Efficient tryptic proteolysis accelerated by laser radiation for peptide mapping in proteome analysis

Article abstract of DOI:10.1002/anie.201004152

Back to basics: Coupled with MALDI-TOF MS, laser-assisted proteolysis (see schematic illustration) enabled rapid protein digestion and peptide mapping without the need for enzyme immobilization to increase the efficiency of tryptic digestion. Protein solutions containing trypsin were digested in less than a minute upon irradiation at 808 nm with a laser.

Full text of DOI:10.1002/anie.201004152

Angewandte
Chemie
DOI: 10.1002/anie.201004152
Fast Proteolysis
Efficient Tryptic Proteolysis Accelerated by Laser Radiation for
Peptide Mapping in Proteome Analysis**
Guoping Yao, Chunhui Deng,* Xiangmin Zhang, and Pengyuan Yang
Proteomics has become one of the fastest-developing areas of
biological research.[1,2] It aims to provide a global perspective
of changes in the amount of a protein through the character-
ization of a large number of proteins.[3] The commonly used
strategy for protein identification consists of protein digestion
and subsequent peptide-mass measurements based on mass
spectrometry (MS). Because the conventional in-solution
digestion of proteins is prone to such intrinsic limitations as
prolonged digestion time, autolysis, and sample loss, the
development of novel methods for highly efficient proteolysis
is of high importance for MS-based peptide mapping.
Many efforts to increase tryptic-digestion efficiency by
immobilization of the enzyme on various substrates have
been reported.[4–17] The benefits of the use of immobilized
enzyme molecules for the characterization of proteins include
the reusability and stability of the enzyme, the higher
efficiency of the digestion of protein analytes, and the lack
of enzyme-autolysis products. In a previous study,[18] we
successfully developed magnetic carbonaceous microspheres
as a new substrate for enzyme immobilization and applied
them to fast protein digestion. Digestion was complete within
30 minutes. The efficiency of conventional in-solution pro-
teolysis could also be accelerated by microwave irradiation,
ultrasonic waves, and infrared (IR) radiation. Juan et al.[19]
demonstrated that the optimum conditions for in-gel micro-
wave-assisted tryptic digestion was treatment at 195 W for
5 minutes. In 2007, Rial-Otero et al.[20] demonstrated that
ultrasonic waves could decrease the digestion time of conven-
tional in-solution proteolysis to 1 minute. More recently,
Chen and co-workers used IR radiation as an energy source to
promote in-solution tryptic proteolysis: digestion was com-
plete within 5 minutes.[21]
attracted increasing attention.[22–26] This near-infrared
coherent light can penetrate readily through water with
little energy deposition even under high power and prolonged
irradiation. When the near-infrared laser beam is applied
under certain thresholds, organized tissue will not sustain
irreversible damage. Furthermore, if the laser power is high
enough, it may accelerate heat accumulation and raise the
tissue temperature in the process. Besides photothermal
effects, the high-energy near-IR irradiation could excite
overtone or harmonic vibrations of the chemical bonds
within the organic components of the tissue. These vibrations
might lead to more cleavage sites of proteins exposed to
trypsin and thus result in easier cleavage of peptide bonds and
better digestion efficiency. It should therefore be feasible to
employ an 808 nm laser to enhance the efficiency of conven-
tional in-solution proteolysis with little damage to the
enzyme.
In this study, protein solutions containing trypsin were
allowed to digest directly with the assistance of laser
irradiation (808 nm) both in sealed transparent Eppendorf
tubes and on the spots of a stainless steel MALDI plate. Laser
irradiation (808 nm) was also employed to enhance the
efficiency of the in-gel tryptic digestion of proteins separated
by SDS-PAGE. The laser-assisted proteolysis technique was
coupled with MALDI-TOF MS for protein digestion and
peptide identification. High digestion efficiency was observed
for both standard proteins and real protein samples. Thus, this
novel digestion method has promise for high-throughput
protein identification.
A continuous laser was used to irradiate the protein
sample perpendicularly (Figure 1). The laser used in our study
was a diode infrared laser module (CNI, Changchun, China).
It emits a wavelength of 808 nm with a power of 5 W. The
laser-beam output was coupled to a fiber-lens system, which
led to a spot diameter at the sample of 5 mm. The resulting
irradiance was 25.5 WcmÀ2. The distance between the fiber
lens and the liquid surface was fixed at 4 cm.
