Identification of α2 macroglobulin as a major serum ghrelin esterase

Article abstract of DOI:10.1002/anie.201104512

Fishing for a protein: An analogue of the appetite-stimulating hormone ghrelin containing a phosphonofluoridate moiety and a terminal alkyne functions as a probe to capture the protein α₂ macroglobulin (red; see picture). The extraction of the

Full text of DOI:10.1002/anie.201104512

DOI: 10.1002/anie.201104512
Enzymes
Identification of a2 Macroglobulin as a Major Serum Ghrelin
Esterase**
Lisa M. Eubanks, G. Neil Stowe, Sandra De Lamo Marin, Alexander V. Mayorov,
Mark S. Hixon, and Kim D. Janda*
Obesity or excessive body weight leads to a morbidity, known
collectively as metabolic syndrome, and afflicts around one
billion people worldwide.[1] Reduction in calorific input
continues to be the most effective means of treatment, with
approaches that range from simple dieting to the extreme of
bariatric surgery.[2] Regulating the desire to eat through
pharmaceutical intervention is a lofty goal with a troubled
history, mostly because of an inability to separate regulation
of satiety from the bodyꢀs fundamental reward system. The
end result, depending on the mechanism, is that potential
therapeutic agents become drugs of abuse or lead to
depression.[2b]
Ser3 with an eight-carbon fatty acid chain to generate the
mature ghrelin protein, which is 28 amino acids in length.
Ghrelin is the only known protein to contain an n-octanoyl
moiety and, importantly, this posttranslational modification is
essential for binding to and activation of GHSRla (Fig-
ure 1a).
The profound role ghrelin plays in feelings of satiety and
the fact that it is considerably upstream of the bodyꢀs reward
system has lead to intense interest in the biochemical pathway
of this hormone. Logically, therapeutic intervention in ghrelin
regulation has attracted the interest of the pharmaceutical
industry in the pursuit of anti-obesity agents.[2b,3]
Figure 1. a) Mature ghrelin (R=23 amino acids). b) Ghrelin-activity-
based probe 1. c) Labeling of serum proteins by a copper(I)-catalyzed
azide–alkyne cycloaddition reaction. G=glycine, F=phenylalanine,
L=leucine, S=serine. TCEP=tris(2-carboxyethyl)phosphine, TBTA =
tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine.
Ghrelin is an appetite-stimulating hormone secreted from
endocrine cells in the stomach. Ghrelin was first identified in
1999 by Kojima et al. as the endogenous ligand for the
growth-hormone receptor 1a (GHSR1a), a G-coupled recep-
tor.[4] It has been well established that the potent growth
hormone secretagogue plays an important metabolic role in
regulating food intake and energy homeostasis;[5] more
recently, ghrelin has been implicated in the regulation of
glucose metabolism.[6] Ghrelin is synthesized as a 117 amino
acid polypeptide (preproghrelin), which is translocated
through the endoplasmic reticulum membrane followed by
proteolytic cleavage of the N-terminal 23 amino acid signal
sequence to produce proghrelin (94 amino acids). Subsequent
posttranslational modification events include cleavage of
proghrelin after residue Arg28 and octanoylation of residue
Under fasting conditions, ghrelin production is increased
and correlates with a higher level of circulating ghrelin in
serum; conversely, serum levels of ghrelin decrease immedi-
ately after food intake. The enzyme that is responsible for the
posttranslational acylation and activation of ghrelin was
recently identified as the acyltransferase enzyme ghrelin O-
acyltransferase.[7] However, the enzymes and pathways
involved in ghrelin deacylation and proteolysis are poorly
understood.
Ghrelin has been reported to be deacylated and proteo-
lyzed into smaller peptide fragments by serum and tissue
homogenates.[8] Two enzymes have been proposed to parti-
cipate in the deacylation of ghrelin in serum, liver carboxyl-
esterase (rat serum) and butyrylcholinesterase (human
serum).[8] Additionally, acyl-protein thioesterase 1/lysophos-
pholipase I has been isolated from both rat stomach homo-
genates and bovine serum; a recombinant form of this enzyme
deacylates ghrelin in vitro.[9]
[*] Dr. L. M. Eubanks, Dr. G. N. Stowe, Dr. S. De Lamo Marin,
Dr. A. V. Mayorov, Prof. K. D. Janda
Departments of Chemistry and Immunology
The Skaggs Institute for Chemical Biology and
The Worm Institute for Research and Medicine
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla CA 92037 (USA)
E-mail: kdjanda@scripps.edu
Dr. M. S. Hixon
The above-mentioned studies relied on the identification
of potential deacylating enzymes from analytically separated
serum or homogenated fractions, followed by analysis of the
proteins present in the active samples. Our laboratory
pursued a different approach by use of a mechanism-based
probe that could irreversibly bind to ghrelin-deacylating
enzymes directly within serum. Upon conjugation of a
Department of Discovery Biology Takeda San Diego, Inc.
