Electrochemical assay of plasmin activity and its kinetic analysis

The multifaceted role of proteases and modern analytical methods for investigation of their catalytic activity

We discuss the diverse functions of proteases in the context of their biotechnological and medical significance, as well as analytical approaches used to determine the functional activity of these enzymes. An insight into modern approaches to studying the kinetics and specificity of proteases, based on spectral (absorption, fluorescence), mass spectrometric, immunological, calorimetric, and electrochemical methods of analysis is given. We also examine in detail electrochemical systems for determining the activity and specificity of proteases. Particular attention is given to exploring innovative electrochemical systems based on the detection of the electrochemical oxidation signal of amino acid residues, thereby eliminating the need for extra redox labels in the process of peptide synthesis. In the review, we highlight the main prospects for the further development of electrochemical systems for the study of biotechnologically and medically significant proteases, which will enable the miniaturization of the analytical process for determining the catalytic activity of these enzymes.

Peptide-based self-assembled monolayers (SAMs): what peptides can do for SAMs and vice versa

Self-assembled monolayers (SAMs) represent highly ordered molecular materials with versatile biochemical features and multidisciplinary applications. Research on SAMs has made much progress since the early begginings of Au substrates and alkanethiols, and numerous examples of peptide-displaying SAMs can be found in the literature. Peptides, presenting increasing structural complexity, stimuli-responsiveness, and biological relevance, represent versatile functional components in SAMs-based platforms. This review examines the major findings and progress made on the use of peptide building blocks displayed as part of SAMs with specific functions, such as selective cell adhesion, migration and differentiation, biomolecular binding, advanced biosensing, molecular electronics, antimicrobial, osteointegrative and antifouling surfaces, among others. Peptide selection and design, functionalisation strategies, as well as structural and functional characteristics from selected examples are discussed. Additionally, advanced fabrication methods for dynamic peptide spatiotemporal presentation are presented, as well as a number of characterisation techniques. All together, these features and approaches enable the preparation and use of increasingly complex peptide-based SAMs to mimic and study biological processes, and provide convergent platforms for high throughput screening discovery and validation of promising therapeutics and technologies.

Application of Multiharmonic QCM-D for Detection of Plasmin at Hydrophobic Surfaces Modified by β-Casein

Plasmin protease plays an important role in many processes in living systems, including milk. Monitoring plasmin activity is important for control of the nutritional quality of milk and other dairy products. We designed a biosensor to detect the proteolytic activity of plasmin, using multiharmonic quartz crystal microbalance with dissipation (QCM-D). The β-casein immobilized on the hydrophobic surface of 1-dodecanethiol on the AT-cut quartz crystal was used to monitor plasmin activity. We demonstrated detection of plasmin in a concentration range of 0.1–20 nM, with the limit of detection about 0.13 ± 0.01 nM. The analysis of viscoelastic properties of the β-casein layer showed rapid changes of shear elasticity modulus, μ, and coefficient of viscosity, η, at plasmin sub-nanomolar concentrations, followed by modest changes at nanomolar concentrations, indicating multilayer architecture β-casein. A comparative analysis of viscoelastic properties of β-casein layers following plasmin and trypsin cleavage showed that the higher effect of trypsin was due to larger potential cleavage sites of β-casein.

Surface-enhanced Raman scattering-based detection of plasmin activity by specific peptide substrate

In this study, a new surface-enhanced Raman scattering (SERS)-based method has been developed for the detection of plasmin activity. Firstly, different peptide sequences, which are specific to plasmin, were examined. Then, SERS substrates were prepared by chosen peptide substrate. Enzyme activity was determined by pursuing the reduction of DTNB band at 1331 cm^-1 with Raman spectroscopy. The reduction in SERS intensity was related to the plasmin activity, and changes in SERS intensity vs. plasmin concentration graph was obtained. Limit of detection (LOD) and limit of quantification (LOQ) values were calculated as 2.14 U/mL and 6.42 U/mL, respectively. Intra- and inter-day repeatability results were determined as 1.45% and 1.47% relative standard deviation (RSD). Also, recovery of the method was determined for the plasmin spiked milk samples. The results demonstrated that the proposed method could be successfully used to detect the plasmin activity in milk samples.

