Development of a bioreactor based on trypsin immobilized on monolithic support for the on-line digestion and identification of proteins

A miniaturised semi-dynamic in-vitro model of human digestion

Reliable in-vitro digestion models that are able to successfully replicate the conditions found in the human gastrointestinal tract are key to assess the fate and efficiency of new formulations aimed for oral consumption. However, current in-vitro models either lack the capability to replicate crucial dynamics of digestion or require large volumes of sample/reagents, which can be scarce when working with nanomaterials under development. Here, we propose a miniaturised digestion system, a digestion-chip, based on incubation chambers integrated on a polymethylmethacrylate device. The digestion-chip incorporates key dynamic features of human digestion, such as gradual acidification and gradual addition of enzymes and simulated fluids in the gastric phase, and controlled gastric emptying, while maintaining low complexity and using small volumes of sample and reagents. In addition, the new approach integrates real-time automated closed-loop control of two key parameters, pH and temperature, during the two main phases of digestion (gastric and intestinal) with an accuracy down to ± 0.1 °C and ± 0.2 pH points. The experimental results demonstrate that the digestion-chip successfully replicates the gold standard static digestion INFOGEST protocol and that the semi-dynamic digestion kinetics can be reliably fitted to a first kinetic order model. These devices can be easily adapted to dynamic features in an automated, sensorised, and inexpensive platform and will enable reliable, low-cost and efficient assessment of the bioaccessibility of new and expensive drugs, bioactive ingredients or nanoengineered materials aimed for oral consumption, thereby avoiding unnecessary animal testing.

Real-time bottom-up characterization of protein mixtures enabled by online microdroplet-assisted enzymatic digestion (MAED)

Enzymatic digestion remains one of the "rate-determining steps" in the bottom-up analysis of proteins. However, by performing digestion in microdroplets generated from electrosonic spray, the reaction could be accelerated to a timescale lower than milliseconds. Here, we describe a simple and rapid online digestion platform named online microdroplet-assisted enzymatic digestion (MAED). It involves the integration of intact protein separation with enzymatic digestion in microdroplets. Via online MAED, various protein standards, including an antibody standard, were characterized in a bottom-up manner without prior digestion, and high sequence coverages were obtained. We further extended the application of online MAED to a more complex sample, mouse brain extract, where protein identifications were successfully yielded. Compared with the conventional bottom-up approach, a more comprehensive characterization could be obtained particularly for low molecular weight proteins. In short, we provide a rapid and alternative bottom-up analysis in a top-down fashion as well as a new possibility for microdroplet chemistry.

Development of novel enzyme immobilization methods employing formaldehyde or triethoxysilylbutyraldehyde to fabricate immobilized enzyme microreactors for peptide mapping

The digestion of proteins with proteolytic enzymes has expedited the analysis of peptide mapping. Here, we compared the digestion efficiency of soluble chymotrypsin (CT) with two immobilized CT preparations using bovine serum albumin (BSA) as the substrate. An efficient method of immobilizing chymotrypsin using formaldehyde (FA) was optimized and the conditions were applied to assess a novel immobilization reagent, triethoxysilylbutaraldehyde (TESB). Efforts to determine the best enzyme-to-substrate (E : S) ratios during digestion of denatured BSA with single-use FA-CT enzyme particles were performed by adjusting the amount of substrate used. An E : S ratio of 10 : 1 was found to be best based on the LC-MS/MS analysis data showing sequence coverage of 67%. Fabrication of immobilized enzyme microreactors (IMERs) was carried out using both (3-aminopropyl)triethoxysilane (APTES) with the idealized conditions with FA, as well as the novel procedure utilizing TESB for a proof of concept open-tubular IMER. It was found that the FA-APTES IMER had a sequence coverage of 6%, while the TESB IMER had 29% sequence coverage from MS analysis. The application of TESB in enzyme immobilization has the potential to facilitate a greater degree of enzymatic digestion with higher sequence coverage than traditional immobilization or crosslinking reagents for bottom-up proteomics.

