432 J. Phys. Chem. A, Vol. 102, No. 2, 1998
Das
Conclusions
These results indicate that the oxidation of ITyOH follows
the trend for phenolic compounds; formation of the phenoxyl
radical followed by its dimerization that remain unaffected in
•
the presence of O2 or excess parent compound. For the OH
radical, although the rate constant for the reaction is similar for
tyrosine and 3,5-diiodotyrosine, the transient yields suggest
different pathways. It appears that, with each additional iodine
substitution, the extent of formation of the cyclohexadienyl
radical is increasingly disfavored, and instead, simultaneous loss
of a molecule of water is favored. Thus, going from tyrosine
to 3-iodotyrosine and 3,5-diiodotyrosine, the extent of formation
•
of the OH adduct decreases from >50%13 to ≈30% and
≈15%,20 respectively. This decrease may result from addition
of •OH onto the iodine atoms, as is known to occur in aliphatic
iodine compounds,23 followed by rapid loss of water to give
Figure 9. Absorption spectrum of ITyOH2• transient 8 µs after pulse;
(b) pH 1.5 with HClO4 and (O) pH 5 with 1 M KH2PO4, absorbed
dose 20 Gy. Matrix: 0.2 M t-BuOH, N2 saturated, [ITyOH] ) 0.8 mM.
•
the phenoxyl radical. The reaction of OH with iodobenzene
has been shown to yield a radical cation in acidic solutions.35
If such a species is formed from ITyOH, it is likely to lose the
-OH proton rapidly and form the phenoxyl radical. The
reduction potential value of ITyOH at physiological pH also
suggests cellular oxidation to be fast and irreversible, resulting
in loss of active ITyOH under oxidative stress.
•
•
Inset: variation of (1/τ1/2
)
for ITyOH2 decay vs [ITyOH2 ]max
first
(proportional to the absorbed dose).
of the one-electron reduction potential of the tyrosyl phenoxyl
radical as compared to that of the 3-iodotyrosyl phenoxyl radical
at pH > 11 supports the electron-transfer mechanism in
Scheme 1.
Reduction of ITyOH, although very rapid with the primary
reducing radicals, is quite slow with secondary reducing radicals.
Since, in living cells, the primary reducing radicals are efficiently
scavenged by other chemical species present in the surrounding
(e.g., omnipresent O2), the chance of survival for ITyOH under
Reaction with H•. Near pH 5, the H• radical was generated
in deoxygenated solutions containing 0.2 M tert-butyl alcohol
and 1 M KH2PO4.34 Below pH 2, H•reactions were studied in
deoxygenated solutions; G(H•) ) 0.34 in both cases. Transient
spectra obtained from H• reactions are presented in Figure 9.
The spectral and kinetic parameters are listed in Table 3. The
rate constants for the reaction of H• with ITyOH were measured
(i) directly from the formation kinetics at 360 nm and also from
(ii) competition kinetics with respect to methylviologen and
thionine by following the kinetics of the formation of the
appropriate transients arising out of these solutes.7,10 The rate
constant values indicate that H• reaction with ITyOH is
independent of pH. From the reported reactions of H• with
tyrosine17 and 3,5-diiodotyrosine31 and the close matching of
the transient spectra at both pH 1.5 and 5, formation of the
-
a reductive stress is high. For eaq reactions, although the k
values are of similar order of magnitude for 3-iodo- and 3,5-
diiodotyrosines, only in the case of 3-iodotyrosine is the
semioxidized transient formed following reduction (Scheme 1).
Since the initial deiodination step occurs with both these species,
the subsequent reactions of the hydroxy-iodo-phenyl radical
anion from 3,5-diiodotyrosine are also expected to produce the
ITyO• as the secondary transient (following similar reactions
as in Scheme 1). Probably, due to a small difference between
reduction potentials of the respective phenoxyl radicals (ITyO•
and I2TyO•),20 further oxidation of 3,5-diiodotyrosine by ITyO•
was not observed earlier.31 Thus, favorable energetics in
aqueous medium makes this mode of reaction unique for
3-iodotyrosine between the iodine-substituted tyrosines. Mim-
icking a situation where both 3-iodo- and 3,5-diiodotyrosines
are present in the same matrix, as reported during hormone
synthesis in the thyroid cells, eaq- reactions may show interesting
results.
•
H-adduct, i.e., ITyOH2 , is indicated. In γ-radiolysis experi-
ments, neither I- nor NH3 was detected as end products after
radiolysis. The second-order transient decay characteristics
(Figure 9 inset) indicated that it does not react with ITyOH,
and instead a radical-radical route is favored. The possible
•
reactions are the dimerization and the reaction between ITyOH2
and the â-hydroxy radical derived from tert-butyl alcohol used
to scavenge the OH radicals. The effect of O2 on ITyOH2
could not be quantified in this study.
Acknowledgment. I thank Dr. P. Neta for reviewing the
manuscript and offering many helpful comments and sugges-
tions. I fondly recall many lively discussions with my colleagues
Dr. Sambhu Nath Guha and Dr. Devidas Basappa Naik at BARC
Chemistry group and thank them for many invaluable sugges-
tions during the course of this study. I also thank Mr.Vijendra
Rao for the technical support at the LINAC facility.
•
•
Reactions with Other Reducing Radicals. These include
the superoxide anion, O2- (an important intermediate in living
systems formed mainly by enzymatic processes),16 generated
in O2-saturated 0.1 M HCO2- solution at pH > 6; the carboxyl
radical anion (CO2-), produced from N2O-saturated 0.1 M
-
HCO2 solution; and the 1-hydroxy-1-methylethyl radical (or
References and Notes
its anion) from N2O-saturated 1.0 M isopropyl alcohol. ITyOH
-
(1) Address for correspondence in 1997-1998: Chemistry 222 #A-
261, Physical and Chemical Properties Division, NIST, Gaithersburg, MD
20899. Fax: (301)-975-3672.
(2) West, E. S.; Todd, W. R.; Mason, H. S; van Bruggen. J. T. Textbook
of Biochemistry, 4th ed.; The Macmillan Company: London, 1966; p 271.
(3) Eberhardt, N. L.; Apriletti, J. M.; Baxter, J. D. In Biochemical
Action of Hormones; Litwack, G., Ed.; Academic Press: New York, 1980;
Vol. VII, Chapter 9.
was found to be unreactive toward O2 radical. However, its
reduction via the deiodination route was confirmed in γ-radi-
olysis studies with the latter two radicals. From the yields of
I- and assuming competition between the reduction process and
the radical-radical reaction, the rate constants with both the
radicals were estimated to be <106 M-1 s-1. Ammonia was
not detected as an end product, indicating the absence of the
deamination reaction.
(4) Oppenheimer, J. H.; Samuels, H. H. Molecular Basis of Thyroid
Hormone Action; Academic Press: New York, 1983.