adults and children

Increased exposure to fluoride, which notably affects bone metabolism, is a global concern. However, the correlations and sensitivity of bone metabolism to fluoride remain controversial. In this cross-sectional study, 549 children (aged 7-12 years) and 504 adults (≥ 18 years old) were recruited in the high-fluoride areas of the Henan Province. Urinary fluoride (UF) level was determined using a fluoride electrode. Fasting venous blood serum was collected to measure bone metabolism biomarkers. The selected bone metabolism biomarkers for children included bone alkaline phosphatase (BALP), serum alkaline phosphatase (ALP), osteocalcin (OCN), calcitonin (CT), parathyroid hormone (PTH), phosphorus (P5+), and calcium (Ca2+). For adults, the biomarkers included ALP, CT, PTH, β-CrossLaps (β-CTX), P5+, and Ca2+. The correlations between UF and bone metabolism biomarkers were analyzed using binary logistic regression, a trend test, a generalized additive model, and threshold effect analysis. Regression analysis indicated a significant correlation between serum OCN, PTH, and UF levels in children aged 7-9 years. Serum OCN, PTH, and BALP contents were significantly correlated with UF in boys (P < 0.05). Furthermore, the interaction between age and UF affected serum P5+ and PTH (P < 0.05). The generalized additive model revealed nonlinear dose-response relationships between P5+, BALP, and UF contents in children (P < 0.05). Serum OCN level was linearly correlated with the UF concentration (P < 0.05). Similarly, a significant correlation was observed between β-CTX and UF levels in adults. In addition, significant correlations were observed between UF-age and serum Ca2+, β-CTX, and PTH contents. There was a non-linear correlation between serum Ca2+, P5+, and β- CTX and UF levels (P < 0.05). Overall, serum OCN, BALP, and P5+ levels can serve as sensitive bone metabolism biomarkers in children, while β-CTX, P5+, and Ca2+ can be considered fluoride-sensitive bone metabolism biomarkers in adults.

Functional analysis of single enzymes combining programmable molecular circuits with droplet-based microfluidics

The analysis of proteins at the single-molecule level reveals heterogeneous behaviours that are masked in ensemble-averaged techniques. The digital quantification of enzymes traditionally involves the observation and counting of single molecules partitioned into microcompartments via the conversion of a profluorescent substrate. This strategy, based on linear signal amplification, is limited to a few enzymes with sufficiently high turnover rate. Here we show that combining the sensitivity of an exponential molecular amplifier with the modularity of DNA-enzyme circuits and droplet readout makes it possible to specifically detect, at the single-molecule level, virtually any D(R)NA-related enzymatic activity. This strategy, denoted digital PUMA (Programmable Ultrasensitive Molecular Amplifier), is validated for more than a dozen different enzymes, including many with slow catalytic rate, and down to the extreme limit of apparent single turnover for Streptococcus pyogenes Cas9. Digital counting uniquely yields absolute molar quantification and reveals a large fraction of inactive catalysts in all tested commercial preparations. By monitoring the amplification reaction from single enzyme molecules in real time, we also extract the distribution of activity among the catalyst population, revealing alternative inactivation pathways under various stresses. Our approach dramatically expands the number of enzymes that can benefit from quantification and functional analysis at single-molecule resolution. We anticipate digital PUMA will serve as a versatile framework for accurate enzyme quantification in diagnosis or biotechnological applications. These digital assays may also be utilized to study the origin of protein functional heterogeneity.

Advances in single molecule arrays (SIMOA) for ultra-sensitive detection of biomolecules

In the contemporary era of scientific and medical advancements, the accurate and ultra-sensitive detection of proteins, nucleic acids and metabolites plays a pivotal role in disease diagnosis and treatment monitoring. Single-molecule detection technologies play a great role in achieving this goal. In recent years, digital detection methods based on single molecule arrays (SIMOA) have brought groundbreaking contributions to the field of single-molecule detection. By confining the target molecules to femtoliter-sized containers, the SIMOA technology achieves detection sensitivity of attomolar. This review delves into the historical evolution and fundamentals of SIMOA technology, summarizes various approaches to optimize its performance, and describes the applications of SIMOA for the ultrasensitive detection of biomarkers for diseases such as cancer, COVID-19, and neurological disorders, as well as in DNA detection. Currently, some SIMOA technologies have been realized for high-throughput and multiplexed detection. It is believed that SIMOA technology will play a significant role in medical monitoring and disease prevention in the future.