Affinity monolith chromatography: A review of general principles and recent developments

Affinity monolith chromatography (AMC) is a liquid chromatographic technique that utilizes a monolithic support with a biological ligand or related binding agent to isolate, enrich, or detect a target analyte in a complex matrix. The target-specific interaction exhibited by the binding agents makes AMC attractive for the separation or detection of a wide range of compounds. This article will review the basic principles of AMC and recent developments in this field. The supports used in AMC will be discussed, including organic, inorganic, hybrid, carbohydrate, and cryogel monoliths. Schemes for attaching binding agents to these monoliths will be examined as well, such as covalent immobilization, biospecific adsorption, entrapment, molecular imprinting, and coordination methods. An overview will then be given of binding agents that have recently been used in AMC, along with their applications. These applications will include bioaffinity chromatography, immunoaffinity chromatography, immobilized metal-ion affinity chromatography, and dye-ligand or biomimetic affinity chromatography. The use of AMC in chiral separations and biointeraction studies will also be discussed.

Immobilization of trypsin onto porous methacrylate-based monolith for flow-through protein digestion and its potential application to chiral separation using liquid chromatography

Monolithic columns for analytical applications have attracted the researcher's attention. In this work, the laboratory-made organic-polymer monolithic column is modified with trypsin and further applied as a nanobiocatalyst microreactor and a stationary phase for separating chiral compounds by liquid chromatography. The monolith was synthesized by in-situ copolymerization of glycidyl methacrylate (GMA) and ethylene glycol dimethacrylate (EDMA) or trimethylolpropane trimethacrylate (TRIM) as a crosslinking agent, with porogen of 1,4-butanediol/propanol/water (4:7:1 v/v) and AIBN as the radical polymerization initiator inside PEEK and silicosteel tubings (1.0 mm i.d × 100 mm) at 60 °C for 12 h. A total monomer ratio (%T) and crosslinking agent (%C) of 40:25 and 28:12 were applied to prepare poly-(GMA-co-EDMA) and poly-(GMA-co-TRIM), respectively. The produced monoliths were further modified by introducing trypsin (10 mg/L) through the ring-opening reaction of the epoxide group existing in the monolithic column. The trypsin-immobilized poly-(GMA-co-EDMA) monolithic column was applied as the nanobiocatalyst microreactor for online/flow-through and rapid digestion of β-casein sample into its peptide fragments. The trypsin-immobilized poly-(GMA-co-TRIM) column has potential application to be used as the HPLC stationary phase for the separation of R/S-citronellal enantiomers.

Study of the geometry of open channels in a layer-bed-type microfluidic immobilized enzyme reactor

This paper aims at studying open channel geometries in a layer-bed-type immobilized enzyme reactor with computer-aided simulations. The main properties of these reactors are their simple channel pattern, simple immobilization procedure, regenerability, and disposability; all these features make these devices one of the simplest yet efficient enzymatic microreactors. The high surface-to-volume ratio of the reactor was achieved using narrow (25–75 μm wide) channels. The simulation demonstrated that curves support the mixing of solutions in the channel even in strong laminar flow conditions; thus, it is worth including several curves in the channel system. In the three different designs of microreactor proposed, the lengths of the channels were identical, but in two reactors, the liquid flow was split to 8 or 32 parallel streams at the inlet of the reactor. Despite their overall higher volumetric flow rate, the split-flow structures are advantageous due to the increased contact time. Saliva samples were used to test the efficiencies of the digestions in the microreactors.

Tryptic digestion of human serum for proteomic mass spectrometry automated by centrifugal microfluidics

Mass spectrometry has become an important analytical tool for protein research studies to identify, characterise and quantify proteins with unmatched sensitivity in a highly parallel manner. When transferred into clinical routine, the cumbersome and error-prone sample preparation workflows present a major bottleneck. In this work, we demonstrate tryptic digestion of human serum that is fully automated by centrifugal microfluidics. The automated workflow comprises denaturation, digestion and acidification. The input sample volume is 1.3 μl only. A triplicate of human serum was digested with the developed microfluidic chip as well as with a manual reference workflow on three consecutive days to assess the performance of our system. After desalting and liquid chromatography tandem mass spectrometry, a total of 604 proteins were identified in the samples digested with the microfluidic chip and 602 proteins with the reference workflow. Protein quantitation was performed using the Hi3 method, yielding a 7.6% lower median intensity CV for automatically digested samples compared to samples digested with the reference workflow. Additionally, 17% more proteins were quantitated with less than 30% CV in the samples from the microfluidic chip, compared to the manual control samples. This improvement can be attributed to the accurate liquid metering with all volume CVs below 1.5% on the microfluidic chip. The presented automation solution is attractive for laboratories in need of robust automation of sample preparation from small volumes as well as for labs with a low or medium throughput that does not allow for large investments in robotic systems.

