Identification of the main intermediate precursor of L-ergothioneine biosynthesis in human biological specimens

Article abstract of DOI:10.3390/molecules21101298

A capillary electrophoresis coupled to tandem mass spectrometry (CE-MS/MS) has been used to make a qualitative determination of hercynine - the main precursor of L-ergothioneine biosynthesis - in some key human biological specimens, such as urine, whole blood, plasma, and saliva. From semiquantitative analysis results, the highest concentrations of hercynine were detected in saliva and whole blood, whereas much lower concentrations were measured in urine and plasma. Whole blood was the biological matrix with the highest concentration of L-ergothioneine followed by plasma, saliva, and urine. The antioxidant effects attributed to L-ergothioneine, along with its peculiar antioxidant mechanism, offer a possible explanation for the presence of the hercynine, as well as its concentration, in the considered biological matrices.

Full text of DOI:10.3390/molecules21101298

molecules
Article
Identification of the Main Intermediate Precursor of
L-Ergothioneine Biosynthesis in Human
Biological Specimens
1
,
2
1
2
2
Salvatore Sotgia *, Arduino A. Mangoni , Mauro Forteschi , Rhys B. Murphy , David Elliot ,
1
1,3
1,4
and Angelo Zinellu 1
Elisabetta Sotgiu , Gianfranco Pintus , Ciriaco Carru
1
Department of Biomedical Sciences, School of Medicine, University of Sassari, Sassari 07100, Italy;
mforteschi@gmail.com (M.F.); eli.sotgiu@alice.it (E.S.); gpintus@uniss.it (G.P.); carru@uniss.it (C.C.);
azinellu@uniss.it (A.Z.)
Department of Clinical Pharmacology, School of Medicine, Flinders University and Flinders Medical Centre,
Adelaide SA 5042, Australia; arduino.mangoni@flinders.edu.au (A.A.M.);
2
rhys.murphy@flinders.edu.au (R.B.M.); david.elliot@flinders.edu.au (D.E.)
Department of Biomedical Sciences, College of Health Sciences, Qatar University, Doha 2713, Qatar
Quality Control Unit, University Hospital Sassari (AOU), Sassari 07100, Italy
Correspondence: ssotgia@uniss.it; Tel.: +39-079-229-775; Fax: +39-079-228-120
3
4
*
Academic Editor: Derek J. McPhee
Received: 18 August 2016; Accepted: 23 September 2016; Published: 28 September 2016
Abstract: A capillary electrophoresis coupled to tandem mass spectrometry (CE–MS/MS) has been
used to make a qualitative determination of hercynine—the main precursor of L-ergothioneine
biosynthesis—in some key human biological specimens, such as urine, whole blood, plasma, and
saliva. From semiquantitative analysis results, the highest concentrations of hercynine were detected
in saliva and whole blood, whereas much lower concentrations were measured in urine and plasma.
Whole blood was the biological matrix with the highest concentration of L-ergothioneine followed
by plasma, saliva, and urine. The antioxidant effects attributed to L-ergothioneine, along with its
peculiar antioxidant mechanism, offer a possible explanation for the presence of the hercynine, as well
as its concentration, in the considered biological matrices.
Keywords: aminothione; capillary electrophoresis; mass spectrometry; quaternary ammonium;
dismutation
1
. Introduction
L-ergothioneine (ERT; 2-mercaptohistidine trimethylbetaine) is an unusual hydrophilic
low-molecular-weight thiol biosynthesized exclusively by some fungi, including edible mushrooms,
and bacteria such as mycobacteria and cyanobacteria [ ]. In mammals, ERT is acquired solely
by dietary sources [ ]. Through a specific organic protein transporter (ETT), ERT accumulates in
mitochondria, cells, and tissues normally exposed to oxidative stress and involved in the inflammatory
response process [ ]. Unexpected high levels of ERT in red blood cells has been observed in some
autoimmune disorders [ ], such as rheumatoid arthritis and Crohn’s disease, in pre-eclamptic
women [ ] and, recently, in amniotic fluid of pregnant sheep after transfer of vitrified/thawed in vitro
produced embryos [ ]. Similar to glutathione (GSH), several observations suggest that ERT might
act as antioxidant and scavenger of oxidizing species [ ]. The antioxidant effect of ERT is generally
described as part of a cycle where, following oxidation to disulphide or to mixed disulphide, ERT
is regenerated by disulphide reduction, as is the case for other alkylthiols [10 11]. However, due
1–3
1,4
1,3
5,6
7
8
9
,
to the tautomeric equilibrium of ERT between thiol and thione forms, with the latter predominant
at physiological pH [12], ERT shows a peculiar stability and reactivity compared to other naturally
Molecules 2016, 21, 1298; doi:10.3390/molecules21101298
www.mdpi.com/journal/molecules