New Insights on Plasmin Long Term Stability and the Mechanism of Its Activity Inhibition Analyzed by Quartz Crystal Microbalance

We used the research quartz crystal microbalance (RQCM) to monitor regulatory effects of plasmin and trypsin in the presence of their inhibitor α₂-antiplasmin. The gold surface of quartz crystals was modified with a β-casein layer that served as a substrate for protease digestion. The addition of plasmin or trypsin as well as their mixtures with α₂-antiplasmin resulted in an increase of resonant frequency, f, and in a decrease of motional resistance, R_m, depending on the molar ratio of protease: antiplasmin. At equimolar concentrations of protease and α₂-antiplasmin (5 nM:5 nM) full inhibition of protease activity took place. Monitoring of plasmin activity on an hourly and daily basis revealed a prominent effect of autolysis and decrease of plasmin activity in freshly activated samples. The degree of inhibition as well as plasmin half-life (t_1/2 = 2.48 ± 0.28 days) connected with its degradation was determined.

Quantitative Detection of Cathepsin B Activity in Neutral pH Buffers Using Gold Microelectrode Arrays: Toward Direct Multiplex Analyses of Extracellular Proteases in Human Serum

Proteases are critical signaling molecules and prognostic biomarkers for many diseases including cancer. There is a strong demand for multiplex bioanalytical techniques that can rapidly detect the activity of extracellular proteases with high sensitivity and specificity. This study demonstrates an activity-based electrochemical biosensor of a 3 × 3 gold microelectrode array for the detection of cathepsin B activity in human serum diluted in a neutral buffer. Proteolysis of ferrocene-labeled peptide substrates functionalized on 200 × 200 μm microelectrodes is measured simultaneously over the nine channels by AC voltammetry. The protease activity is represented by the inverse of the exponential decay time constant (1/τ), which equals to (k_cat/K_M)[CB] based on the Michaelis-Menten model. An enhanced activity of the recombinant human cathepsin B (rhCB) is observed in a low-ionic-strength phosphate buffer at pH = 7.4, giving a very low limit of detection of 8.49 × 10^-4 s^-1 for activity and 57.1 pM for the active rhCB concentration that is comparable to affinity-based enzyme-linked immunosorbent assay (ELISA). The cathepsin B presented in the human serum sample is validated by ELISA, which mainly detects the inactive proenzyme, while the electrochemical biosensor specifically measures the active cathepsin B and shows significantly higher decay rates when rhCB and human serum are activated. Analyses of the kinetic electrochemical measurements with spiked active cathepsin B in human serum provide further assessment of the protease activity in the complex sample. This study lays the foundation to develop the gold microelectrode array into a multiplex biosensor for rapid detection of the activity of extracellular proteases toward cancer diagnosis and treatment assessment.

Application of high-resolution ultrasonic spectroscopy for detection of the plasmin activity toward β-casein

High-resolution ultrasonic spectroscopy (HR-US) was applied for precise detection of plasmin activity towards β-casein in buffer at pH 7.8 and 37 °C. The evolution of ultrasonic velocity and ultrasonic attenuation measured at 15.5 MHz is related to the concentration of peptide bonds hydrolyzed and loss of β-casein aggregates, respectively. The ultrasonic assay presents sensitive and direct activity-based quantification of plasmin levels in milk. The variation in plasmin concentration between HR-US and ELISA method owed to the differing detection principles. The real-time ultrasonic profiles of hydrolysis were utilized to describe the kinetic aspect of plasmin activity. The non-linear activity curve was fitted with classic and inverse Michaelis-Menten type models. Within 1-8.6 mg·mL^-1 β-casein, the V_max and K_M obtained were (6.30 ± 2.21) × 10^-5 mol.kg^-1·min^-1 and 10.33 ± 3.50 mg·mL^-1, respectively. The maximum peptide bond cleaved was 5-6 (2.7% degree of hydrolysis) achieved at 1 mg·mL^-1 β-casein.

