J. Biol. Macromol., 9 (1), 13-22 (2009)

ArticleUV-B irradiation-induced electron transfer between 3-hydroxykynurenine andtryptophan
Yosuke Nakazawaa, Makoto Takehanab*, Mikako Okab, Fukuko Shibuyaa,
Jun’ichi Katakawaa and Yoh Sanoaa

Faculty of Pharmaceutical Sciences, Setsunan University, 45-1, Nagaotoge-cho, Hirakata,
Osaka 573-0101, Japanb
Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
Received January 6, 2009; Accepted February 17, 2009

Kynurenine metabolites are derived from the oxidation of tryptophan (Trp) and are
involved in many diseases. Previous reports have suggested that some kynurenine metabolites
produce superoxide radicals and singlet oxygen molecules under UV illumination. We usedelectron spin resonance (ESR) spectroscopy to analyze the formation of radicals following
ultraviolet-B (UV-B) irradiation of Trp and kynurenine metabolites. As the results, strong
phenoxyl radical formation was detected following UV-B irradiation of 3-hydroxy-kynuirenine(3HK), a kynurenine metabolite. Alpha-crystallin, the major lens protein, undergoes many
post-translational modifications, including oxidation. Oxidation occurs selectively at methionine,
histidine, Trp and lysine residues and associated with the development of age-related cataracts.
We examined the effect of these amino acids, as well as tyrosine (which forms tyrosyl radicals
in vivo) and aspartic acid (which undergoes racemization following exposure to UV irradiation),on radical formation by 3HK. The combination of Trp and 3HK generated a stronger ESR signal
than did 3HK alone. The phenoxyl radical formed by 3HK was strongly induced by UV
illumination in the presence of Trp. These results indicated that 3HK could easily produce thephenoxyl radicals via electron-transfer interactions with Trp, which act as a photosensitizer
induced UV excitation. In addition, this radical promoted photo-oxidation and may enhance theability of 3HK to cause disease.
Keywords: 3-hydroxykynurenine; electron-transfer interaction; photo-oxidation* Corresponding author : Makoto Takehana

Abbreviations: UV, ultraviolet; ESR, electron spin

Tel: +81-3-5400-2663.

resonance;

Fax: +81-3-5400-2693.

E-mail: takehana-mk@pha.keio.ac.jp

3HK,3-hydroxy-kynurenine;

NFK,

N-formyl- kynurenine; Kyn, kynurenine; AA, anthranilic
acid;

3HAA,

3-hydroxy-anthranilic

acid;

Trp,

tryptophan; Met, methionine; His, histidine; Lys, lysine;
Tyr, tyrosine; Asp, aspartic acid;14

Radical related interaction

Introduction

O

OH

COOH

C

While most of the pharmacological

H2N

CH

H2N

CH

CH2

CH2

C

O

interest in tryptophan (Trp) has centered on

OHC
HN

the conversion of this amino acid to

N
H

NFK

Trp

5-hydroxytryptamine (5-HT), the majority of
Trp is metabolized via a different route (i.e.,throughtheUltraviolet-B

kynuninine(UV-B)

pathway).

and

This

H2
C

2HN

O
C

CH

process(NFK),

which

acid

3-hydroxykynurenine
3-hydroxy-anthranilic

Kyn

is

(AA),

(3HK)
acid

NH2

O

3HK

Glu

3HKG

yieldsA series of enzymatic reactions convert Kyn
anthranilic

O
C

COOH

NH2

COOH
NH2

NH2

immediately hydrolyzed to kynurenine (Kyn).
to

H2
C
CH

OH

COOH

N-formyl-kynurenine

2HN

O
C

COOH

NH2

COOH

H2
C
CH

ultraviolet-C(UV-C) irradiation induce the oxidation of
Trp.

2HN

(3HAA)

OH

AA

Scheme3HAA

1.

Schematic representation

of

the

and

biochemical pathway from Try to AA, 3HKG and[1].

3HAA. All steps are enzymatically catalyzed.