Molecules 2016, 21, 1298
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occurring alkylthiols [13]. Consequently, the mechanism through which ERT performs its antioxidant
effect differs from that of alkylthiols. As outlined in the past by Melville [14] and recently demonstrated
by Servillo et al. [15], ERT can indeed oxidize to disulphide (ESSE). However, because of ESSE
instability at physiological pH—and contrary to other disulphides—ESSE undergoes a progressive
decomposition by disproportionation, rather than a reductive reconversion to ERT [15,16]. As a result,
without reducing agents, ESSE can both generate hercynine (ERY) and regenerate ERT [15]. ERY,
a betaine derived from histidine (see Figure 1), is the biological precursor of ERT [17]. Although ERY
has been found in association with ERT in some organisms and biological matrix such as mushrooms,
king crab, erythrocytes of cattle, and seminal fluid of boar [14], its distribution in living organisms is
virtually unknown [18].
Figure 1. Chemical structures of analytes and internal standard.
Similar to ERT, mammals cannot synthetize ERY. Therefore, the presence of ERY in the body might
be ascribed to ERT oxidation. Notably, ETT shows lower affinity for ERY vs. ERT [19]. Therefore, ERY
is more easily excreted by cells whereas ERT is avidly accumulated [15,20]. Thus, the determination
of ERY in biological fluids could provide significant advantages over ERT in detecting oxidative
stress states involving ERT. To our knowledge, however, no studies have specifically determined the
occurrence of this imidazole derivative in human biological specimens. For this purpose, considering
the lack of published analytical methods for its determination, the development of an ad hoc assay
is needed. In this context, ERY might potentially show the same shortcomings of other betaine-like
compounds. The quaternary moiety confers, in fact, a polar character to ERY. This might prevent
a good chromatographic separation on reversed-phase columns, without the use of ion-pairing
reagents as additives. In addition, the optical features of ERY, principally due to the imidazole
ring, might not be enough to measure relatively low concentrations. It may therefore be necessary
to develop a derivatization reaction to improve the limit of detection and the limit of quantification.
Alternatively, field-amplified sample injection (FASI) capillary electrophoresis could overcome both
the chromatographic and the sensitivity issues. However, it must also be considered that ERY is not