Thioester-Based Coupled Fluorogenic Assays in Microdevice for the Detection of Single-Molecule Enzyme Activities of Esterases with Specified Substrate Recognition

Single-molecule enzyme activity assay is a platform that enables the analysis of enzyme activities at single proteoform level. The limitation of the targetable enzymes is the major drawback of the assay, but the general assay platform is reported to study single-molecule enzyme activities of esterases based on the coupled assay using thioesters as substrate analogues. The coupled assay is realized by developing highly water-soluble thiol-reacting probes based on phosphonate-substituted boron dipyrromethene (BODIPY). The system enables the detection of cholinesterase activities in blood samples at single-molecule level, and it is shown that the dissecting alterations of single-molecule esterase activities can serve as an informative platform for activity-based diagnosis.

Ratio-fluorescent and naked-eye visualized dual-channel sensing strategy for Cu2+ and alkaline phosphatase activity assay

The levels of Cu2+ and alkaline phosphatase (ALP) are the important indicators of the developed stage of the relative diseases. Herein, a binary ratio-fluorescent and smartphone-assisted visual strategy basing on 4'-aminomethyl-4, 5', 8-trimethylpsoralen (AMT) and the oxidation of o-phenylenediamine was developed. Under the action of Cu2+, the fluorescent molecule, 3-diaminophenazine (DAP) formed which can act as a fluorescent acceptor of the ratio-fluorescent sensor. The emission spectrum of AMT overlapped with the excitation spectrum of DAP and, thus, it can act as the fluorescent donor of the ratio-fluorescent sensor. With the increasing concentration of Cu2+ and ALP, the fluorescent intensity of AMT decreased and the fluorescent intensity of DAP increased. The dual-emission reverse change ratio-fluorescent sensor realized the sensitive detection Cu2+ and ALP with the detection limits of 2 nM and 0.03 U/mL, respectively. In addition, the acceptable recoveries were obtained when the Cu2+ and ALP in spiked samples were detected. Furthermore, the relative activity of ALP was assessed by increasing the concentrations of the inhibitor Na3VO4 and IC50 of 25 μM was obtained. Importantly, the target concentration-dependent color change of DAP allowed us to utilize R/B ratio values to design the smartphone-assisting visual detection model of Cu2+ and ALP activity with the detection limits of 0.1 μM and 0.18 U/mL. This simple, flexible, dual-mode sensor strategy has a potential for disease diagnosis and drug screening.

Activatable Near-Infrared Versatile Fluorescent and Chemiluminescent Dyes Based on the Dicyanomethylene-4H-pyran Scaffold: From Design to Imaging and Theranostics

Activatable fluorescent and chemiluminescent dyes with near-infrared emission have indispensable roles in the fields of bioimaging, molecular prodrugs, and phototheranostic agents. As one of the most popular fluorophore scaffolds, the dicyanomethylene-4H-pyran scaffold has been applied to fabricate a large number of versatile activatable optical dyes for analytes detection and diseases diagnosis and treatment by virtue of its high photostability, large Stokes shift, considerable two-photon absorption cross-section, and structural modifiability. This review discusses the molecular design strategies, recognition mechanisms, and both in vitro and in vivo bio-applications (especially for diagnosis and therapy of tumors) of activatable dicyanomethylene-4H-pyran dyes. The final section describes the current shortcomings and future development prospects of this topic.

Identification of activity-based biomarkers for early-stage pancreatic tumors in blood using single-molecule enzyme activity screening

Single-molecule enzyme activity-based enzyme profiling (SEAP) is a methodology to globally analyze protein functions in living samples at the single-molecule level. It has been previously applied to detect functional alterations in phosphatases and glycosidases. Here, we expand the potential for activity-based biomarker discovery by developing a semi-automated synthesis platform for fluorogenic probes that can detect various peptidases and protease activities at the single-molecule level. The peptidase/protease probes were prepared on the basis of a 7-amino-4-methylcoumarin fluorophore. The introduction of a phosphonic acid to the core scaffold made the probe suitable for use in a microdevice-based assay, while phosphonic acid served as the handle for the affinity separation of the probe using Phos-tag. Using this semi-automated scheme, 48 fluorogenic probes for the single-molecule peptidase/protease activity analysis were prepared. Activity-based screening using blood samples revealed altered single-molecule activity profiles of CD13 and DPP4 in blood samples of patients with early-stage pancreatic tumors. The study shows the power of single-molecule enzyme activity screening to discover biomarkers on the basis of the functional alterations of proteins.