Subsequently, 3HK is glucosylated to form
3-hydroxykynurenine o-glucoside (3HKG)

lens are kynurenine metabolites (e.g., Kyn(Scheme 1). Levels of 3HK are increased inand 3HK). These molecules prevent lightthe brains of people with Huntington’s

below 400 nm from reaching the retina.

disease [2, 3], human immunodeficiency

Proteins containing 3HK are oxidized byvirus [4] and Parkinson’s disease [5]. In

phenoxyl,

addition, bladder cancer and cataracts areanion radicals [9]. It has been suggested thatinduced by 3HK [6, 7], and investigators

the photodynamic actions of kynurenine

have proposed that these diseases stem in part

metabolites contribute to the development offrom the ability of kynurenine metabolites tocataracts. One of the best-characterizedinteract with and modify other proteins, suchinteractions of kynurenine metabolites withas glutamate receptors or lens proteins.

proteins is the lens protein α-crystallin,

The optical lens is exposed to sunlighthydroperoxyl

andsuperoxide

which comprises up to 50% of the total

highlyprotein mass of the mammalian lens and thisconducive to photochemical events such asprotein is comprised from two subunits, αA-

direct photo-oxidation (i.e., a type 1 event)and αB-crystallins. The protein has been

and indirect oxidation (i.e., a type 2 event)many[8]. The lens has an efficient UV protection

modifications associated with age, including

system that acts to ameliorate the effects ofphosphorylation [10], racemization [11 - 13],

photo-oxidation. The major UV filters in thedeamination [14] and oxidation. Oxidation

every

day,

makingthistissuereportsofpost-translational

Y. Nakazawa et al.

15

Table 1. Oxidative modifications of αA- andMaterials and methods

αB-crystallins in human and bovine.

Materials

Amino acid

Modification

Referencein vivo oxidation
photooxidation
photooxidation
photooxidation[15 - 17]
[15, 17][15, 17][17]

photooxidationin vivo oxidation[15, 17][15]

αA-crystallin

L-tryptophan,
L-histidine,

Met1
His7, 97, 154
Trp9
Lys166αB-crystallin

L-lysine,

L-tyrosine

andL-aspartic acid were obtained from Wako
Pure Chemical Ind., Ltd. (Osaka, Japan).
DL-kynurenine,

Trp9
Met68

L-methionine,

anthranilicacid,

DL-3-hydroxykynurenine

and3-hydroxy-anthranilic acid were purchased
occurs selectively at methionine (Met) [15 17], histidine (His) [15, 17], Trp [15, 17] andlysine (Lys) [17] residues (Table 1). Tyrosine(Tyr) possesses an aromatic side chain thatcan directly absorb UV light and serves as atargetforatype 1andatype 2

photo-oxidation [18, 19]. In addition, aspartic
acid (Asp) undergoes racemization andisomerization in response to UV-B and UV-C
irradiation, forming D-isomers and β-aspartic
acid [12, 13]. Thus, Tyr and Asp residues
may interact via mechanisms other thanphoto-oxidation or in vivo oxidation in
response to UV irradiation.
Here, we used ESR spectroscopy toinvestigate

theformationofradicalsfollowing exposure of Trp and kynurenine
metabolites to UV-B irradiation. We also
compared the ESR spectra of 3HK alone orin mixtures with various amino acids todetermine which amino acids could modulate
the ESR spectrum of 3HK in response to
UV-B irradiation. Finally, we sought tocharacterize the electron-transfer interactions
between various amino acids and 3HK.

from Sigma Chemical Co. (MO, USA).
Sodium phosphate buffer (pH 7.2) was
prepared using analytical reagent grade
NaH2PO4 and Na2HPO4, which were bothobtained from Wako Pure Chemical Ind. Ltd.
Ultraviolet-B (UV-B) irradiationTrp,kynurenine

metabolites,

andmixtures of 3HK and amino acids weredissolved in 50 mM sodium phosphate buffer(pH 7.2) and irradiated with UV-B light (312
nm)

using

Spectroline(EB280C/J;

Spectronics co., NY, USA) at 4°C.
Electron spin resonance (ESR) spectroscopy
The ESR spectra were measured 10
minutes after UV irradiation using an X-band
ESR spectrometer (JES-TE300, JEOL Datum
co. Ltd., Tokyo, Japan) and recorded using an
ESR monitor (ES-PRIT425, Ver. 1.9, JEOL
Datum co. Ltd.), as described previously [20],with minor modifications. All measurements
were carried out at 18°C with a radio
frequency of 100 kHz, a microwave power of
5 mW, a modulation amplitude of 0.1 mT, atime constant of 1 second, a scan speed of 10
mT/ 4 minutes, and an amplitude of 4 x 1000.