Molecules 2016, 21, 1298
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commercially available, hence the need for in-house synthesis. In this context, the specificity and
sensitivity of mass spectrometry technology provides undeniable advantages over other analytical
options. Using this approach, synthesized ERY can be detected without an extensive purification step.
In the same manner, ERY can also be identified at high sensitivity in the samples. Thus, following this
strategy, we used a capillary electrophoresis coupled to tandem mass spectrometry (CE–MS/MS) to
make a qualitative determination of ERY in key human biological specimens, such as urine, whole
blood, plasma, and saliva.
2
. Results and Discussion
Both ERY and ERT were detectable in all the human samples examined (Table 1).
Table 1. Concentrations of the analytes in the human biological samples. Values are the mean of three
replicates (average of nine readings per type).
Samples
ERT (µmol/L ± SD)
ERY (µmol/L ± SD)
Whole Blood (N = 3)
Plasma (N = 3)
Urine (N = 3)
66 ± 2.2
1.1 ± 0.3
0.2 ± 0.2
0.6 ± 0.2
1.3 ± 0.1
0.2 ± 0.1
0.5 ± 0.3
2.2 ± 0.5
Saliva (N = 3)
Semiquantitative analysis revealed that the highest concentrations of ERY were detected in
saliva and whole blood, whereas much lower concentrations were measured in urine and plasma.
As expected, the highest concentration of ERT was detected in whole blood, with lower concentrations
in plasma, saliva, and urine. The relatively high concentrations of ERT in whole blood vs. other
body fluids, primarily due to accumulation in red blood cells (RBCs) [21], further supports its almost
exclusive intracellular distribution and poor tissue turnover [12]. ERT stems from erythroblasts that,
like other myeloid precursor cells, strongly express the ETT transporter [13]. Thus, ERT is high in the
early life of RBCs and, because of the inability of mature RBCs to take up additional ERT, declines as the
age of erythrocytes increases. Under normal conditions, however, ERT drops only very slowly over the
entire erythrocyte life span [13
it might serve as a powerful catalytic scavenger of oxidizing species that are not free radicals [13
,
14]. Although the physiological role of ERT is not yet fully elucidated,
22].
,
In particular, as observed in in vitro experiments, ERT in erythrocytes reduces ferryl hemoglobin
thus avoiding its detrimental effect on promoting autocatalytic oxidation of native hemoglobin to
methemoglobin [13,23]. The observation of the synchronized uptake of ERT and iron by means,
respectively, of the ETT and the transferrin receptor CD71 [13]—both heavily expressed in nucleated
erythrocyte progenitor cells—provides further support for this role. Regardless of the biological role of
ERT, the unstable ESSE appears to be the main product of its oxidation. Dismutation of ESSE, in turn,
leads both to ERY production and to reduction back to ERT [15]. In particular, from 2 mol of ESSE, 3 mol
of ERT and 1 mol of ERY are formed [15]. The concentrations of the latter in whole blood, therefore,
could be a marker of the redox activity of ERT. Thus, according to the semiquantitative analysis of ERY
(
Table 1) and to the stoichiometry of redox reaction [15], the indirect evaluation of the ESSE shows
whole blood concentrations of about 2.6 mol/L. In turn, ERT concentrations from the hydrolytic
breakdown of ESSE were approximately 4 µmol/L, while the ERT involved in the redox process was
about 8% (~5.2 mol/L) of its total amount in the RBCs. Although data in literature are not available
µ
µ
for a comparison, these small concentrations are consistent with physiological conditions, as expected
in the apparently healthy subjects enrolled in this study. Baseline conditions can also explain the
relatively low concentrations of ERY detected in plasma, which likely represent the balance between
cellular efflux and excretion in urine. With a whole blood-to-plasma ratio of 9, our data suggest that
the efflux of ERY from RBCs occurs slowly. It is difficult to establish whether this unexpected finding
can be generalized for other types of cells. If it was, it could be explained by the interaction of ERY with
ETT. Although the ETT-mediated transport of ERY is 25-fold less than ERT [19], it might still be enough