Electrochemical Activity Assay for Protease Analysis Using Carbon Nanofiber Nanoelectrode Arrays

There is a strong demand for bioanalytical techniques to rapidly detect protease activities with high sensitivity and high specificity. This study reports an activity-based electrochemical method toward this goal. Nanoelectrode arrays (NEAs) fabricated with embedded vertically aligned carbon nanofibers (VACNFs) are functionalized with specific peptide substrates containing a ferrocene (Fc) tag. The kinetic proteolysis curves are measured with continuously repeated ac voltammetry, from which the catalytic activity is derived as the inverse of the exponential decay time constant based on a heterogeneous Michaelis-Menten model. Comparison of three peptide substrates with different lengths reveals that the hexapeptide H2N-(CH2)4-CO-Pro-Leu-Arg-Phe-Gly-Ala-NH-CH2-Fc is the optimal probe for cathepsin B. The activity strongly depends on temperature and is the highest around the body temperature. With the optimized peptide substrate and measuring conditions, the limit of detection of cathepsin B activity and concentration can reach 2.49 × 10-4 s-1 and 0.32 nM, respectively. The peptide substrates show high specificity to the cognate proteases, with negligible cross-reactions among three cancer-related proteases cathepsin B, ADAM10, and ADAM17. This electrochemical method can be developed into multiplex chips for rapid profiling of protease activities in cancer diagnosis and treatment monitoring.

Peptide-based biosensors: From self-assembled interfaces to molecular probes in electrochemical assays

Redox-tagged peptides have emerged as functional materials with multiple applications in the area of sensing and biosensing applications due to their high stability, excellent redox properties and versatility of biomolecular interactions. They allow direct observation of molecular interactions in a wide range of affinity and enzymatic assays and act as electron mediators. Short helical peptides possess the ability to self-assemble in specific configurations with the possibility to develop in highly-ordered, stable 1D, 2D and 3D architectures in a hierarchical controlled manner. We provide here a brief overview of the electrochemical techniques available to study the electron transfer in peptide films with particular interest in developing biosensors with immobilized peptide motifs, for biological and clinical applications.

Quantitative electrochemical detection of cathepsin B activity in complex tissue lysates using enhanced AC voltammetry at carbon nanofiber nanoelectrode arrays

The proteolytic activity of a cancer-related enzyme cathepsin B is measured with alternating current voltammetry (ACV) using ferrocene (Fc) labeled tetrapeptides attached to nanoelectrode arrays (NEAs) fabricated with vertically aligned carbon nanofibers (VACNFs). This combination enables the use of high AC frequencies (~1kHz) with enhanced electrochemical signals. The specific proteolysis of the Fc-peptide by cathepsin B produces decay in the ACV peak current versus the reaction time. The exponential component of the raw data can be extracted and defined as the "extracted proteolytic signal" which allows consistent quantitative analyses using a heterogeneous Michaelis-Menten model. A "specificity constant" kcat/KM = (3.68 ± 0.50) × 10(4)M(-1)s(-1) for purified cathepsin B was obtained. The detections of cathepsin B activity in different concentrations of whole lysate of human breast tissue, tissue lysate spiked with varied concentrations of cathepsin B, and the tissue lysate after immunoprecipitation showed that there is ~13.4 nM higher cathepsin B concentration in 29.1 µg mL(-1) of whole tissue lysate than the immunoprecipitated sample. The well-defined regular VACNF NEAs by e-beam lithography show a much faster kinetics for cathepsin B proteolysis with kcat/KM = 9.2 × 10(4)M(-1)s(-1). These results illustrate the potential of this technique as a portable multiplex electronic system for cancer diagnosis by rapid protease profiling of serum or blood samples.

Electrochemical Protease Biosensor Based on Enhanced AC Voltammetry Using Carbon Nanofiber Nanoelectrode Arrays

We report an electrochemical method for measuring the activity of proteases using nanoelectrode arrays (NEAs) fabricated with vertically aligned carbon nanofibers (VACNFs). The VACNFs of ~150 nm in diameter and 3 to 5 μm in length were grown on conductive substrates and encapsulated in SiO2 matrix. After polishing and plasma etching, controlled VACNF tips are exposed to form an embedded VACNF NEA. Two types of tetrapeptides specific to cancer-mediated proteases legumain and cathepsin B are covalently attached to the exposed VACNF tip, with a ferrocene (Fc) moiety linked at the distal end. The redox signal of Fc can be measured with AC voltammetry (ACV) at ~1 kHz frequency on VACNF NEAs, showing distinct properties from macroscopic glassy carbon electrodes due to VACNF's unique interior structure. The enhanced ACV properties enable the kinetic measurements of proteolytic cleavage of the surface-attached tetrapeptides by proteases, further validated with a fluorescence assay. The data can be analyzed with a heterogeneous Michaelis-Menten model, giving "specificity constant" kcat /Km as (4.3 ± 0.8) × 104 M-1s-1 for cathepsin B and (1.13 ± 0.38) × 104 M-1s-1 for legumain. This method could be developed as portable multiplex electronic techniques for rapid cancer diagnosis and treatment monitoring.