Radical related interaction

Results(a) 2000

Analysis of tryptophan metabolites by ESR

Intensity

16

spectroscopy-2000
334.00We dissolved Trp and kynurenine
in 50 mM sodium phosphate buffer (pH 7.2)

0

54 kJ/m ) at 4°C. Radical formation wasmeasured 10 minutes after UV irradiation,using ESR spectroscopy. Radicals were not-2000
334.00weaker ESR signal (Fig. 1). The g-values andhyperfine coupling constants detected in thisstudy are identical to those previously

Intensity

irradiation of 3HAA produced a relatively

Magnetic Field (mT)

339.00Magnetic Field (mT)

339.00Magnetic Field (mT)

339.00Magnetic Field (mT)

339.00(d) 20000-2000
334.00(e) 2000
Intensity

3HK produced a strong ESR signal, while

339.000-2000
334.00detected following UV irradiation of Trp,
Kyn and AA. However, UV irradiation ofMagnetic Field (mT)(c) 2000
Intensity

2

Intensity

metabolites (e.g., Kyn, 3HK, AA and 3HAA)(b) 2000and subjected them to UV-B irradiation (i.e.,

1mT

0

0

-2000
334.00determined for the phenoxyl radicals of 3HK,
indicating that these molecules are the same
species (i.e., as determined with g = 2.004
and hyperfine coupling constant = 0.53 mT)[20 - 22].
To determine if the height of the ESR
peaks of the 3HK spectrum correlated with
UV dose or 3HK concentration, we irradiated
1 mM 3HK with varying doses of UV-B light(i.e., 18, 54 and 90 kJ/m2) and measured
radical formation by ESR. Radical formation
increased gradually with increasing doses of

Figure 1. ESR spectra of the Trp and kynurenine
metabolites. ESR spectra were detected following
UV-B irradiation (312 nm, 54 kJ/m2) at 4°C of (a)Trp (1 mM), (b) Kyn (1 mM), (c) 3HK (1 mM), (d)AA (1 mM) or (e) 3HAA (1 mM) in 50 mM sodium
phosphate buffer (pH 7.2). Spectra were averages of
two scans. Recording conditions: temperature, 18°C; radio frequency, 100 kHz; microwave power, 5
mW; modulation amplitude, 0.1 mT.

4).

UV (Fig. 2). When we exposed different
concentrations of 3HK (i.e., 0.25, 0.5 and 1.0mM) to a single dose of UV-B irradiation (i.e.,

Interaction between 3HK and amino acids
Previous reports have shown that the

54 kJ/m ), the heights of the ESR peaks

Met, His, Trp and Lys residues of α-crystallin

increased linearly with increasing 3HK

undergo oxidative modification (Table 1) andconcentration (Fig. 3) and were directlythat the oxidation of α-crystallin by 3HK

proportionate with 3HK concentrations (Fig.

may contribute to cataractogenesis [6]. Tyr

2

Y. Nakazawa et al.

2000
0

-2000
334.00339.00-2000
334.00Intensity
Magnetic Field (mT)

339.00-2000
334.0000Intensity

2000

Intensity

Magnetic Field (mT)

339.00Magnetic Field (mT)

339.00Magnetic Field (mT)

339.000(c) 2000-2000
334.001mT0(b) 20000-2000
334.00(c)

Magnetic Field (mT)

2000

Intensity(b)(a) 2000

1mT

Intensity

Intensity(a)17

Magnetic Field (mT)