Laser radiation has been shown to modulate many
biological progresses. It offers great promise for lesion
treatment and selectivity of tissue destruction. In particular,
lasers emitting a wavelength of approximately 800 nm have
Three replicate experiments were carried out for the
optimal reaction time (see Figure S1 in the Supporting
Information). The average sequence coverage found upon
comparison with structures recorded in the Swiss-Prot data-
base was 42% for bovine serum albumin (BSA; Figure 2a),
89% for myoglobin (see Figure S2a in the Supporting
Information), and 83% for cytochrome c (see Figure S3a in
the Supporting Information). For comparison purposes, we
also measured the MALDI mass spectra of the digests
obtained by conventional in-solution digestion of the proteins
for 12 hours (Figure 2b; see also Figures S2b and S3b in the
Supporting Information). Detailed identification results are
listed in Tables S1, S2, and S3 of the Supporting Information.
[*] G. Yao, Prof. C. Deng, Prof. X. Zhang, Prof. P. Yang
Department of Chemistry and Institutes of Biomedical Sciences
Fudan University
Shanghai 200433 (China)
Fax: (+86)21-6564-1740
E-mail: chdeng@fudan.edu.cn
[**] This research was supported by the National Natural Science
Foundation of China (Projects: 20875017, 21075022, and
20735005), the Technological Innovation Program of Shanghai
(09JC1401100), the National Basic Research Priorities Program
(Project: 2007CB914100/3), and the Shanghai Leading Academic
Discipline Project (B109).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004152.
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peptide sequences, there
can be no significant diver-
gence in digestion selectiv-
ity between laser-assisted
in-solution digestion and
traditional digestion for
12 h (see Figure S4 in the
Supporting Information).
To investigate the
detection limit of this pro-
posed approach, we con-
ducted the laser-assisted
in-solution digestion of
cytochrome c at concentra-
tions of 100, 50, and
25 ngmLÀ1 (see Figure S5
in the Supporting Informa-
tion). Proteolysis was still
efficient when the concen-
tration of the protein was
decreased to 25 ngmLÀ1
(see Figure S5c in the Sup-
porting Information). If
the protein concentration
was less than 25 ngmLÀ1,
the mass-spectral peaks of
the tryptic peptides of
cytochrome c would be
barely detectable. Four,
six, and 11 tryptic peptides
were matched, with a cor-
responding amino acid
sequence coverage of 36,
56, and 77% for the three
protein concentrations of
25, 50, and 100 ngmLÀ1,
respectively (see Table S4
in the Supporting Informa-
tion). The results indicated
Figure 1. Schematic illustration of laser-assisted in-solution proteolysis.
that
the
proposed
approach can be used for
the fast digestion of pro-
teins at low concentrations.
We next sought to
demonstrate the feasibility
of laser-assisted on-plate
proteolysis. Following the
Figure 2. The MALDI mass spectra of digests of BSA obtained by a) laser-assisted in-solution digestion at 378C
for 45 s (power: 5W) and b) conventional in-solution digestion at 378C for 12 h. The peaks of all matched
peptides are labeled with mass values. Conditions: 1 mL trypsin (0.1 mgmLÀ1)/20 mL proteins (0.2 mgmLÀ1).
I= % intensity.
When laser (808 nm) radiation was used to accelerate in-
solution digestion, the number of matched peptides and the
sequence coverage increased from 23 to 28 and from 36 to
42% for BSA, from 12 to 14 and from 83 to 89% for
myoglobin, and from 10 to 12 and from 81 to 83% for
cytochrome c (Table 1). The results indicate that the effi-
ciency of in-solution proteolysis was substantially enhanced
by laser radiation, whereas the digestion time was signifi-
cantly reduced from 12 hours for conventional in-solution
digestion to less than 1 minute for the present laser-assisted
(808 nm) digestion. Given the overlap in the results observed
for these two different digestion methods in terms of matched
laser-assisted on-plate digestion of BSA and myoglobin,
MALDI mass spectra of the digests were obtained (see
Figure S6 in the Supporting Information). Three replicate
experiments were carried out for the optimal reaction time
(see Figure S7 in the Supporting Information), and an average
sequence coverage of 35% for BSA and 90% for myoglobin
was obtained against the Swiss-Prot database (Table 1).
The intrinsic effects of laser irradiation on tryptic
proteolysis are difficult to determine because of the photo-
thermal effects resulting from high-power laser irradiation. To
solve this problem, we chilled the reaction mixture in an ice
bath to reduce the catalytic activity that arises from thermal
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Table 1: Summary of MALDI-TOF MS results for digestion products
obtained by different proteolysis methods.
in trypsin solution at 48C for 30 minutes. Subsequent laser-
assisted in-gel digestion involved digestion of the treated gel
pieces in transparent Eppendorf tubes with the assistance of
an 808 nm laser.