Kinetics of enzyme action on surface-attached substrates: a practical guide to progress curve analysis in any kinetic situation

In the present work, exact kinetic equations describing the action of an enzyme in solution on a substrate attached to a surface have been derived in the framework of the Michaelis-Menten mechanism but without resorting to the often-used steady-state approximation. The here-derived kinetic equations are cast in a workable format, allowing us to introduce a simple and universal procedure for the quantitative analysis of enzyme surface kinetics that is valid for any kinetic situation. The results presented here should allow experimentalists studying the kinetics of enzyme action on immobilized substrates to analyze their data in a perfectly rigorous way.

Optimizing electrode-attached redox-peptide systems for kinetic characterization of protease action on immobilized substrates. Observation of dissimilar behavior of trypsin and thrombin enzymes

In this work, we experimentally address the issue of optimizing gold electrode attached ferrocene (Fc)-peptide systems for kinetic measurements of protease action. Considering human α-thrombin and bovine trypsin as proteases of interest, we show that the recurring problem of incomplete cleavage of the peptide layer by these enzymes can be solved by using ultraflat template-stripped gold, instead of polished polycrystalline gold, as the Fc-peptide bearing electrode material. We describe how these fragile surfaces can be mounted in a rotating disk configuration so that enzyme mass transfer no longer limits the overall measured cleavage kinetics. Finally, we demonstrate that, once the system has been optimized, in situ real-time cyclic voltammetry monitoring of the protease action can yield high-quality kinetic data, showing no sign of interfering effects. The cleavage progress curves then closely match the Langmuirian variation expected for a kinetically controlled surface process. Global fit of the progress curves yield accurate values of the peptide cleavage rate for both trypsin and thrombin. It is shown that, whereas trypsin action on the surface-attached peptide closely follows Michaelis-Menten kinetics, thrombin displays a specific and unexpected behavior characterized by a nearly enzyme-concentration-independent cleavage rate in the subnanomolar enzyme concentration range. The reason for this behavior has still to be clarified, but its occurrence may limit the sensitivity of thrombin sensors based on Fc-peptide layers.

Cleavage-sensing redox peptide monolayers for the rapid measurement of the proteolytic activity of trypsin and alpha-thrombin enzymes

Ferrocene (Fc)-labeled peptides are end-grafted onto gold electrodes via a flexible polyethylene glycol (PEG) linker, and their ability to act as substrates for proteolytic enzymes trypsin and alpha-thrombin is investigated by cyclic voltammetry. It is shown that whereas a short Fc-tetrapeptide substrate is rapidly cleaved by trypsin, a longer Fc-heptapeptide substrate is required for alpha-thrombin detection. However, in both cases it is observed that not all of the Fc-peptide chains present on the electrode surface are cleavable by the proteases and that the cleavage yield is actually controlled by the surface coverage in the Fc-peptide. Surface dilution of the Fc-peptide using a backfilling molecule such as MCH (6-mercapto-1-hexanol) was required to obtain a cleavage yield larger than 80%. The kinetics of Fc-peptide cleavage by trypsin or alpha-thrombin is then shown to be adequately described by Michaelis Menten kinetics, allowing enzymatic constants k(cat) and K(M) to be determined. The obtained rate constant values showed that the affinity of the enzymes for their respective Fc-peptide substrates is very high (i.e., low K(M) values) whereas that for the cleavage step itself is relatively low (low k(cat) values). Partial compensation of these parameters yields a fast response of the Fc-peptide electrodes to the proteases in solution in the 1-1000 nM range. The type of molecule used to backfill the Fc-peptide layers, either MCH or PEG(6) chains, is shown to modulate the activity of the proteases versus the Fc-peptide layers: in particular, the PEG(6) diluent is specifically shown to decrease the ability of alpha-thrombin to cleave its Fc-peptide substrate whereas trypsin activity is unaffected by the presence of PEG chains.