339.00-2000
334.00Figure 2. ESR spectra of 3HK (1 mM) in 50 mM

Figure 3. ESR spectra of different concentrations of

sodium phosphate buffer (pH 7.2) exposed to different

3HK in 50 mM sodium phosphate buffer (pH 7.2)

doses of UV-B irradiation (312 nm). The doses of

exposed to UV-B irradiation (312 nm, 54 kJ/m2). The

UV-B used were (a) 18, (b) 54 and (c) 90 kJ/m2.

concentrations of 3HK were (a) 0.25 mM, (b) 0.5 mM

Radicalsand (c) 1.0 mM. Radicals were detected 10 minutes

weredetected

10 minutes

after

UV

irradiation, and spectra were averages of two scan.

after

UV irradiation, and recording conditions:

Recording conditions: temperature, 18 °C; radio

temperature, 18 °C; radio frequency, 100 kHz;

frequency, 100 kHz; microwave power, 5 mW;

microwave power, 5 mW; modulation amplitude, 0.1

modulation amplitude, 0.1 mT.

mT. Spectra were averages of two scans.

has been shown to form the tyrosyl radicals

Asp. The mixture of His and 3HK displayed

and act as an electron generator [23], and Aspa weak ESR spectrum, compared with that ofundergo in vivo racemization in response to

3HK alone. His is oxidated during the initial

UV irradiation [12, 13]. To determine which

formation of one or more endoperoxides andamino acids influenced radical formation by

via the consumption of radicals during His

3HK, we compared the ESR spectra of 3HK

ring opening [19, 23]. In contrast, the ESR

alone to the spectra of 3HK mixtures

spectra of 3HK mixtures containing Trp were

containing Met, His, Trp, Lys, Tyr or Asp instronger than that of 3HK alone (Fig. 5),

50 mM phosphate buffer (pH 7.2), in

indicating that a strong electron-transfer

response to UV irradiation (i.e., 54 kJ/m2).

interaction occurs between 3HK and Trp in

We did not detect radical formation by any

response to UV illumination.

individual amino acids in response to UV
irradiation (data not shown). The ESR
spectrum of 3HK alone was similar to that of
3HK mixtures containing Met, Lys, Tyr or

Discussion
Here, it was examined the formation of

Height of the peaks

18

Radical related interaction

2500

and 3HAA [24]. Our data reveal that 3HAA

2000

generates a weak radical (Fig. 1e). In general,1500

the presence of carboxyl group in a phenyl
ring will weaken the phenoxyl radical and

1000

render it unstable, because radicals are often
500

abstracted by reactions in which protons are
0
0

0.2

0.4

0.6

0.8

1.0

1.2

present in the carboxyl group.

3HK concentrations (mM)

Age-related cataracts are associatedFigure 4. Quantification of ESR spectra. The heights

with the radical-related oxidation of lensof the ESR peaks at each 3HK concentration were

proteins [25]. Several studies have reported

calculated by determining the difference between thethe photo-oxidation and in vivo oxidation of

maximum intensity and the minimum intensity of the

α-crystallin (Table 1). The effects of various

spectra. The height of ESR peaks and the 3HK

amino acids, including Tyr and Asp, on

concentrations were correlated.

radical formation by 3HK in response to
UV-B irradiation was examined. We foundradicals by Trp and kynurenine metabolites

that Met, Lys, Tyr and Asp had no effect onfollowing UV-B irradiation. In addition, we

the ESR spectrum of 3HK (Fig. 5a, 5d, 5einvestigated the interactions of a specificand 5f).
Takemoto et al. previously reported

metabolite, 3HK, with various amino acids todetermine which amino acids modulated

that

UV-B induced radical formation. To detect

αA-crystallin

the formation of radicals in response to theoxidation [16]. This modification occurs soon

UV irradiation with oxygen of Trp andafter synthesis of the protein, and has also

kynurenine

ESR

been reported in fetal bovine lens tissues.

spectroscopy, because the lens is continually

These results suggest that the in vivo

exposed to UV rays with oxygen in vivo.