An insight into chiral monolithic stationary phases for enantioselective high-performance liquid chromatography applications

In this review, three main classes of chiral monolithic stationary phases, namely silica-, organic polymer-, and hybrid-based monolithic stationary phases, are covered. Their preparations, applications, and advantages compared with the conventional-packed and open-tubular capillary columns are discussed. A detailed description of the different types and techniques used for the introduction of chiral selectors into the monolithic matrices such as immobilization, functionalization, coating, encapsulation, and bonding. Special emphasis is given to the recent developments of chiral selectors in HPLC monolithic stationary phases during the past 18 years.

CE with multi-walled carbon nanotubes (MWCNTs). Part II. SDS coated functionalized MWCNTs as pseudo-stationary phases in nanoparticle EKC - Retention behaviors of small and large solutes

In this study, functionalized multi-walled carbon nanotubes (MWCNTs), namely hydroxylated MWCNTs (MWCNT-OH), carboxylated MWCNTs (MWCNT-COOH) and sulfonated MWCNTs (MWCNT-SO₃H) coated with sodium dodecyl sulfate (SDS) were demonstrated as effective pseudo-stationary phases (PSPs) in the separation of various species by the nanoparticle capillary electrokinetic chromatography (NPEKC) mode of capillary electrophoresis (CE). Due to the significant increase in their surface charge density in the presence of SDS, the three SDS coated MWCNTs yielded high performance separation for herbicides, barbiturates, dansyl-DL-amino acids (Dns-AAs), dipeptides and proteins by NPEKC. In addition, high resolution tryptic peptide maps of three standard proteins including myoglobin, cytochrome C and lysozyme were readily obtained. The three PSPs systems yielded high plate numbers that spanned a wide range of values depending on the type of species. The values of the observed selectivity factors (i.e., α values) were significantly different among the three PSPs for solutes that underwent strong interactions with the SDS coated functionalized MWCNTs while for negatively charged solutes (e.g., Dns-AAs) of the same charge sign as the PSPs the α values were about the same on the three different PSPs indicating weak association with the PSPs and signaling separation based chiefly on the differences in electro-migration arising from differences in charge-to-mass ratios.

Stabilization of Immobilized Enzymes via the Chaperone-Like Activity of Mixed Lipid Bilayers

Biomimetic lipid bilayers represent intriguing materials for enzyme immobilization, which is critical for many biotechnological applications. Here, through the creation of mixed lipid bilayers, the retention of immobilized enzyme structures and catalytic activity are dramatically enhanced. The enhancement in the retention of enzyme structures, which correlated with an increase in enzyme activity, is observed using dynamic single-molecule (SM) fluorescence methods. The results of SM analysis specifically show that lipid bilayers composed of mixtures of 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl- sn-glycero-3-phospho-(1'- rac-glycerol) (DOPG) stabilize the folded state of nitroreductase (NfsB), increasing the rate of refolding relative to unfolding of enzyme molecules on the bilayer surface. Remarkably, for optimal compositions with 15-50% DOPG, over 95% of NfsB remains folded while the activity of the enzyme is increased as much as 2 times over that in solution. Within this range of DOPG, the strength of the interaction of folded and unfolded NfsB with the bilayer surface was also significantly altered, which was evident by the change in the diffusion of folded and unfolded NfsB in the bilayer. Ultimately, these findings provide direct evidence for the chaperone-like activity of mixed DOPG/DOPC lipid bilayers, which can be controlled by tuning the fraction of DOPG in the bilayer.

Acrylate-based poly-high internal phase emulsions for effective enzyme immobilization and activity retention: from computationally-assisted synthesis to pharmaceutical applications

PolyHIPE functional materials were chemically conjugated with a model enzyme. It retained its activity upon flow as demonstrated by the conversion of a specific substrate.

Affinity monolith chromatography: A review of general principles and applications

Affinity monolith chromatography, or AMC, is a liquid chromatographic method in which the support is a monolith and the stationary phase is a biological-binding agent or related mimic. AMC has become popular for the isolation of biochemicals, for the measurement of various analytes, and for studying biological interactions. This review will examine the principles and applications of AMC. The materials that have been used to prepare AMC columns will be discussed, which have included various organic polymers, silica, agarose, and cryogels. Immobilization schemes that have been used in AMC will also be considered. Various binding agents and applications that have been reported for AMC will then be described. These applications will include the use of AMC for bioaffinity chromatography, immunoaffinity chromatography, dye-ligand affinity chromatography, and immobilized metal-ion affinity chromatography. The use of AMC with chiral stationary phases and as a tool to characterize biological interactions will also be examined.