Fluorescent assay for alkaline phosphatase by integrating strand displacement amplification with DNAzyme-catalytic recycling cleavage of molecular beacons

A fluorescence sensing method for the quantification of alkaline phosphatase (ALP) was developed by integrating the strand displacement amplification with DNAzyme-catalytic recycling cleavage of molecular beacons. ALP can hydrolyze a 3'-phosphoralated primer into a 3'-hydroxy primer which can initiate the strand displacement amplification to produce the Mg2+-dependent DNAzyme. The DNAzyme can then catalyze the cleavage of the DNA molecular beacon labeled with FAM fluorophore at its 5'-end and BHQ1 quencher at its 3'-end, turning on the fluorescence of FAM fluorophore. The content of ALP in a sample can be deduced from the measured fluorescence intensity. Due to the cascading nature of its amplification strategy, the proposed method achieved sensitive and specific ALP detection in human serum samples. Its results were in good consistent with the corresponding values obtained by a commercial ALP detection kit. The limit of detection of the proposed method for ALP is about 0.015 U/L, lower than some methods recently reported in literature, demonstrating its potential for ALP detection in biomedical research and clinical diagnosis.

Digital Cascade Assays for ADP- or ATP-Producing Enzymes Using a Femtoliter Reactor Array Device

Digital enzyme assays are emerging biosensing methods for highly sensitive quantitative analysis of biomolecules with single-molecule detection sensitivity. However, current digital enzyme assays require a fluorogenic substrate for detection, which limits the applicability of this method to certain enzymes. ATPases and kinases are representative enzymes for which fluorogenic substrates are not available; however, these enzymes form large domains and play a central role in biology. In this study, we implemented a fluorogenic cascade reaction in a femtoliter reactor array device to develop a digital bioassay platform for ATPases and kinases. The digital cascade assay enabled quantitative measurement of the single-molecule activity of F1-ATPase, the catalytic portion of ATP synthase. We also demonstrated a digital assay for human choline kinase α. Furthermore, we developed a digital cascade assay for ATP-synthesizing enzymes and demonstrated a digital assay for pyruvate kinase. These results show the high versatility of this assay platform. Thus, the digital cascade assay has great potential for the highly sensitive detection and accurate characterization of various ADP- and ATP-producing enzymes, such as kinases, which may serve as disease biomarkers.

Pd(II)-based coordination polymer nanosheets for ratiometric colorimetric and photothermal dual-mode assay of serum alkaline phosphatase

Fabrication of a multi-signal readout assay with high sensitivity and selectivity is highly desirable for clinical and biochemical analysis, but remains a challenge due to laborious procedures, large-scale instruments, and inadequate accuracy. Herein, a straightforward, rapid, and portable detection platform based on palladium(II) methylene blue (MB) coordination polymer nanosheets (PdMBCP NSs) was unveiled for the ratiometric dual-mode detection of alkaline phosphatase (ALP) with temperature and colorimetric signal readout properties. The sensing mechanism is the ALP-catalyzed generation of ascorbic acid for competitive binding and etching PdMBCP NSs to release free MB in a quantitive means for detection. Specifically, ALP addition led to the decrease of temperature signal readout from the decomposed PdMBCP NSs under 808 nm laser excitation, and simultaneous increase of the temperature from the generated MB with a 660 nm laser, together with the corresponding absorbance changes at both wavelengths. Notably, this ratiometric nanosensor exhibited a detection limit of 0.013 U/L (colorimetric) and 0.095 U/L (photothermal) within 10 min, respectively. The reliability and satisfactory sensing performance of the developed method were further confirmed by clinic serum samples. Therefore, this study provides a new insight for the development of dual-signal sensing platforms for convenient, universal, and accurate detection of ALP.

