An Allosteric-Probe for Detection of Alkaline Phosphatase Activity and Its Application in Immunoassay

The development of biosensors for alkaline phosphatase activity detection based on a phosphorylated DNA probe

Alkaline phosphatase (ALP) is a zinc-containing metalloprotein that shows very great significance in clinical diagnosis, which can catalyze the hydrolysis of phosphorylated species. ALP has the potential to serve as a valuable biomarker for detecting liver dysfunction and bone diseases. On the other hand, ALP is an efficient biocatalyst to amplify detection signals in the enzyme-linked assay. It has always been a major research focus to develop novel biosensors that can detect ALP activity with high selectivity and sensitivity. There have been numerous reports on the development of biosensors to determine ALP activity using a phosphorylated DNA probe. Among them, various beneficial strategies, such as λ exonuclease-mediated cleavage reaction, terminal deoxynucleotidyl transferase-triggered DNA polymerization, and Klenow fragment polymerase-catalyzed elongation, are employed to generate amplified and more intuitive signal. This review discusses and summarizes the development and advances of biosensors for ALP activity detection that use a well-designed phosphorylated DNA probe, aiming to provide some guidelines for the design of more sophisticated sensing strategies that exhibit improved sensitivity, selectivity, and adaptability in detecting ALP activity.

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.

Unnatural nucleotide-based rkDNA probe combined with graphene oxide for detection of alkaline phosphatase activity

In this study we developed a fluorescent double-stranded DNA, incorporating an unnatural dU^rk nucleotide, that we used as a probe for the detection of alkaline phosphatase (ALP) based on enzymatic cleavage of the non-fluorescent complementary strand. Primer extension performed using the unnatural nucleotide triphosphate dU^rkTP and the natural deoxynucleotide triphosphates dATP, dCTP, and dGTP provided a simple fluorescent DNA strand that hybridized with the 5́-monophosphate non-fluorescent complementary strand. When applying the 5́-phosphate recognition and cleavage properties of lambda exonuclease (λ-exo), this probe could bind to graphene oxide (GO) and quench the fluorescence (in the absence of ALP) or not bind to GO and retain its fluorescence (in the presence of ALP). We obtained strongly fluorescent DNA strands through simple incorporation of multiple A sites in the complementary sequence, thereby increasing the number of dU^rk residues during primer extension. This unnatural nucleotide-based rkDNA probing system exhibited high fluorescence differentiation for discriminating the status of ALP. This rkDNA-GO probing system appears to be a promising tool for monitoring the activity of disease-associated enzymes.

3'-Terminal Repair-Powered Dendritic Nanoassembly of Polyadenine Molecular Beacons for One-Step Quantification of Alkaline Phosphatase in Human Serum

Alkaline phosphatase (ALP) is an important hydrolase with crucial roles in biological processes, and the dysregulation of ALP may cause various human diseases. The conventional ALP assays usually involve cumbersome procedures with poor sensitivity. Herein, taking advantage of intrinsic superiorities of molecular beacons (MBs) and unique features of terminal deoxynucleotidyl transferase (TdT), we demonstrate for the first time the 3'-terminal repair-powered dendritic nanoassembly of polyadenine (A) MBs for one-step quantification of ALP in human serum. When ALP is present, it catalyzes 3'-terminal dephosphorylation of poly-A MBs to induce TdT-mediated template-free polymerization, generating long chains of polythymidine (T) sequences. The long poly-T chains can function as the anchoring templates to hybridize with many poly-A MBs, leading to the unfolding of loop structures and the dissociation of FAM/BHQ1 pairs (the 1st amplification stage). Subsequently, all 3'-hydroxylated poly-A MBs can be extended with the assistance of TdT to generate the branched long poly-T chains, leading to the hybridization of more poly-A MBs and the dissociation of more FAM/BHQ1 pairs (the 2nd amplification stage). Through multiple rounds of extension, assembly, and activation of poly-A MBs, dendritic DNA nanostructures are automatically formed, resulting in the dissociation of abundant fluorophores from the FAM/BHQ1 pairs to generate an exponentially amplified fluorescence signal (the nth amplification stage). This strategy possesses high sensitivity and excellent specificity, and the detection limit can reach 1 cell. Moreover, it can evaluate kinetic parameters, screen inhibitors, estimate cellular inhibition effects, and measure ALP in human serums.