Multichannel Open Tubular Enzyme Reactor Online Coupled with Mass Spectrometry for Detecting Ricin

For counterterrorism purposes, a selective nano liquid chromatography-mass spectrometry (nanoLC-MS) platform was developed for detecting the highly lethal protein ricin from castor bean extract. Manual sample preparation steps were omitted by implementing a trypsin/Lys-C enzyme-immobilized multichannel reactor (MCR) consisting of 126 channels (8 μm inner diameter in all channels) that performed online digestion of proteins (5 min reaction time, instead of 4-16 h in previous in-solution methods). Reduction and alkylation steps were not required. The MCR allowed identification of ricin by signature peptides in all targeted mode injections performed, with a complete absence of carry-over in blank injections. The MCRs (interior volume ≈ 1 μL) have very low backpressure, allowing for trivial online coupling with commercial nanoLC-MS systems. The open tubular nature of the MCRs allowed for repeatable within/between-reactor preparation and performance.

Thiol-ene Monolithic Pepsin Microreactor with a 3D-Printed Interface for Efficient UPLC-MS Peptide Mapping Analyses

To improve the sample handling, and reduce cost and preparation time, of peptide mapping LC-MS workflows in protein analytical research, we here investigate the possibility of replacing conventional enzymatic digestion methods with a polymer microfluidic chip based enzyme reactor. Off-stoichiometric thiol-ene is utilized as both bulk material and as a monolithic stationary phase for immobilization of the proteolytic enzyme pepsin. The digestion efficiency of the, thiol-ene based, immobilized enzyme reactor (IMER) is compared to that of a conventional, agarose packed bed, pepsin IMER column commonly used in LC-MS based protein analyses. The chip IMER is found to rival the conventional column in terms of digestion efficiency at comparable residence time and, using a 3D-printed interface, be directly interfaceable with LC-MS.

Preparation and characterization of a packed bead immobilized trypsin reactor integrated into a PDMS microfluidic chip for rapid protein digestion

This paper demonstrates the design, efficiency and applicability of a simple, inexpensive and high sample throughput microchip immobilized enzymatic reactor (IMER) for rapid protein digestion. The IMER contains conventional silica particles with covalently immobilized trypsin packed inside of a poly(dimethylsiloxane) (PDMS) microchip channel (10mm×1mm×35µm). The microchip consists of 9 different channels, enabling 9 simultaneous protein digestions. Trypsin was covalently immobilized using carbodiimide activation, the ideal trypsin/silica particle ratio (i.e. measured mass ratio before the immobilization reaction) was determined. The amount of immobilized trypsin was 10-15μg trypsin for 1mg silica particle. Migration times of CZE peptide maps showed good repeatability and reproducibility (RSD%=0.02-0.31%). The IMER maintained its activity for 2 months, in this period it was used effectively for rapid proteolysis. Four proteins (myoglobin, lysozyme, hemoglobin and albumin) in a wide size range (15-70kDa) were digested to demonstrate the applicability of the reactor. Their CZE peptide maps were compared to peptide maps obtained from standard in-solution digestion of the four proteins. The number of peptide peaks correlated well with the theoretically expected peptide number in both cases, the peak patterns of the electropherograms were similar, however, digestion with the microchip IMER requires only <10s, while in-solution digestion takes 16h. LC-MS/MS peptide mapping was also carried out, the four proteins were identified with satisfying sequence coverages (29-50%), trypsin autolysis peptides were not detected. The protein content of human serum was digested with the IMER and with in-solution digestion.

Recent Progress in Monolithic Silica Columns for High-Speed and High-Selectivity Separations

Monolithic silica columns have greater (through-pore size)/(skeleton size) ratios than particulate columns and fixed support structures in a column for chemical modification, resulting in high-efficiency columns and stationary phases. This review looks at how the size range of monolithic silica columns has been expanded, how high-efficiency monolithic silica columns have been realized, and how various methods of silica surface functionalization, leading to selective stationary phases, have been developed on monolithic silica supports, and provides information on the current status of these columns. Also discussed are the practical aspects of monolithic silica columns, including how their versatility can be improved by the preparation of small-sized structural features (sub-micron) and columns (1 mm ID or smaller) and by optimizing reaction conditions for in situ chemical modification with various restrictions, with an emphasis on recent research results for both topics.