In-depth analysis of biocatalysts by microfluidics: An emerging source of data for machine learning

Nowadays, the vastly increasing demand for novel biotechnological products is supported by the continuous development of biocatalytic applications which provide sustainable green alternatives to chemical processes. The success of a biocatalytic application is critically dependent on how quickly we can identify and characterize enzyme variants fitting the conditions of industrial processes. While miniaturization and parallelization have dramatically increased the throughput of next-generation sequencing systems, the subsequent characterization of the obtained candidates is still a limiting process in identifying the desired biocatalysts. Only a few commercial microfluidic systems for enzyme analysis are currently available, and the transformation of numerous published prototypes into commercial platforms is still to be streamlined. This review presents the state-of-the-art, recent trends, and perspectives in applying microfluidic tools in the functional and structural analysis of biocatalysts. We discuss the advantages and disadvantages of available technologies, their reproducibility and robustness, and readiness for routine laboratory use. We also highlight the unexplored potential of microfluidics to leverage the power of machine learning for biocatalyst development.

DNA-Programmed Tuning of the Growth and Enzyme-Like Activity of a Bimetallic Nanozyme and Its Biosensing Applications

Nanozymes, which combine the merits of both nanomaterials and natural enzymes, have aroused tremendous attention as new representatives of artificial enzyme mimics. However, it still remains to be a great challenge to rationally engineer the morphologies and surface properties of nanostructures that lead to the desired enzyme-like activities. Here, we report a DNA-programming seed-growth strategy to mediate the growth of platinum nanoparticles (PtNPs) on gold bipyramids (AuBPs) for the synthesis of a bimetallic nanozyme. We find that the preparation of a bimetallic nanozyme is in a sequence-dependent manner, and the encoding of a polyT sequence allows the successful formation of bimetallic nanohybrids with greatly enhanced peroxidase-like activity. We further observe that the morphologies and optical properties of T15-mediated Au/Pt nanostructures (Au/T15/Pt) change over the reaction time, and the nanozymatic activity can be tuned by controlling the experimental conditions. As a concept application, Au/T15/Pt nanozymes are used to establish a simple, sensitive, and selective colorimetric assay for the determination of ascorbic acid (AA), alkaline phosphatase (ALP), and the inhibitor sodium vanadate (Na₃VO₄), demonstrating excellent analytical performance. This work provides a new avenue for the rational design of bimetallic nanozymes for biosensing applications.

Ultrasensitive alkaline phosphatase activity assay based on controllable signal probe production coupled with the cathodic photoelectrochemical analysis

A highly sensitive and selective split-type perovskite-based photoelectrochemical (PEC) platform was developed for measuring alkaline phosphatase (ALP) activity in milk and serum samples. ALP in the test sample hydrolyzed 2-phosphate sesquimagnesium salt hydrate (AAPS) in a 96-microwell plate to produce ascorbic acid (AA), a PEC electron donor. The resulting AA, which could preferentially annihilate the photogenerated holes, indirectly reflects ALP activity. The PEC used a cetyltrimethylammonium bromide (CTAB)-functionalized CH₃NH₃PbI₃ (CTAB@CH₃NH₃PbI₃) film as the cathode to monitor the controlled AA production. Due to the excellent photoelectric characteristics of the CH₃NH₃PbI₃ perovskite and the split-type assay, excellent sensitivity and selectivity for ALP detection were obtained. Under the optimum experimental conditions, ALP activity with a limit of detection (LOD) of 2.6 × 10^-4 U/L in a linear dynamic range of 10^-3 ∼ 10² U/L was obtained. With its sensitive, rapid, and high-throughput detection capabilities, this split-type and label-free PEC platform has great potential for use in food and biomedical analysis.

Self-assembly of protein-DNA superstructures for alkaline phosphatase detection in blood

We designed a paper-based analytical device by integrating horseradish peroxidase (HRP)-encapsulated 3D DNA for visual detection of alkaline phosphatase (ALP). This device allows on-paper sample pre-treatment, target recognition and signal readout, enabling simple (without additional pre-treatment of blood samples) and rapid (within 23 min) determination of ALP in clinical samples.

Ultrasensitive Automatic Detection of Small Molecules by Membrane Imaging of Single Molecule Assays

Determination of trace amounts of targets or even a single molecule target has always been a challenge in the detection field. Digital measurement methods established for single molecule counting of proteins, such as single molecule arrays (Simoa) or dropcast single molecule assays (dSimoa), are not suitable for detecting small molecule, because of the limited category of small molecule antibodies and the weak signal that can be captured. To address this issue, we have developed a strategy for single molecule detection of small molecules, called small molecule detection with single molecule assays (smSimoa). In this strategy, an aptamer is used as a recognition element, and an addressable DNA Nanoflower (DNF) attached on the magnetic beads surface, which exhibit fluorescence imaging, is employed as the output signal. Accompanied by digital imaging and automated counting analysis, E2 at the attomolar level can be measured. The smSimoa breaks the barrier of small molecule detection concentration and provides a basis for high throughput detection of multiple substances with fluorescence encoded magnetic beads.