oxidation of Met is not related to aging orStrongdetected

UV irradiation. Our results are consistentfollowing the UV-B irradiation of 3HK at 54

with those of previous studies demonstratingkJ/m2 (Fig. 1c). The extent of radical

thatformation was dependent on UV dose (Fig.

radical-mediated2) and 3HK concentration (Figs. 3 and 4) and

response to UV irradiation. Previous studies,

the ESR spectra were similar to those of 3HK,

performed in the dark, reported that 3HK

H2O2 and horseradish peroxidase (HRP) [20].

binds to Lys residues via Michael addition [6,Ichijima

that25]. In our study, Lys did not affect the ESR

radical-related reaction are readily induced

spectrum of 3HK (Fig. 5d), suggesting thatand mediated by the phenoxyl groups of 3HK

Lys residues and 3HK do not interact via a

metabolites

radical

and

using

formation

Iwata

was

speculated

the

Met

N-terminus
is

does

Met

modified

notresidueofin vivo byparticipate in thereactions

of

3HK

in

Y. Nakazawa et al.

2000(d) 2000

1mT

Intensity

Intensity(a)0(b) 2000

(e)

0

-2000
334.00339.00-2000
334.00339.00Magnetic Field (mT)

339.00Magnetic Field (mT)

339.00Magnetic Field (mT)

339.000

2000

Intensity

0

Magnetic Field (mT)

2000

-2000
334.00(f)

2000

Intensity(c)Magnetic Field (mT)

0

-2000
334.00339.00Intensity

Magnetic Field (mT)Intensity-2000
334.00

19

0

-2000
334.00Figure 5. The ESR spectra of 3HK mixtures containing various amino acids in response to UV-B irradiation(i.e., 312 nm, 54 kJ/m2). Mixtures of 1 mM 3HK and (a) 10 mM Met, (b) 10 mM His, (c) 10 mM Trp, (d) 10
mM Lys, (e) 1 mM Tyr or (f) 10 mM Asp were subjected to UV-B irradiation. Radicals were detected 10
minutes after UV irradiation at 18 °C with a radio frequency of 100 kHz, a microwave power of 5 mW and a
modulation amplitude of 0.1 mT.

radical-mediated mechanism. Tyr is known topathwaysform the tyrosyl radical in vivo, however, we

Various His residues of α-crystallin are

could not detected the radicals following UVknown to undergo photo-oxidation (Table 1).

irradiation by 3HK (Fig. 5e). This process

Uroporphyrin, as a singlet oxygen generator,

may reflect the 10 µs lifespan of thehas contributes to the photo-oxidation of His

transitional state of Tyr [26]. UV-B and UV-C

residues [18]. His residues are readily

irradiation have been reported to trigger theoxidized in the presence of peroxide species

increased racemization and isomerization ofand are known to consume peroxide species

Asp residues in α-crystallin, and suggested

during the ring opening process [19, 23]. In

that the racemization of Asp is dependent on

the current study, the peak ESR spectra for a

the rate of succinimide formation [12, 13]. It

3HK mixture containing His were lower than

could not detect radical formation by Asp

those of 3HK alone under UV illumination

following UV-B irradiation (data not shown),with oxygen (Fig. 5b). These data suggest

suggesting that the conversion to succinimide

that the unpaired electrons of the phenoxyl

is not related to radical reactions. Theseradicals

observations

thetransferred to His residues and consumed

racemization and isomerization of Asp,

during the His ring opening. In contrast, thealthough catalyzed by UV light, progress via

peakfurthersuggest

that

other

than

generated

ESR

spectraradical

by

of

3HK

3HK

reactions.

may

be

mixturesRadical related interaction

20

containing Trp were higher than those ofphenomenoncan

3HK alone (Fig. 5c). In this case, singlet

photo-oxidation

oxygen molecules were re-distributed to Trp

contribute to disease.

and

be

inducedbycansubsequently

to form tryptophanyl radicals. Radiolysis
experiments have shown that two types of
species are formed when phenoxyl radicals