Sample preparation strategies for targeted proteomics via proteotypic peptides in human blood using liquid chromatography tandem mass spectrometry

The simultaneous quantification of protein concentrations via proteotypic peptides in human blood by liquid chromatography coupled to quadrupole MS/MS is an important field of bioanalytical research with a high potential for routine diagnostic applications. This review summarizes currently available sample preparation procedures and trends for absolute protein quantification in blood using LC-MS/MS. It discusses approaches of transferring established qualitative protocols to a quantitative analysis regarding their reliability and reproducibility. Techniques used to enhance method sensitivity such as the depletion of high-abundant proteins or the immunoaffinity enrichment of proteins and peptides are described. Furthermore, workflows for (i) protein denaturation, (ii) disulfide bridge reduction and (iii) thiol alkylation as well as (iv) enzymatic digestion for absolute protein quantification are presented. The main focus is on the tryptic digestion as a bottleneck of protein quantification via proteotypic peptides. Conclusively, requirements for a high-throughput application are discussed.

Immobilization of lambda exonuclease onto polymer micropillar arrays for the solid-phase digestion of dsDNAs

The process of immobilizing enzymes onto solid supports for bioreactions has some compelling advantages compared to their solution-based counterpart including the facile separation of enzyme from products, elimination of enzyme autodigestion, and increased enzyme stability and activity. We report the immobilization of λ-exonuclease onto poly(methylmethacrylate) (PMMA) micropillars populated within a microfluidic device for the on-chip digestion of double-stranded DNA. Enzyme immobilization was successfully accomplished using 3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) coupling to carboxylic acid functionalized PMMA micropillars. Our results suggest that the efficiency for the catalysis of dsDNA digestion using λ-exonuclease, including its processivity and reaction rate, were higher when the enzyme was attached to a solid support compared to the free solution digestion. We obtained a clipping rate of 1.0 × 10³ nucleotides s^–1 for the digestion of λ-DNA (48.5 kbp) by λ-exonuclease. The kinetic behavior of the solid-phase reactor could be described by a fractal Michaelis–Menten model with a catalytic efficiency nearly 17% better than the homogeneous solution-phase reaction. The results from this work will have important ramifications in new single-molecule DNA sequencing strategies that employ free mononucleotide identification.

Immobilized purine nucleoside phosphorylase from Aeromonas hydrophila as an on-line enzyme reactor for biocatalytic applications

We described the development of a biochromatographic system which uses a purine nucleoside phosphorylase from Aeromonas hydrophila (AhPNP) for the evaluation of the substrate specificity on nucleoside libraries. AhPNP has been covalently immobilized on a fused silica Open Tubular Capillary (OTC) via Schiff base chemistry. The resulting bioreactor has been characterized by the determination of kinetic constants (Km and Vmax) for a natural substrate (inosine) and then assayed versus all natural purine (deoxy)ribonucleosides and a small library of 6-substituted purine ribosides. Characterization of the bioreactor has been carried out through a bidimensional chromatographic system with the sample on-line transfer from the bioreactor to the analytical column for the separation and quantification of substrate and product. Comparison with the soluble enzyme showed that the AhPNP-based bioreactor is reliable as the same ranking order, with respect to the standard activity assay, was obtained. The stability of the IMER was also assessed and the system was found to be stable up to 60 reactions.

A fluorescence turn on trypsin assay based on aqueous polyfluorene

A new method based on the electrostatic interaction of a novel anionic water soluble polymer P1 with a positively charged polypeptide Arg₆ was developed for a continuous and real time turn on assay for the enzymatic activity of trypsin under alkaline conditions with a limit of detection of 0.17 nM. This method was also able to screen the inhibitors of trypsin. P1 fluorescence intensity was significantly decreased by the positively charged Arg₆ due to the electrostatic interaction, whereas the enzymatic action recovered P1 fluorescence due to the fragmentation of Arg₆ into small positively charged fragments and these were unable to quench the P1 fluorescence. Therefore, by triggering the fluorescence intensity change, it was possible to assay the enzymatic activity. Use of water soluble conjugated polymer P1 and no labeling on the substrate enhances the utility of this method significantly.