Enzyme-based digital bioassay technology - key strategies and future perspectives

Digital bioassays based on single-molecule enzyme reactions represent a new class of bioanalytical methods that enable the highly sensitive detection of biomolecules in a quantitative manner. Since the first reports of these methods in the 2000s, there has been significant growth in this new bioanalytical strategy. The principal strategy of this method is to compartmentalize target molecules in micron-sized reactors at the single-molecule level and count the number of microreactors showing positive signals originating from the target molecule. A representative application of digital bioassay is the digital enzyme-linked immunosorbent assay (ELISA). Owing to their versatility, various types of digital ELISAs have been actively developed. In addition, some disease markers and viruses possess catalytic activity, and digital bioassays for such enzymes and viruses have, thus, been developed. Currently, with the emergence of new microreactor technologies, the targets of this methodology are expanding from simple enzymes to more complex systems, such as membrane transporters and cell-free gene expression. In addition, multiplex or multiparametric digital bioassays have been developed to assess precisely the heterogeneities in sample molecules/systems that are obscured by ensemble measurements. In this review, we first introduce the basic concepts of digital bioassays and introduce a range of digital bioassays. Finally, we discuss the perspectives of new classes of digital bioassays and emerging fields based on digital bioassay technology.

Leucine Alters Blood Parameters and Regulates Hepatic Protein Synthesis via mTOR Activation in Intrauterine Growth Restriction Piglets

Neonatal piglets often suffer low birth weights and poor growth performance accompanied by the disruption of protein metabolism, when intrauterine growth restriction (IUGR) takes place during pregnancy, leading to a higher mortality and bigger economic loss than expected. Leucine has been proposed to function as a nutritional signal regulating protein synthesis in numerous studies. The aim of this study was to determine the effect of dietary leucine supplementation on the blood parameters and hepatic protein metabolism in IUGR piglets. Weaned piglets were assigned to one of four to treatments in a 2 × 2 factorial arrangement: (1) piglets fed a basal diet with normal birth weight; (2) piglets fed a basal diet plus 0.35% L-leucine with normal birth weight; (3) IUGR piglets fed a basal diet with low birth weight; (4) IUGR piglets fed a basal diet plus 0.35% L-leucine with low birth weight. The results showed that IUGR decreased serum aspartate aminotransferase and alkaline phosphatase activities, increased serum cortisol and prostaglandin E2 levels at 35 days of age (P < 0.05), suggesting the occurrence of liver dysfunction and stress response. Leucine supplementation increased serum alkaline phosphatase activity, and decreased serum cortisol levels at 35 days of age (P < 0.05). IUGR decreased the lysozyme activity and complement 3 level in serum (P < 0.05), which were prevented by dietary leucine supplementation. IUGR piglets showed increased hepatic DNA contents while showing reduced RNA/DNA ratio (P < 0.05). Piglets supplied with leucine had decreased RNA/DNA ratio in the liver (P < 0.05). Leucine supplementation stimulated hepatic protein anabolism through up-regulating protein synthesis related genes expression and activating the phosphorylation of mammalian/mechanistic target of rapamycin (mTOR) (P < 0.05). Moreover, IUGR inhibited the mRNA expression of hepatic protein degradation related genes, indicating a compensatory mechanism for the metabolic response. Dietary leucine supplementation attenuated the suppression of the protein catabolism induced by IUGR in liver. These results demonstrate that dietary leucine supplementation could alter the blood parameters, alleviated the disrupted protein metabolism induced by IUGR via enhanced mTOR phosphorylation to promote protein synthesis in weaned piglets.