Acknowledgements

react with Trp: phenoxyl radical adducts [27]The authors are grateful to Dr. Kimie

and tryptophanyl radicals in equilibrium with

Imaication radicals [28]. We did not detect the

spectrometer. The authors acknowledge theformation of radicals in the presence of Trp

support in part by Minoru Wada, Nozomu

alone under UV illumination; however, thisHoriuchi,may reflect the lifetime of tryptophanyl

Kitahigashi.

forassistance

Saki

with

Fujiwara

the

and

ESR

Miyukiradicals (i.e., 8 to 20 µs) [29]. It was
hypothesized that it may be easier for 3HK togenerate phenoxyl radicals when the ESR

References

spectra of 3HK mixtures containing Trp[1] T.W. Stone, Inhibitors of the kynurenine

become stronger, because the excited state ofpathway, European Journal of Medicinal

Trp residues (i.e. or tryptophanyl radicals) act

Chemistry 35 (2000), 179-86.

as photosensitizers to phenoxyl radical that[2] G.P. Reynolds and S.J. Pearson, Increasedallow

brain 3-hydroxykynurenine in Huntington'stheelectron-transfer

reactiontoproceed between the excited Trp molecules

disease, Lancet 2 (1989), 979-980.

and 3HK.

[3] S.J. Pearson and G.P. Reynolds, Increased

In summary, among the Trp andconcentrationsofkynurenine metabolites tested in this study,

3-hydroxykynurenine,in

3HK was the only molecule capable ofdisease, Neuroscience Letters 144 (1992),

forming phenoxyl radicals in response to

119-201.

UV-B irradiation. The peak heights of the[4] A.M. Sardar, J.E. Bell and G.P. Reynolds,

3HK ESR spectra were higher when a

Increased concentrations of the neurotoxin

mixture of Trp and 3HK was subjected to UV

3-hydroxykynurenine in the frontal cortex ofirradiation. However, Met, Lys, Tyr, and Asp

HIV-1-positive

had no effect on phenoxyl radical formation

Neurochemistry 64 (1995), 932-935.

by 3HK. We suggest that the formation of[5] T. Ogawa, W.R. Matson, M.F. Beal, R.H.

phenoxyl radicals by 3HK occurs in response

Myers, E.D. Bird, P. Milbury and S. Saso,

to daily sunlight exposure and that radical

Kynurenine

transformation occurs in the presence of Trp

Parkinson's disease, Neurology 42 (1992),

via

1702-1706.

electron-transfer

interactions.

This

brain

patients,

pathwayaneurotoxin,
Huntington's

Journalabnormalities

ofinY. Nakazawa et al.

21[6] A. Korlimbinis, P.G. Hains, R.J. Truscottalpha crystallin, Molecular Vision 10 (2004),

and J.A. Aquilina, 3-Hydroxykynurenine

814-820.

oxidizes alpha crystallin: potential role in[14] C.E. Voorter, W.A. de haard-Hoekman,cataractogenesis, Biochemistry 45 (2006),

P.J. van den Oetelaar, H. Bloemendal and1852-1860.

W.W. de Jong, Spontaneous peptide bond

[7] P.Dhir, N.J. Akhtar, T.X. Sun and J.J.

cleavage in aging alpha crystallin through aLiang,

succinimide intermediate, The Journal of

Photooxidized

products

ofrecombinant alpha-A crystallin and W9F

Biological

Chemistry

263(1998),

mutant, Photochemistry and Photobiology 69

19020-19023.

(1999), 329-335.

[15] M. McDermott, R. Chiesa, J.E. Roberts
Truscott,and J. Dillon, Photooxidation of specificPhoto-oxidation of proteins and its role inresidues in alpha crystallin polypeptides,

cataractogenesis, Journal of Photochemistry

Biochemistry 30 (1991), 8653-8660.

and Photobiology B: Biology 63 (2001),[16] L. Takemoto, J. Horwitz and T. Emmons,114-125.

Oxidation of the N-terminal methionine of[9] J. Neuzil and R. Stocker, Bilirubinlens alpha-A crystallin, Current Eye Researchattenuates radical-mediated damage to serum

11 (1992), 651-655.

albumin, FEBS Letters 331 (1993), 281-284.