Affinity monolith chromatography: a review of principles and recent analytical applications

Affinity monolith chromatography (AMC) is a type of liquid chromatography that uses a monolithic support and a biologically related binding agent as a stationary phase. AMC is a powerful method for the selective separation, analysis, or study of specific target compounds in a sample. This review discusses the basic principles of AMC and recent developments and applications of this method, with particular emphasis being given to work that has appeared in the last 5 years. Various materials that have been used to prepare columns for AMC are examined, including organic monoliths, silica monoliths, agarose monoliths, and cryogels. These supports have been used in AMC for formats that have ranged from traditional columns to disks, microcolumns, and capillaries. Many binding agents have also been employed in AMC, such as antibodies, enzymes, proteins, lectins, immobilized metal ions, and dyes. Some applications that have been reported with these binding agents in AMC are bioaffinity chromatography, immunoaffinity chromatography or immunoextraction, immobilized-metal-ion affinity chromatography, dye-ligand affinity chromatography, chiral separations, and biointeraction studies. Examples are presented from fields that include analytical chemistry, pharmaceutical analysis, clinical testing, and biotechnology. Current trends and possible directions in AMC are also discussed.

Flow-based systems for rapid and high-precision enzyme kinetics studies

Enzyme kinetics studies normally focus on the initial rate of enzymatic reaction. However, the manual operation of steps of the conventional enzyme kinetics method has some drawbacks. Errors can result from the imprecise time control and time necessary for manual changing the reaction cuvettes into and out of the detector. By using the automatic flow-based analytical systems, enzyme kinetics studies can be carried out at real-time initial rate avoiding the potential errors inherent in manual operation. Flow-based systems have been developed to provide rapid, low-volume, and high-precision analyses that effectively replace the many tedious and high volume requirements of conventional wet chemistry analyses. This article presents various arrangements of flow-based techniques and their potential use in future enzyme kinetics applications.

Highly efficient enzyme reactors containing trypsin and endoproteinase LysC immobilized on porous polymer monolith coupled to MS suitable for analysis of antibodies

Capillary enzymatic microreactors containing trypsin and endoproteinase LysC immobilized on a porous polymer monolith have been prepared and used for the characterization and identification of proteins such as cytochrome c, bovine serum albumin, and high-molecular weight human immunoglobulin G. The hydrophilicity of diol functionalities originating from the hydrolyzed poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith was not sufficient to avoid adsorption of hydrophobic albumin in a highly aqueous mobile phase. Therefore, this monolith was first hydrophilized via photografting of poly(ethylene glycol) methacrylate followed by photografting of a 4-vinyl-2,2-dimethylazlactone to provide the pore surface with reactive functionalities required for immobilization. This new approach reduced the undesired nonspecific adsorption of proteins and peptides and facilitated control of both the enzyme immobilization and protein digestion processes. The enzymatic reactors were coupled off-line with MALDI/TOF MS and/or on-line with ESI/TOF MS. Experimental conditions for digestion were optimized using cytochrome c and bovine serum albumin as model proteins. The optimized reactors were then integrated into a multidimensional system comprised of a monolithic capillary enzyme reactor, an in-line nanoLC separation of peptides using a poly(lauryl methacrylate-co-ethylene dimethacrylate) monolithic column, and ESI/TOF MS. With the use of this system, immunoglobulin G was digested at room temperature in 6 min to an extent similar to that achieved with soluble enzyme at 37 degrees C after 24 h.

Pressure-assisted tryptic digestion using a syringe

A simple and effective digestion method was developed using a syringe. A 3 mL syringe was used to apply a pressure of 6 atm to expedite tryptic digestion. Application of a pressure of 6 atm during digestion resulted in better digestion efficiency than digestion under atmospheric pressure. The protein peaks in the matrix-assisted laser desorption/ionization mass spectra of three model proteins (cytochrome c, horse heart myoglobin, and bovine serum albumin (BSA)) completely disappeared within 30 min at 37 degrees C under a pressure of 6 atm, with greater numbers of peptides observed in 30 min pressure-assisted digestion than in overnight atmospheric pressure digestion. This is mostly due to the miscleaved peptides. Similar sequence coverages were obtained for 30 min pressure-assisted digestion and overnight atmospheric pressure digestion of the three model proteins (92% vs. 88% for cytochrome c, 100% vs. 97% for horse heart myoglobin, and 53% vs. 53% for BSA).