10410 Science Center Drive, San Diego, CA 92121 (USA)
[**] This work was supported by The Skaggs Institute for Chemical
Biology and the National Institute of Health under grant number
R01-DK092191.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104512.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10699

Communications
reporter tag to the now enzyme-bound probe, deacylating
enzymes could be identified by proteomics.
Within serum, inactivation of ghrelin is thought to occur
predominately by way of deacylation. In addition, the work of
De Vriese et al., in which the stability of ghrelin in serum was
monitored in the presence of a variety of known hydrolase
inhibitors, strongly implicated serine hydrolases as the
dominant participants in ghrelin degradation.[8] The serine
hydrolases are a large family of enzymes that represent more
than 2% of the eukaryotic proteome and share a common
chemical mechanism involving an activated serine as part of a
catalytic diad or triad. These enzymes hydrolyze ester and
amine bonds in small-molecule and protein substrates.
Carboxylesterases and cholinesterases also belong to the
serine hydrolase family.
By virtue of their catalytic mechanism, serine hydrolases
are susceptible to the phosphonofluoridate warhead when it is
incorporated within a chemical structure with sufficient
recognition elements for the target enzyme to attempt to
hydrolyze it. Thus, phosphonofluoridates have found wide use
in the design of irreversible inhibitors of serine hydrolases and
have become a valuable tool in the production of activity-
based probes for this family of enzymes.[10] By use of this
approach, we generated a “bait” ghrelin-like molecule that
contains a phosphonofluoridate warhead to capture serum-
based serine hydrolase enzymes that participate in the
deacylation of circulating ghrelin.
To design an appropriate ghrelin analogue, we noted that
the amino acid sequence of ghrelin is highly conserved among
mammals and the first ten amino acids are identical. Short
truncated peptides that contain the first four to five residues
of ghrelin are potent agonists of GHSR1a with efficiencies
similar to the full-length protein; the tetrapeptide Gly-Ser-
Ser(n-octanoyl)-Phe is considered to be the active core of
ghrelin.[11] However, while the activities of the truncated
tetra- and pentaghrelin peptides are essentially equivalent,
the binding affinity of the tetrapeptide is increased approx-
imately 16-fold by the addition of the fifth residue, Leu.[11]
The octanoyl chain of ghrelin also undergoes an important
binding interaction with GHSR1a and is accommodated in a
hydrophobic pocket of the receptor.[12] Therefore, ghrelin
probe 1 was designed as a short peptide that contains the first
five amino acids of ghrelin with a phosphonofluoridate
warhead at the Ser3 residue in place of the physiological n-
octanoyl modification (Figure 1b). A key feature of the probe
is the conservation of the hydrocarbon chain length within 1,
this factor may also be important for the binding of and
selectivity toward ghrelin deacylating enzymes. Lastly, an
alkyne group at the end of the hydrocarbon chain allows the
conjugation of an azide reporter tag by a copper(I)-catalyzed
azide–alkyne cycloaddition reaction, or “click” chemistry, for
subsequent analysis and protein identification (Figure 1c).[13]
The chemical synthesis of 1 was achieved as shown in
Scheme 1. Primary alcohol 2 was prepared from 2-heptyn-1-ol
by following the procedure of Li and OꢀDoherty,[14] and
subsequent tosylation gave 3.[15] The tosyl group of 3 was
displaced with LiBr to give the intermediate bromide, which
was, without further purification, heated with tris(trimethyl-
silyl) phosphite to give phosphonic acid 4 after hydrolysis of
Scheme 1. Synthesis of phosphonofluoridate 1. Reagents and condi-
tions: a) TsCl (1.1 equiv), NEt3 (2.1 equiv), DMAP (catalytic), CH2Cl2,
08C to RT, 16 h, 80%; b) LiBr (4 equiv), acetone, 14 h; c) P(OSiMe3)3
(1.0 equiv), 1508C, 8 h; d) H2O, RT, 4 h, 22% (2 steps); e) 0.1m TBAF,
DMF, RT, 2ꢀ15 min; f) 4 (5.5 equiv), DIC (12 equiv), DIPEA (5 equiv),
DMAP (5.5 equiv), DMF, RT, 2 days; g) DAST (4.0 equiv), CH2Cl2, RT,
2 h; h) 95% TFA, 2.5% TIPS, 2.5% H2O. DAST=(diethylamino)sulfur
trifluoride, DIC=diisopropylcarbodiimide, DIPEA=diisopropylethyl-
amine, DMAP=4-dimethylaminopyridine, DMF=N,N-dimethylform-
amide, TBAF= tetra-n-butylammonium fluoride, TFA=trifluoroacetic
acid, TIPS=triisopropylsilyl, Ts=toluene-4-sulfonyl.