Brightness Analysis per Moving Particle: In Situ Analysis of Alkaline Phosphatase in Living Cells

In situ quantitative analysis of enzymes such as phosphatase is important to understand a number of involved biological processes ranging from various metabolisms to signal transduction and cellular regulation. In this paper, a novel in situ measurement strategy was proposed to detect alkaline phosphatase (ALP) activity in different locations within single living cells. The principle is based on the measurement of the resonance light scattering brightness ratio (SBR) per moving nanoparticle that forms in an ALP-related chemical reaction. In the method, a novel resonance light scattering correlation spectroscopy (RLSCS) system was developed using two lasers for illumination or two detection channels. Using the gold nanoparticles (AuNPs) as probes, the Au@Ag nanoparticles (Au@Ag NPs) formed due to the ALP-catalyzed hydrolysis of ascorbic acid 2-phosphate (AAP) and the subsequent reduction-deposition reaction of Ag ions that occurred on the AuNPs. The SBR value per moving particle was determined based on the obtained RLS intensity traces and RLSCS curves. The SBR value was found to be not influenced by the intracellular viscosity and size that was confirmed in the experiments. The linear relation between the SBR and ALP activity was established and applied to detect ALP activity and evaluate the inhibition of different drugs. Finally, the method was successfully used to in situ measure ALP activity within living cells. The method overcomes the shortcoming of conventional methods that lack quantitative analysis and are susceptible to intracellular viscosity.

Monitoring protein conformational changes using fluorescent nanoantennas

Understanding the relationship between protein structural dynamics and function is crucial for both basic research and biotechnology. However, methods for studying the fast dynamics of structural changes are limited. Here, we introduce fluorescent nanoantennas as a spectroscopic technique to sense and report protein conformational changes through noncovalent dye-protein interactions. Using experiments and molecular simulations, we detect and characterize five distinct conformational states of intestinal alkaline phosphatase, including the transient enzyme-substrate complex. We also explored the universality of the nanoantenna strategy with another model protein, Protein G and its interaction with antibodies, and demonstrated a rapid screening strategy to identify efficient nanoantennas. These versatile nanoantennas can be used with diverse dyes to monitor small and large conformational changes, suggesting that they could be used to characterize diverse protein movements or in high-throughput screening applications.

Sensitive ratiometric fluorescence probe based on chitosan carbon dots and calcein for Alkaline phosphatase detection and bioimaging in cancer cells

Alkaline phosphatase (ALP) is a commonly used marker in clinical practice, and this enzyme is a key indicator for diagnosing various diseases. In this study, we describe the development of a reliable and novel fluorescent assay for ALP detection based on chitosan carbon dots (C-CDs, peak emission, 412 nm) and calcein (peak emission, 512 nm). In the presence of Eu³⁺ (which binds calcein), the fluorescence intensity of calcein is quenched. Utilizing the ALP-triggered generation of phosphate ions (PO₄^3-) from the substrate p-nitrophenyl phosphate (pNPP), the Eu³⁺ ions bind PO₄^3- (which shows a higher affinity toward Eu³⁺ than calcein), and the fluorescence of calcein is recovered. As a consequence, C-CDs fluorescence is decreased by inner filter effect (IFE). Exploiting these changes in the fluorescence intensity ratio of C-CDs and calcein, we developed a high sensitivity, accurate, and easily synthesized ratiometric fluorescence probe. Our novel fluorescent bioassay demonstrates good linear relationship in the 0.09-0.8 mU mL^-1 range, with a low detection limit of 0.013 mU mL^-1. The excellent applicability of this novel assay in HepG2 cells and human serum samples demonstrates that our novel method has excellent biomedical research and disease diagnosis prospects.

Nano-octahedral bimetallic Fe/Eu-MOF preparation and dual model sensing of serum alkaline phosphatase (ALP) based on its peroxidase-like property and fluorescence

Herein a nano-scale bimetallic Fe/Eu-MOF with a regular octahedral structure was synthesized for the first time. The synthesized Fe/Eu-MOF has both peroxidase-like activity and fluorescence properties. Fe/Eu-MOF can catalyze H₂O₂ to oxidize the chromogenic substrate TMB to produce blue oxTMB, which has ultraviolet absorption at 652 nm. Unexpectedly, the generated oxTMB can effectively quench the fluorescence of the catalyst Fe/Eu-MOF at 450 nm. The quenching mechanism is mainly the internal filtration effect (IFE), accompanied by static quenching (SQE), Förster resonance energy transfer (FRET) and photoelectron transfer (PET). Fe/Eu-MOF has a high affinity for sodium pyrophosphate (PPi). PPi can be adsorbed to the surface of Fe/Eu-MOF, destroying the structure of Fe/Eu-MOF and inhibiting its catalytic activity, resulting in a decrease in UV absorbance and the decline of fluorescence quenching. In contrast, phosphoric acid (Pi) has almost no effect on the reaction system. Alkaline phosphatase (ALP) can catalyze the hydrolysis of PPi to Pi, thereby reducing the inhibitory effect of PPi. Based on this, we successfully constructed a dual-mode ALP sensor with high selectivity. The linear ranges based on the 652 nm absorption or the fluorescence detection are from 1 to 200 U/L, and the detection limits are 0.6 for the absorption method and 0.9 U/L for the fluorescence method, respectively.