[17] J. Dillon, R. Chiesa, R.H. Wang and M.

[10]

Differential

McDermott, Molecular changes during thephosphorylation of alpha-A crystallin inphotooxidation of alpha crystallin in thehuman lens of different age. Experimental

presence of uroporphyrin, Photochemistry

Eye Research 62 (1996), 499-504.

and Photobiology 57 (1993), 526-530.

[11] N. Fujii, K. Satoh, K. Harada and Y.

[18] M.J. Davies and B.C. Gilbert, Free

Ishibashi, Simultaneous stereoinversion andradicalisomerization

rearrangements

[8]

M.J.

Davies

L.J.

and

R.J.

Takemoto,

atspecific

aspartic

acid

reactions:infragmentation
aqueousandsolution,

residues in alpha-A crystallin from human

Advances in Detailed Reaction Mechanisms 1

lens, Journal of Biochemistry 116 (1994),(1991), 35-81.

663-669.

[19] A. Wright, C.L. Hawkins and M.J.

[12] N. Fujii, Y. Momose, Y. Ishibasi, T.

Davies, Singlet oxygen-mediated protein

Uemura, M. Takita and M. Takehana,oxidation: evidence for the formation ofSpecific racemization and isomerization ofreactive peroxides, Redox Report 5 (2000)

the aspartyl residue of alpha-A crystallin due

159-161.

to UV-B irradiation,[20] T. Ishii, H. Iwahashi, R. Sugata and R.

Experimental

Eye

Rresearch 65 (1997), 99-104.

Kido,

Formationof[13] N. Fujii, H. Uchida and T. Saito, The

hydroxanthommatin-derived radical in thedamaging effect of UV-C irradiation on lensoxidation of 3-hydroxykynurenine, Archives

22

Radical related interactionof Biochemistry and Biophysics 294 (1992),

19 (1987), 157-163.

616-622.

[25] A. Korlimbinis and R.J. Truscott,[21] H. Iwahashi and T. Ishii, Detection of

Identification of 3-hydroxykynurenine bound

the

of

to proteins in the human lens. A possible role

3-hydroxykynurenine

using

in age-related nuclear cataract, Biochemistryhigh-performanceliquid45 (2006), 1950-60.

oxidative

products[26] D.V. Bent and E. Hayon, Excited statechromatography-electrochemical
absorption

chemistry of aromatic amino acids andresonance

related peptides. I. Tyrosine, Journal of thespectrometry and high-performance liquid

American Chemical Society 97 (1975),

chromatography-electrochemical

2599-2606.

detection-ultraviolet
detection-electron

spin

detection-ultraviolet

absorption[27] R.C. Armstrong and A.J. Swallow,

detection-mass

JournalPulse- and gamma-radiolysis of aqueous

spectrometry,of

Chromatography, A 773 (1997), 23-31.

solutions of tryptophan, Radiation Research[22] S. Vazquez, B. Garner, M.M. Sheil and40 (1969), 563-579.

R.J. Truscott. Characterisation of the major[28] M.L. Posener, G.E. Adams, P. Wardman

autoxidationofand R.B. Cundall, Mechanism of tryptophan

physiologicaloxidation by some inorganic radical-anions: aconditions, Free Radical Research 32 (2000),

pulse radiolysis study, Journal of the

11-23.

Chemical Society, Faraday Transactions 1 72

[23] S.H. Chang, G.M. Teshima, T. Milby, B.

(1976), 2231-2239.

Gillece-Castro

Canova-Davis,[29] D.V. Bent and E. Hayon, Excited state

Metal-catalyzed photooxidation of histidine

chemistry of aromatic amino acids andinrelated peptides. III. Tryptophan, Journal of

products

3-hydroxykynurenine under

human

and

growthE.

hormone,Analytical

Biochemistry 244 (1997), 221-227.

the American Chemical Society 97 (1975),[24] H. Ichijima and S. Iwata, Changes of

2599-2606.

lens

crystallins

photosensitizedwithtryptophan metabolites, Ophthalmic ResearchCommunicated by Kanaori Kenji