the trimethylsilyl group. The TBDMS protecting group of
peptide 5, which was still attached to the resin from the solid-
phase synthesis, was selectively removed by using TBAF,
followed by Mitsunobu coupling of partially deprotected
peptide 6 with phosphonic acid 4 to give resin-attached
phosphonic acid modified peptide 7. Compound 7 was then
converted to the phosphonofluoridate with DAST, followed
by TFA-mediated cleavage from the resin to give crude
phosphonofluoridate-modified peptide 1 after precipitation
by diethyl ether. While phosphonofluoridate 1 could be
purified by preparative HPLC, all attempts to concentrate the
HPLC fractions that contained the phosphonofluoridate
product resulted in hydrolysis to the corresponding phos-
phonic acid modified peptide. As a result, crude phosphono-
fluoridate 1 was used for all labeling experiments.
For the enzyme capture phase, probe 1 was incubated with
albumin-depleted rat serum for 90 min at 378C to allow cross-
linking between the mechanism-based probe and reactive
serum enzymes. The samples were then subjected to a
cycloaddition reaction with a rhodamine–azide tag (RhN3),
and the labeled proteins within the serum sample were
subsequently separated by SDS-PAGE and visualized by
fluorescence imaging (Figure 2a).[13b] Two dominate bands
were excised and in-gel trypsin digested, and the resulting
peptide fragments were analyzed by nanoLC-MS/MS. The
results were searched against protein databases and the two
labeled proteins were putatively identified.
One of the proteins identified was rat liver carboxylester-
ase (E.C.3.1.1.1). Notably, this enzyme has been proposed
previously to play a major role in the deacylation of ghrelin in
rat serum.[8] Studies conducted with commercially purified
porcine liver carboxylesterase validated the ability of this
enzyme to catalyze the deacylation reaction.[8] The identifi-
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value of ghrelin–MCA with varying concentrations of 1; a
dose-dependent inhibition response was observed, thereby
verifying that this fluorophosphonate pentapeptide probe was
an effective mimic of the acylated ghrelin substrate. Addi-
tionally, labeling studies that involved purified human a2M
and probe 1 further validated the ability of the human protein
to react with the acylated ghrelin mimic (Figure 2b).
A number of protease inhibitors that are known to inhibit
hydrolytic enzymes in serum were evaluated.[8] The mecha-
nism-based serine hydrolase probes phenylmethylsulfonyl
fluoride (PMSF), 4-(aminophenyl)methanesulfonyl fluoride
(p-APMSF), and bis(p-nitrophenyl) phosphate (BNPP) were
evaluated as inhibitors of the deacylating catalytic activity of
a2M. Other compounds such as ethylendiaminetetraacetate
(EDTA), eserine, and NaF were not expected to inhibit the
deacylating activity of a2M, thereby providing additional
evidence that a2M is a serine hydrolase. All compounds were
tested at approximately the Km value of the substrate. Only
two compounds displayed significant inhibition at a concen-
tration of 1 mm or less after preincubation for 30 min. BNPP, a
carboxylesterase inhibitor, showed 40% inhibition at 1 mm
and 0% inhibition at 100 mm. p-APMSF, a serine protease
inhibitor, showed 100% inhibition at 50 mm and 80%
inhibition at 5 mm, thus making p-APMSF the most potent
inhibitor of the screen. p-APMSF is an irreversible inhibitor
that is mechanistically analogous to phosphonofluoridate
compounds. While it was not explicitly characterized, we can
place a lower limit on the p-APMSF/a2M specificity constant
of 470mÀ1 sÀ1 (see the Supporting Information).
Irreversible inhibitors can titrate the number of catalytic
sites present in an enzyme sample, thus making them a
reliable method for quantitating the amount of active enzyme
present in a sample.[19] The instability of ghrelin probe 1 made
its precise quantitation difficult, therefore we selected p-
APMSF as a titrant to identify the number of catalytic sites
within the a2M tetramer. The protein a2M is a homotetramer
and, in principle, each subunit could contain a functional
active site. Titration experiments conducted at a concentra-
tion of 2 mm tetramer/8 mm monomer of a2M and varying
concentrations of p-APMSF (0–10 mm) are shown in Figure 3.
