Advances in Analysis of Milk Proteases Activity at Surfaces and in a Volume by Acoustic Methods

Qualitative and quantitative protease activity tests based on protein degradation in three-dimensional structures

The pattern of the activity of proteases is related to distinct physiological states of living organisms. Often activity changes of a certain protease can be assigned to a specific disease. Hence, they are useful biomarkers and a simple and fast determination method of their activity could be a valuable tool for the efficient monitoring of numerous diseases. Here, two different methods for the qualitative and quantitative determination of protease activity are demonstrated using the model system of proteinase K. The first test system is based on a protein-modified and colored 3D silica structure that changes color when exposed to the enzyme. This method has also been used for the detection of matrix metallo-protease 2 (MMP2) with gelatine as protease substrate on the plates. The second detection system uses the decrease in the voltammetric signal of a cytochrome c/DNA multilayer electrode after incubation with a protease to quantitatively determine its proteolytic activity. While activities down to 0.15 U/ml can be detected with the first method, the second one provides detection limits of about 0.03U/ml (for proteinase K.) The functionality of both systems can be demonstrated and ways for further enhancement of sensitivity have been elucidated.

An Electronic "Tongue" Based on Multimode Multidirectional Acoustic Plate Wave Propagation

This paper theoretically and experimentally demonstrates the possibility of detecting the five basic tastes (salt, sweet, sour, umami, and bitter) using a variety of higher-order acoustic waves propagating in piezoelectric plates. Aqueous solutions of sodium chloride (NaCl), glucose (C6H12O6), citric acid (C6H8O7), monosodium glutamate (C5H8NO4Na), and sagebrush were used as chemicals for the simulation of each taste. These liquids differed from each other in terms of their physical properties such as density, viscosity, electrical conductivity, and permittivity. As a total acoustic response to the simultaneous action of all liquid parameters on all acoustic modes in a given frequency range, a change in the propagation losses (ΔS12) of the specified wave compared with distilled water was used. Based on experimental measurements, the corresponding orientation histograms of the ΔS12 were plotted for different types of acoustic waves. It was found that these histograms for different substances are individual and differ in shape, area, and position of their extremes. Theoretically, it has been shown that the influence of different liquids on different acoustic modes is due to both the electrical and mechanical properties of the liquids themselves and the mechanical polarization of the corresponding modes. Despite the fact that the mechanical properties of the used liquids are close to each other, the attenuation of different modes in their presence is not only due to the difference in their electrical parameters. The proposed approach to creating a multi-parametric multimode acoustic electronic tongue and obtaining a set of histograms for typical liquids will allow for the development of devices for the operational analysis of food, medicines, gasoline, aircraft fuel, and other liquid substances without the need for detailed chemical analysis.

Human proinsulin production in the milk of transgenic cattle

BACKGROUND

The worldwide growing demand for human insulin for treating diabetes could be supplied by transgenic animals producing insulin in their milk.

METHODS AND RESULTS

Pseudo-lentivirus containing the bovine β-casein promoter and human insulin sequences was used to produce modified adult fibroblasts, and the cells were used for nuclear transfer. Transgenic embryos were transferred to recipient cows, and one pregnancy was produced. Recombinant protein in milk was evaluated using western blotting and mass spectrometry. One transgenic cow was generated, and in milk analysis, two bands were observed in western blotting with a molecular mass corresponding to the proinsulin and insulin. The mass spectrometry analysis showed the presence of human insulin more than proinsulin in the milk, and it identified proteases in the transgenic milk that could convert proinsulin into insulin and insulin-degrading enzyme that could degrade the recombinant protein.

CONCLUSION

The methodologies used for generating the transgenic cow allowed the detection of the production of recombinant protein in the milk at low relative expression compared to milk proteins, using mass spectrometry, which was efficient for detecting recombinant protein with low expression in milk. Milk proteases could act on protein processing converting recombinant protein to functional protein. On the other hand, some milk proteases could act in degrading the recombinant protein.

Bulk and Surface Acoustic Wave Biosensors for Milk Analysis

Milk and dairy products are common foods and, therefore, are subject to regular controls. Such controls cover both the identification and quantification of specific components and the determination of physical parameters. Components include the usual milk ingredients, mainly carbohydrates, proteins, and fat, and any impurities that may be present. The latter range from small molecules, such as drug residues, to large molecules, e.g., protein-based toxins, to pathogenic microorganisms. Physical parameters of interest include viscosity as an indicator of milk gelation. Bulk and surface acoustic wave sensors, such as quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices, can principally be used for both types of analysis, with the actual application mainly depending on the device coating and the test format. This review summarizes the achievements of acoustic sensor devices used for milk analysis applications, including the determination of physical liquid parameters and the detection of low- and high-molecular-weight analytes and microorganisms. It is shown how the various requirements resulting from the respective analytes and the complex sample matrix are addressed, and to what extent the analytical demands, e.g., with regard to legal limits, are met.

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.