An Exonuclease I-Aided Turn-Off Fluorescent Strategy for Alkaline Phosphatase Assay Based on Terminal Protection and Copper Nanoparticles

As an important DNA 3'-phosphatase, alkaline phosphatase can repair damaged DNA caused by replication and recombination. It is essential to measure the level of alkaline phosphatase to indicate some potential diseases, such as cancer, related to alkaline phosphatase. Here, we designed a simple and fast method to detect alkaline phosphatase quantitively. When alkaline phosphatase is present, the resulting poly T-DNA with a 3'-hydroxyl end was cleaved by exonuclease I, prohibiting the formation of fluorescent copper nanoparticles. However, the fluorescent copper nanoparticles can be monitored with the absence of alkaline phosphatase. Hence, we can detect alkaline phosphatase with this turn-off strategy. The proposed method is able to quantify the concentration of alkaline phosphatase with the LOD of 0.0098 U/L. Furthermore, we utilized this method to measure the effects of inhibitor Na₃VO₄ on alkaline phosphatase. In addition, it was successfully applied to quantify the level of alkaline phosphatase in human serum. The proposed strategy is sensitive, selective, cost effective, and timesaving, having a great potential to detect alkaline phosphatase quantitatively in clinical diagnosis.

A path-choice-based biosensor to detect the activity of the alkaline phosphatase as the switch

Alkaline phosphatase (ALP), which converts the phosphate group (-PO₄) in the substrate to the hydroxyl group (-OH), is a useful tool in the biological analysis, a good indicator of dissolved inorganic phosphorus levels and an important biomarker for several diseases. In conventional designs for ALP detection, both the interferent with a -PO₄ and the target with a -OH will go into the sensing path and give out the undesired background and the desired signal respectively. This limited the sensitivity of the method and required the complicated design to achieve a satisfying limit of detection (LOD) of ALP. Here, we provided a new sensing strategy for ALP detection design. We designed a path-choice-based biosensor with two DNA tracks in which ALP works as the switch to guide the reaction path of lambda exonuclease (λ exo). The path-choice character enlarged the difference between signal and background by separating the interferent removing path and the target sensing path. The substrate preference of ALP and λ exo was studied to optimize the structure of DNA tracks. The path-choice-based biosensor achieved simple, fast (30 min), sensitive (LOD 0.014 U L^-1) and selective detection of the activity of ALP. The method has been applied to detect the activity of ALP in cell lysates, which shows the potential application in ALP-related biological research.

Detecting acid phosphatase enzymatic activity with phenol as a chemical exchange saturation transfer magnetic resonance imaging contrast agent (PhenolCEST MRI)

Phenol contains an exchangeable hydroxyl proton resonant at 4.8 ppm from the resonance frequency of water in the 1H nuclear magnetic resonance (1H NMR) spectrum, enabling itself to be detected at sub-mM concentration by either chemical exchange saturation transfer magnetic resonance imaging (CEST MRI) or exchange-based T2 relaxation enhancement (T2ex) effect under acidic and basic conditions, respectively. We recently investigated the T2ex effects of phenol and its derivatives, but the CEST characteristics of phenols are unknown in detail, and no study on using the natural CEST MRI effects of phenol for detecting enzymatic activity has been conducted. Herein, on the basis of the inherent CEST MR property of phenol, namely phenolCEST, we developed the first MRI approach to detect acid phosphatase (AcP) enzymatic activity. Upon the activity of AcP at pH = 5.0, non-CEST-detectable enzyme substrate phenyl phosphate was converted to CEST-detectable phenol, providing a simple way to quantify AcP activity directly without the need for a second signalling probe. We showed the application of this phenolCEST biosensor for measuring AcP activity in both enzyme solutions and cell lysates of prostate cells. This work opens a door for the utilization of phenolCEST MRI technique in sensor design and development.

Long Wavelength TCF-Based Fluorescent Probe for the Detection of Alkaline Phosphatase in Live Cells

A long wavelength TCF-based fluorescent probe (TCF-ALP) was developed for the detection of alkaline phosphatase (ALP). ALP-mediated hydrolysis of the phosphate group of TCF-ALP resulted in a significant fluorescence "turn on" (58-fold), which was accompanied by a colorimetric response from yellow to purple. TCF-ALP was cell-permeable, which allowed it to be used to image ALP in HeLa cells. Upon addition of bone morphogenic protein 2, TCF-ALP proved capable of imaging endogenously stimulated ALP in myogenic murine C2C12 cells. Overall, TCF-ALP offers promise as an effective fluorescent/colorimetric probe for evaluating phosphatase activity in clinical assays or live cell systems.