Extrapolation of the velocity to the x-intercept gives an active
site concentration of approximately 6 mm. Within experimen-
tal uncertainty, there are four active sites per a2M tetramer.
Could the a2M–ghrelin esterase activity be of physiolog-
ical importance? The plasma half-life of ghrelin has been
Figure 2. Protein labeling with probe 1. a) Serum samples or b) puri-
fied human a2M were incubated with 1 and reacted with RhN3 by a
cycloaddition reaction after which the reactions were analyzed by SDS-
PAGE and visualized by in-gel fluorescence scanning. Lane 1: molec-
ular weight marker, lanes 2 and 3: duplicate labeling reactions.
cation of liver carboxylesterase in our study illustrated the
proof-of-concept and we were encouraged by this positive
result. The second protein that was identified was
a2 macroglobulin (a2M), which is a large homotetrameric
plasma glycoprotein with a molecular weight of approxi-
mately 725 kDa for the human form.[16] a2M can inhibit all
mechanistic classes of proteinases through a serpin-like
mechanism that involves proteinase-mediated proteolysis of
a “bait” region within a2M, followed by immediate and major
conformational changes of a2M, thus ultimately “trapping”
the enzyme.[17] This proteinase-induced conformational
change then triggers the exposure of binding sites for proteins
such as cytokines, growth factors, and hormones.[17] Addition-
ally, the hidden receptor-binding domain of a2M is revealed,
thus leading to receptor-mediated endocytosis and rapid
clearance of the complex from circulation.[17] In general, a2M
plays an important role in the regulation of numerous
biological processes.
The capture of a2M from rat serum by probe 1 (Figure 2a)
suggests that a2M possesses a ghrelin recognition site that
contains an appropriately positioned reactive serine residue.
However, to the best of our knowledge such hydrolytic
activity has never been identified for a2M. To further
delineate this previously undocumented esterolytic activity,
we employed our previously reported HPLC-based ghrelin
hydrolase assay.[18] The substrate for this assay is a full-length
acylated ghrelin that contains the fluorophore 7-methoxy-
coumarin-4-acetic acid (MCA) at the C terminus of the
protein. Solutions containing purified human a2M (2 mm) and
low micromolar concentrations of the ghrelin–MCA substrate
(ca. 3–15 mm) demonstrated that a2M can indeed catalyze the
deacylation of ghrelin. To further characterize this reaction,
steady-state kinetic studies were conducted with a2M and
varying concentrations of ghrelin–MCA. Typical Michaelis–
Menten kinetics were observed with a2M, which displayed a
Michaelis constant (Km) of (24 Æ 3) mm and a rate constant
(kcat) of (2.3 ꢁ 10À2 Æ 0.1 ꢁ 10À2) minÀ1. Probe 1, which was
used to initially isolate the a2M protein from the labeling
studies, was further investigated as an inhibitor of this
reaction. Studies were conducted at approximately the Km
Figure 3. Titration studies with the inhibitor p-APMSF and a2M.
v=velocity, v0 =velocity at [p-APMSF]=0.
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Communications
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estimated to be 240 min, thus affording a first-order rate
constant of 0.003 minÀ1. Therefore, the physiological concen-
tration of ghrelin must be low relative to the Km values of any
preferred substrates for reported esterases.[8] As such, each
esterase is operating under substrate limiting (first-order)
conditions. The overall half-life of ghrelin is then governed by
the sum of the concentrations of each participating enzyme
multiplied by its (kcat/Km)ghrelin. The concentration of plasma
a2M has been reported in the range of 0.25 gLÀ1 to 2 gLÀ1.
Taking a midrange value of 1 gLÀ1 gives a plasma concen-
tration of 1.4 mm, which when coupled with the above-
mentioned ghrelin specificity constant gives a plasma first-
order rate constant of 0.0014 minÀ1, which accounts for half of
the total rate. This calculation thus places a2M as an
important participant in ghrelin metabolism.
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Mechanism-based probes offer an objective approach to
the identification of physiologically relevant enzyme activ-
ities. The method confirmed the alternative approach of
De Vriese et al. by capturing carboxylesterase.[8] On the other
hand, the capture of a2M by ghrelin probe 1 and the
subsequent discovery of ghrelin esterase activity of a2M
were unanticipated and unprecedented, as there are no prior
reports of a2M to possess esterase activity. The studies
reported here constitute one step toward the goal of better
understanding nutrient–hormone interactions that contribute
to metabolic disease states.
Received: June 30, 2011
Published online: September 16, 2011
[16] L. Sottrup-Jensen, T. M. Stepanik, T. Kristensen, D. M. Wierz-
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Keywords: alkynes · azides · click chemistry · enzymes · peptides
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