Analysis of trypsin activity at β-casein layers formed on hydrophobic surfaces using a multiharmonic acoustic method

Proteolysis of milk proteins, such as caseins, caused by milk proteases, can change the organoleptic and nutritional characteristics of milk, and therefore it is essential to monitor this enzymatic activity. We used trypsin as a model protease because of its role as a biomarker for pancreatitis. The aim of this work was to demonstrate the detection of proteolysis of β-casein by trypsin using a multiharmonic quartz crystal microbalance (QCM) biosensor. The β-casein layer was deposited from a 0.1 mg mL^-1 solution on a hydrophobic surface consisting of a self-assembled monolayer of 1-dodecanethiol on the gold electrode of the QCM. The addition of an increasing concentration of trypsin leads to the removal of the casein layer due to proteolysis, and correlates with an increase in the resonant frequency of the QCM. We investigated the effect of trypsin concentrations (0.3-20 nM) on the kinetics of the proteolysis of β-casein and demonstrated that the frequency increase is proportional to the protease concentration. Consequently, an inverse Michaelis-Menten model was used to estimate the Michaelis-Menten constant (K_M = 0.38 ± 0.02 nM) and the limit of detection (LOD = 0.16 ± 0.02 nM). The thickness, mass and viscoelastic properties of the protein adlayer after its formation and following the proteolytic cleavage were evaluated by means of multi-harmonic analysis. We found that β-casein is preferably adsorbed on the hydrophobic surfaces as an asymmetrical double layer, of which the innermost layer was found to be denser and thinner (about 2.37 nm) and the outermost layer was found to be less tightly bound and thicker (about 3.5 nm).

Changes of Viscoelastic Properties of Aptamer-Based Sensing Layers Following Interaction with Listeria innocua

A multiharmonic quartz crystal microbalance (QCM) has been applied to study the viscoelastic properties of the aptamer-based sensing layers at the surface of a QCM transducer covered by neutravidin following interaction with bacteria Listeria innocua. Addition of bacteria in the concentration range 5 × 103-106 CFU/mL resulted in a decrease of resonant frequency and in an increase of dissipation. The frequency decrease has been lower than one would expect considering the dimension of the bacteria. This can be caused by lower penetration depth of the acoustics wave (approximately 120 nm) in comparison with the thickness of the bacterial layer (approximately 500 nm). Addition of E. coli at the surface of neutravidin as well as aptamer layers did not result in significant changes in frequency and dissipation. Using the Kelvin-Voight model the analysis of the viscoelastic properties of the sensing layers was performed and several parameters such as penetration depth, Γ, viscosity coefficient, η, and shear modulus, μ, were determined following various modifications of QCM transducer. The penetration depth decreased following adsorption of the neutravidin layer, which is evidence of the formation of a rigid protein structure. This value did not change significantly following adsorption of aptamers and Listeria innocua. Viscosity coefficient was higher for the neutravidin layer in comparison with the naked QCM transducer in a buffer. However, a further increase of viscosity coefficient took place following attachment of aptamers suggesting their softer structure. The interaction of Listeria innocua with the aptamer layer resulted in slight decrease of viscosity coefficient. The shearing modulus increased for the neutravidin layer and decreased following aptamer adsorption, while a slight increase of µ was observed after the addition of Listeria innocua.

Advances in Electrochemical and Acoustic Aptamer-Based Biosensors and Immunosensors in Diagnostics of Leukemia

Early diagnostics of leukemia is crucial for successful therapy of this disease. Therefore, development of rapid, sensitive, and easy-to-use methods for detection of this disease is of increased interest. Biosensor technology is challenged for this purpose. This review includes a brief description of the methods used in current clinical diagnostics of leukemia and provides recent achievements in sensor technology based on immuno- and DNA aptamer-based electrochemical and acoustic biosensors. The comparative analysis of immuno- and aptamer-based sensors shows a significant advantage of DNA aptasensors over immunosensors in the detection of cancer cells. The acoustic technique is of comparable sensitivity with those based on electrochemical methods; moreover, it is label-free and provides straightforward evaluation of the signal. Several examples of sensor development are provided and discussed.

Detection of Sub-Nanomolar Concentration of Trypsin by Thickness-Shear Mode Acoustic Biosensor and Spectrophotometry

The determination of protease activity is very important for disease diagnosis, drug development, and quality and safety assurance for dairy products. Therefore, the development of low-cost and sensitive methods for assessing protease activity is crucial. We report two approaches for monitoring protease activity: in a volume and at surface, via colorimetric and acoustic wave-based biosensors operated in the thickness-shear mode (TSM), respectively. The TSM sensor was based on a β-casein substrate immobilized on a piezoelectric quartz crystal transducer. After an enzymatic reaction with trypsin, it cleaved the surface-bound β-casein, which increased the resonant frequency of the crystal. The limit of detection (LOD) was 0.48 ± 0.08 nM. A label-free colorimetric assay for trypsin detection has also been performed using β-casein and 6-mercaptohexanol (MCH) functionalized gold nanoparticles (AuNPs/MCH-β-casein). Due to the trypsin cleavage of β-casein, the gold nanoparticles lost shelter, and MCH increased the attractive force between the modified AuNPs. Consequently, AuNPs aggregated, and the red shift of the absorption spectra was observed. Spectrophotometric assay enabled an LOD of 0.42 ± 0.03 nM. The Michaelis-Menten constant, KM, for reverse enzyme reaction has also been estimated by both methods. This value for the colorimetric assay (0.56 ± 0.10 nM) is lower in comparison with those for the TSM sensor (0.92 ± 0.44 nM). This is likely due to the better access of the trypsin to the β-casein substrate at the surface of AuNPs in comparison with those at the TSM transducer.

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.