Digestion
method
Protein
Digestion Sequence
Peptides
time
coverage [%] matched
Figure 4a shows the MALDI mass spectrum of the
extracted digests of 1 mg of BSA (1 mgmLÀ1 ꢀ 1 mL) from gel
pieces obtained by laser-assisted in-gel digestion for 45 s. The
identified peptide residues obtained are presented in Table S6
of the Supporting Information. A total of 24 tryptic peptides
were found to match to BSA, with a corresponding amino
acid sequence coverage of 38%. Conventional in-gel tryptic
digestion gave a comparable result but required a reaction
time of 16 hours (Figure 4b; see Table S6 in the Supporting
Information).
However, over the same irradiation time, the protein-
digestion efficacy of laser-assisted in-gel digestion was not as
good as that of laser-assisted in-solution digestion. This
phenomenon may mostly depend on the specific transmission
mechanism of near-IR light: laser light (808 nm) can better
penetrate samples with a higher water content, and conse-
quently, photons of the 808 nm laser can excite the vibration
of molecules of liquid samples more efficiently. It is therefore
reasonable to assume that a longer irradiation time (e.g. 5 min
or even longer) would result in higher efficiency of laser-
assisted in-gel proteolysis.
The suitability of laser-assisted in-gel proteolysis for the
study of complex proteins was demonstrated by digesting
HSA separated from human serum by SDS-PAGE. The
protein concentration of the sample was 60.0 mgmLÀ1
according to the modified Bradford method. The weight
percentage of HSA in the total protein content of human
serum is in the range of 53.3–70.5%.[29] The darkest protein
band on the polyacrylamide gel after SDS-PAGE of diluted
human serum (1 mgmLÀ1 ꢀ 2 mL) and Coomassie blue staining
(see Figure S11b in the Supporting Information) was excised,
digested, extracted, and finally identified by MALDI-TOF
MS (see Figure S12a in the Supporting Information). A total
of 19 peptides (see Table S7 in the Supporting Information)
were found to match to HSA with a sequence coverage of
26%, and the darkest band on the gel in Figure S11b was
identified to be Coomassie blue stained HSA. The results
were comparable to those obtained by conventional in-gel
tryptic digestion, which required a reaction time of 16 hours
(see Figure S12b and Table S7 in the Supporting Informa-
tion).
In summary, we have demonstrated that laser-assisted
proteolysis coupled with MALDI-TOF MS is a promising
strategy for efficient protein digestion and peptide mapping.
This laser-assisted approach accelerated in-solution/in-gel
digestion and on-plate proteolysis to the extent that they
required only seconds. The efficacy of protein digestion was
comparable to that observed with traditional methods.
Furthermore, this laser-assisted digestion protocol ensured
fast and efficient results even at low protein concentrations of
only 25 ngmLÀ1. The successful digestion of complex samples,
such as rat-brain extract, by laser-accelerated proteolysis
indicates the strong potential of this straightforward, fast,
efficient, and low-cost approach in high-throughput proteome
analysis.
laser-assisted
in-solution
BSA
myoglobin
cytochrome c 30 s
45 s
30 s
42
89
83
28
14
12
conventional
in-solution
BSA
myoglobin
cytochrome c 12 h
12 h
12 h
36
83
81
23
12
10
laser-assisted in BSA
an ice bath
45 s
26
15
laser-assisted
on-plate
BSA
myoglobin
5 s
5 s
35
90
20
13
laser-assisted
in-gel
BSA
HSA
45 s
45 s
38
26
24
19
conventional
in-gel
BSA
HSA
16 h
16 h
37
24
25
19
heating. MALDI mass spectra were recorded of BSA
digested under laser irradiation in an ice bath (see Figure S8
in the Supporting Information). Three replicate experiments
were carried out, and an average sequence coverage of 26%
for BSAwas obtained from the database (Table 1). This result
demonstrates that lasers can trigger significant effects on
tryptic proteolysis. The observed specific laser effect most
likely results from overtones of the vibration, stretching, and
À
À
À
À
À
bending of C H, C C, N H, C O, and O H bonds in protein
chains upon irradiation with the near-infrared laser.[27,28]
To further confirm the feasibility of laser irradiation at
808 nm for the analysis of complex protein mixtures, we
applied this method to rat-brain extract. After reduction and
alkylation, the brain extract was digested by trypsin enzyme
for only 1 minute and directly subjected to LC–MS/MS(ESI)
(the resulting total-ion chromatogram is shown in Figure S9
of the Supporting Information). After a database search
according to the SEQUEST criteria (see the Supporting
Information), 758 peptides and 134 proteins were identified
with p < 0.01 (see Table S5 in the Supporting Information).