Single-molecule enzymology for diagnostics: profiling alkaline phosphatase activity in clinical samples

Enzymes can be used as biomarkers for a variety of diseases. However, profiling enzyme activity in clinical samples is challenging due to the heterogeneity in enzyme activity, and the low abundance of the target enzyme in biofluids. Single-molecule methods can overcome these challenges by providing information on the distribution of enzyme activities in a sample. Here, we describe the concept of using the single-molecule enzymology (SME) method to analyze enzymatic activity in clinical samples. We present recent work focused on measuring alkaline phosphatase isotypes in serum samples using SME. Future work will involve improving and simplifying this technology, and applying it to other enzymes for diagnostics.

Highly Sensitive Homogeneous Electrochemiluminescence Biosensor for Alkaline Phosphatase Detection Based on Click Chemistry-Triggered Branched Hybridization Chain Reaction

Alkaline phosphatase (ALP) has been used as a diagnostic index of clinical diseases since its expression level is closely related to many pathological processes. In this work, a highly sensitive electrochemiluminescence (ECL) method for the determination of ALP based on a click chemistry-induced branched hybridization chain reaction (BHCR) for signal amplification and ultrafiltration technology for the separation of homogeneous amplification products is introduced. ALP can release copper ions from a Cu²⁺/PPi complex by hydrolyzing pyrophosphoric acid, which initiates click chemistry in the system. A BHCR amplification is triggered afterward by the long single-stranded DNA (ssDNA) generated by click chemistry, resulting in a three-dimensional double-stranded DNA (dsDNA) with a large molecular weight. Based on the characteristic that Ru(phen)₃²⁺ can stably insert into the groove of dsDNA, a large amount of Ru(phen)₃²⁺ is retained together with the amplified product after ultrafiltration, and therefore a significantly enhanced ECL signal can be obtained. The test results show that this method can be used for the quantitative determination of ALP ranging from 0.002 to 50 U/L, with a detection limit of 0.7 mU/L. This method has also been confirmed to have good selectivity and anti-interference, and the results of the analysis of the ALP content in the diluted serum samples are satisfactory, showing great application potential in clinical diagnosis.

Multifunctional lanthanide metal-organic framework based ratiometric fluorescence visual detection platform for alkaline phosphatase activity

A turn-on/off ratiometric fluorescence detection platform based on multifunctional lanthanide metal-organic framework (Ln-MOF) and an enzymatic cascade reaction is proposed for alkaline phosphatase (ALP) activity assay. L-phosphotyrosine is hydrolyzed to levodopa (L-dopa) by two steps of enzymatic reaction. L-dopa further reacts with naphthoresorcinol to produce carboxyazamonardine with strong emission at 490 nm. In this process, multifunctional Ln-MOF (Cu@Eu-BTC, BTC is the 1,3,5-benzenetricarboxylic acid) acts not only as a nanozyme to catalyze the fluorogenic reaction between L-dopa and naphthoresorcinol but also as a fluorescence internal standard. The emission of Cu@Eu-BTC at 620 nm is quenched by phosphate anions, and the dual-response ratiometric fluorescence (F₄₉₀/F₆₂₀) can be achieved. A good linear relationship was obtained between Δ(F₄₉₀/F₆₂₀) and ALP activity in the range 0.3-24 U L^-1 with the detection limit of 0.02 U L^-1. In addition, a portable assay tube was designed for visual and point-of-care testing of ALP activity by color variation (ratiometric chromaticity). Both the ratiometric fluorescence detection and the visual detection methods were successfully applied to monitor ALP activity in human serum samples with recovery between 95.5%-109.0% and 94.0%-110.1%, and relative standard deviation less than 8.1% and 9.5%, respectively. As far as we know, this is the first report of ALP activity assay assisted by multifunctional Ln-MOF.Graphical abstract.