Molecules 2016, 21, 1298
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to reduce cellular efflux. However, as ETT is not functional in mature RBCs [13
,14], this hypothesis
is less plausible for this cell type. It has been speculated that ERY is an unstable metabolite with a
short half-life, decomposing spontaneously into histidine and histamine [24]. Therefore, ERY plasma
concentrations may be affected by this metabolic route. The urine-to-plasma ratio of 3, instead, would
support the hypothesis of renal clearance. Unlike ERY, the urine-to-plasma ratio of ERT was 0.15. This
is not surprising considering that ETT is abundantly expressed in the kidneys [13], where presumably
it allows the reabsorption of ERT. More difficult to interpret are the data on the ERT and ERY content
in saliva. Unlike other compartments, such as RBCs or the kidney, salivary glands do not express
ETT [12
,
13]. Therefore, ERY and ERT are likely to be generated in neighboring organs and tissues
13 19]. High
expressing ETT, such as the trachea, lungs and, to a lesser extent, the esophagus [12
,
,
expression of ETT, however, also occurs in skin fibroblast and keratinocytes [25]. Therefore, it is also
possible that similar cells of the oral mucosa express ETT. In this scenario, ERT and ERY in saliva could
derive from the typical exfoliative processes involving the epithelium of the oral mucosa, or from
the presence of traces of blood. Other authors have previously ruled out the potential role of the
microbiota, at least for ERT, in animals [26].
3
. Material and Methods
3
.1. Chemicals
ERT and ammonia (25%) were purchased from Vinci-Biochem (Florence, Italy) and Merck
(
Darmstadt, Germany), respectively. Methanol (HPLC grade), L-2-thiol-histidine (THI), ammonium
acetate, iron(III) chloride hexahydrate (FeCl3 6H O), and formic acid were obtained from Sigma
Aldrich Italy (Milan, Italy). All buffer solutions were filtered through a disposable 0.22
filter purchased from Merck Millipore Italy (Milan, Italy). Disposable ultrafiltration devices, Vivaspin
00 Micro Concentrators, were obtained from Sigma Aldrich Italy. High-purity water, obtained from a
Millipore Milli-Q system, was used throughout the experiments (Merck Millipore Italy).
·
2
µm membrane
5
3
.2. Collection and Processing of Biological Samples
Twelve human biological samples from three healthy volunteers with no medical history—three
for each type (whole blood, plasma, urine, saliva)—were processed for the qualitative analysis by
CE–MS/MS. Prior to the analysis, all samples were spiked with a 65 mol/L THI solution, used
as structural analogues internal standard (IS), then filtered on Vivaspin 500 Micro Concentrators to
remove proteins and molecules up to 10 kDa. For this purpose, 500 L of each sample were added to
L of internal standards and mixed thoroughly by vigorous vortex-mixing. The samples were then
filtered by centrifugation of the filter devices at 21,380 g for 30 min at room temperature. By contrast,
prior to filtration, the whole blood samples were lysed by hypotonic shock with cold water in a
sample/water ratio of 1:5. Then, after vortex-mixing, 500 L of supernatant was processed as above.
µ
µ
10 µ
×
µ
All subjects gave their informed consent for inclusion before they participated in the study. The study
was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the
◦
Ethics Committee of Azienda Asl n 1 di Sassari (2262/CE).
3
.3. Hercynine Synthesis
Because of the commercial unavailability of ERY, to confirm its identification in the biological
samples, ERY was prepared from ERT by oxidation with ferric chloride as described by Barger and
Ewin [27], with some adjustment. Briefly, 5 mg of ERT was mixed with 20 mg of FeCl3 6H O dissolved
·
2
in 1 mL of water. The mixture was initially heated for 30 min in a thermoblock heater, and then
it was cooled to room temperature. Without further purification, the structure was confirmed by
+
CE–MS/MS, and a peak at m/z 198.1 was considered consistent with the parent ion peak [M + H] of
ERY [18]. The structure was further ascertained by MS/MS experiments showing that, in agreement
with previous reports, the main fragment ions were at m/z 60.2, 95.1, and 154.1 [18] (see Figure 2).