The MS/MS spectrum corresponding to the ion at m/z 2797.34
in the precursor mass scan at 139.23 minutes (see Figure S10
in the Supporting Information) is shown in Figure 3. Most
y ions together with b ions produced from the precursor ion
matched together and resulted in the high reliability observed
for the peptide sequence SGPFGQIFRPDNFVFGQS-
GAGNNWAK. These results clearly show that this novel
digestion approach can be used for large-scale proteomic
analysis.
Having demonstrated the suitability of laser irradiation
for in-solution digestion, we next sought to demonstrate that
laser irradiation could enhance the efficiency of in-gel
proteolysis. After SDS-PAGE (see Figure S11 in the Support-
ing Information), the target protein bands were excised, cut
into small pieces, destained, dehydrated, and then rehydrated
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Communications
Figure 3. MS/MS spectrum of the peak at 2797.34 m/z in the precursor mass scan at 139.23 min of rat-brain extract after laser-assisted digestion
by trypsin for 1 min (see Figure S10 in the Supporting Information). The sequence SGPFGQIFRPDNFVFGQSGAGNNWAK could be confirmed to
a high level of certainty on the basis of b ions and y ions.
5 min with ACN (100 mL), removal of ACN). Next, the dehydrated
Experimental Section
gel cubes were rehydrated in 50 mm ammonium bicarbonate solution
(20 mL) containing trypsin (25 ngmLÀ1) and incubated at 48C for
30 min.
Healthy rat brain was provided by Zhongshan Hospital, Fudan
University. The rat-brain tissue was cut into small pieces and cleaned
with cold physiological saline solution (0.9% NaCl) to remove blood
and other contaminants. The tissue debris were then suspended in the
protein-extraction buffer (9.0m urea, 2% 3-[(3-cholamidopropyl)di-
methylammonio]-1-propanesulfonate, 50 mm dithiothreitol, 1.0 mm
phenylmethanesulfonyl fluoride). Lysis was performed by homoge-
nization on ice and aided by vortexing for 30 min. The lysate was
centrifuged at 18000g for 15 min at 48C. The supernatant was
collected, fractionated in aliquots, and stored at À808C until further
analysis. Protein concentration was measured by using the Bradford
assay with BSA as a standard (13 mgmLÀ1) for rat-brain tissue.
SDS-PAGE was performed on 1 mm thick 12% 2-amino-2-
hydroxymethylpropane-1,3-diol (Tris)–glycine polyacrylamide mini-
gels loaded with 1–4 mg of protein. Following electrophoresis, the gel
slab was stained with colloidal Coomassie blue for 4 h and then
destained in water overnight. Protein gel bands of interest were
excised, cut into small cubes (1 mm3), and then subjected to a
repeated washing and dehydration cycle (incubation for 20 min with
50 mm ammonium bicarbonate solution (200 mL) containing 50%
(v/v) acetonitrile (ACN), removal of the solution, incubation for
Laser-assisted in solution/in-gel digestion was performed by the
digestion of protein solutions containing trypsin (1:40) or trypsin-
treated gel pieces in Eppendorf tubes with the aid of laser radiation
(Figure 1). The distance between the fiber lens and the liquid surface
was fixed at 4 cm. After laser-assisted in-solution proteolysis, the
sample solution was deposited directly on a MALDI plate. After
laser-assisted in-gel proteolysis, tryptic peptides were recovered by
repeated extraction (3 ꢀ ) with 50% ACN aqueous solution
/0.1% trifluoroacetic acid (TFA; v/v, 100 mL). The peptide extracts
were lyophilized and dissolved in 50% ACN aqueous solution
/0.1% TFA (10 mL) for analysis by mass spectrometry.
Received: July 7, 2010
Published online: September 24, 2010
Keywords: lasers · mass spectrometry · proteolysis · proteomics
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Figure 4. MALDI mass spectra of the extracted digests of 1 mg of BSA bands (1 mgmLÀ1 ꢀ1 mL) from gel pieces obtained by a) laser-assisted in-gel
digestion at 378C for 45 s and b) conventional in-gel digestion at 378C for 16 h. The peaks of all matched peptides are labeled with mass values.
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