Elucidation and control of low and high active populations of alkaline phosphatase molecules for quantitative digital bioassay

Alkaline phosphatase (ALP), a homo‐dimeric enzyme has been widely used in various bioassays as disease markers and enzyme probes. Recent advancements of digital bioassay revolutionized ALP‐based diagnostic assays as seen in rapid growth of digital ELISA and the emerging multiplex profiling of single‐molecule ALP isomers. However, the intrinsic heterogeneity found among ALP molecules hampers the ALP‐based quantitative digital bioassays. This study aims quantitative analysis of single‐molecule activities of ALP from Escherichia coli and reveals the static heterogeneity in catalytic activity of ALP with two distinct populations: half‐active and fully‐active portions. Digital assays with serial buffer exchange uncovered single‐molecule Michaelis–Menten kinetics of ALP; half‐active molecules have halved values of the catalytic turnover rate, k_cat, and the rate constant of productive binding, k_on, of the fully active molecules. These findings suggest that half‐active ALP molecules are heterogenic dimers composed of inactive and active monomer units, while fully active ALP molecules comprise two active units. Static heterogeneity was also observed for ALP with other origins: calf intestine or shrimp, showing how the findings can be generalized across species. Cell‐free expression of ALP with disulfide bond enhancer and spiked zinc ion resulted in homogenous population of ALP of full activity, implying that inactive monomer units of ALP are deficient in correct disulfide bond formation and zinc ion coordination. These findings provide basis for further study on molecular mechanism and biogenesis of ALP, and also offer the way to prepare homogenous and active populations of ALP for highly quantitative and sensitive bioassays with ALP.

Sensitive colorimetric assay for the determination of alkaline phosphatase activity utilizing nanozyme based on copper nanoparticle-modified Prussian blue

Nanozyme based on Prussian blue nanocubes (PB NCs) loaded with copper nanoparticles (Cu@PB NCs) was synthesized. The peroxidase (POD)-like activity of Cu@PB NCs was studied and utilized for detecting the activity of alkaline phosphatase (ALP). The Cu@PB NCs possess higher POD-like activity compared with PB NCs and natural horseradish peroxidase (HRP) due to the loading of copper nanoparticles. 3,3',5,5'-Tetramethylbenzidine (TMB) can be oxidized to oxTMB in the presence of Cu@PB NCs and H₂O₂, generating blue-colored compound, while introduction of pyrophosphate (PPi) leads to the POD-like activity of Cu@PB NCs decreased obviously. In the presence of ALP, PPi was hydrolyzed and then the POD-like activity of Cu@PB NCs was restored. So, according to the change of the POD-like activity of Cu@PB NCs, a sensitive colorimetric assay for ALP activity was reported. The limit of detection of the assay is 0.08 mU/mL, with linear range from 0.1 to 50 mU/mL. In addition, the assay was also applied for screening the inhibitors of ALP. Nanozyme based on Prussian blue nanocube (PB NCs) loaded with copper nanoparticles was synthesized and utilized for detecting the activity of alkaline phosphatase (ALP).

Multidimensional Digital Bioassay Platform Based on an Air-Sealed Femtoliter Reactor Array Device

Single-molecule experiments have been helping us to get deeper inside biological phenomena by illuminating how individual molecules actually work. Digital bioassay, in which analyte molecules are individually confined in small compartments to be analyzed, is an emerging technology in single-molecule biology and applies to various biological entities (e.g., cells and virus particles). However, digital bioassay is not compatible with multiconditional and multiparametric assays, hindering in-depth understanding of analytes. This is because current digital bioassay lacks a repeatable solution-exchange system that keeps analytes inside compartments. To address this challenge, we developed a digital bioassay platform with easy solution exchanges, called multidimensional (MD) digital bioassay. We immobilized single analytes in arrayed femtoliter (10^-15 L) reactors and sealed them with airflow. The solution in each reactor was stable and showed no cross-talk via solution leakage for more than 2 h, and over 30 rounds of perfect solution exchanges were successfully performed. With multiconditional assays based on our system, we could quantitatively determine inhibitor sensitivities of single influenza A virus particles and single alkaline phosphatase (ALP) molecules, which has never been achieved with conventional digital bioassays. Further, we demonstrated that ALPs from two origins can be precisely distinguished by a single-molecule multiparametric assay with our system, which was also difficult with conventional digital bioassays. Thus, MD digital bioassay is a versatile platform to gain in-depth insight into biological entities in unprecedented resolution.