Molecules 2016, 21, 1298
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Figure 2. MS/MS pattern of synthesized hercynine (ERY).
3
.4. Semiquantitative Analysis
Mass detection was accomplished in positive ion mode by multiple reaction monitoring (MRM)
of the precursor–product ion transitions, m/z 198.1 95.2, 230 127.2, 188.1 142.2 for ERY, ERT, and
→
→
→
THI, respectively. Determination of appropriate MRM transitions and the optimization of parameters,
such as fragmentor voltage or collision energy, was conducted by MassHunter Optimizer Software
(
Agilent Technologies Italy, Milan, Italy). For this purpose, a background electrolyte (BGE) containing
a different analyte each time was used to get a constant concentration of compound in the electrospray.
Since ERY was used as synthesized without further purification, a semiquantification was performed
by comparing the area of a known amount of IS spiked to the samples vs area of the ERY or ERT in the
samples. All of the analyses were performed in triplicate.
3
.5. CE Equipment and Electrophoretic Conditions
CE MS/MS experiments were performed by using an Agilent 7100 Capillary Electrophoresis
System (CE) fitted with a built-in diode-array detector, an autosampler, an air pressure pump, and a
0 kV power supply with a current range up to 300 A. The CE system was coupled via the Agilent
−
3
µ
coaxial sheath-liquid sprayer interface with an Agilent MSD mass spectrometer 6460 Series equipped
with a triple-stage quadrupole analyzer operated with the Agilent G1948B electrospray ionization
(
ESI) source. An Agilent 1200 series isocratic LC pump was used for the sheath-liquid delivery to the
sprayer. CE system operation, MS/MS control, data acquisition, and analysis were run by the Agilent
MassHunter Workstation Software (all Agilent Technologies Italy). Electrophoresis was performed on
an uncoated fused-silica capillary (Beckman Coulter Italy, Milan, Italy). The total/effective capillary
length (to the MS inlet) was 100 cm (inner diameter 75
µ
m) and injection volume was 11.26 nL
◦
(50 mbar × 3 s) for an injection plug length of 4.38 mm. The separation was carried out at 20 C
and 30 kV (10 µA) at normal polarity with an overpressure of 0.1 psi (35 mbar) from inlet to outlet.
The BGE was a 5 mmol/L ammonium acetate buffer adjusted with formic acid at pH 6. After each run,
the capillary was equilibrated with a 1:1 mixture of ammonia/H O for 1 min, H O for 1 min, then
2
2
with BGE for 1 min. To provide a stable electrical connection between the tip of the capillary and the
ground, a sheath-liquid consisting of a 1:1 mixture of 5 mmol/L ammonium acetate/methanol was
delivered isocratically at a flow rate of 6 µL/min to the sprayer through the 1:100 flow splitter of the
LC pump. The capillary voltage in the mass spectrometer was set at 4 kV, dry nitrogen gas was heated
◦
to 300 C and delivered at a flow rate of 10 L/min, while the nitrogen nebulizing gas (N ) pressure
2
was 10 psi.

Molecules 2016, 21, 1298
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4
. Conclusions
This is one of the few reports on the identification of ERY in biological samples. Since no ERY
standard was commercially available, to overcome an expensive time-consuming purification step,
a capillary electrophoresis coupled to tandem mass spectrometry was used to detect ERY by the known
exact m/z value of 198.1 and its fragmentation pattern at m/z 60.2, 95.1, and 154.1. While aware of its
analytical accuracy limitations, this strategy was used to perform a semiquantitative analysis of ERY
and ERT by using THI as a structural analogue internal standard. Both ERT and ERY were detectable
in all the studied samples, at concentrations consistent with physiological conditions. Although at
this stage of the study an alimentary intake of ERY cannot be excluded with certainty, the antioxidant
effects attributed to ERT offer a possible explanation for the presence of the ERY and ERT, as well as
their concentrations, in the specimens considered in this study. The relatively high concentrations of
analytes in the saliva suggest that this easily collectable biological specimen might be used to further
investigate the biological and pathophysiological roles of ERY and ERT, as well as the potential use of
ERY as a marker of the redox activities involving ERT.
During the review process of this work, Cheah et al. [28] reported nanomolar levels of ERY in
whole blood, plasma, and urine. No investigation was performed on the concentrations in saliva.
The semiquantitative approach used in this work can account for concentrations higher than those
reported by the study of Cheah et al. which, on the whole, strengthens our conclusions.
Acknowledgments: Funding was provided by the “Fondazione Banco di Sardegna”, Sassari, Italy, and by the
“
Ministero dell’Università e della Ricerca”, Italy. The funding sources had no influence on the design, execution,
data collection, analysis, drafting of this report, or decision to submit this manuscript. This manuscript has been
drafted by the authors, no external editorial support was used. Professor Arduino A. Mangoni contributed to this
study during a Visiting Professorship at the University of Sassari.
Author Contributions: Conceived and designed the experiments: Salvatore Sotgia, Angelo Zinellu, Ciriaco Carru
and Arduino A. Mangoni. Performed the experiments: Mauro Forteschi, Rhys B. Murphy, David Elliot,
Elisabetta Sotgiu and Gianfranco Pintus. Analyzed data: Salvatore Sotgia, Angelo Zinellu, Ciriaco Carru,
Arduino A. Mangoni, Mauro Forteschi, Rhys B. Murphy, David Elliot, Elisabetta Sotgiu and Gianfranco Pintus.
Wrote the paper: Salvatore Sotgia. All authors read and approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).