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State University of New York, Upstate Medical University, Department of Pediatrics, Syracuse, New York (D.H., A-K.S.); and Syracuse University, Department of Chemistry, Syracuse, New York (J.G., J.C.D.)
The binding of cisplatin to metallothionein (MT) was investigated at
37°C in 10 mM Tris-NO3 (pH
7.4) and 4.62 mM NaCl. The
conditions were chosen to mimic passage of clinical concentrations of
cisplatin through the cytosol. The reactions were monitored by
high-performance liquid chromatography (HPLC), atomic absorption spectroscopy,
and ultraviolet (UV) absorption spectroscopy. The UV data showed that several
reactions occur, the first of which does not affect the absorbance (no
Pt-sulfur bond formation). They also suggested that if [cisplatin] is large
compared with [MT], the rate of subsequent reaction is between first and
second order in [cisplatin] and between zeroth and first order in [MT]. HPLC
eluates with 24 < retention time (tR) < 27 min
contained undialyzable Pt, which increased with reaction time and corresponded
to Pt-thionein product. Eluates with 3 < tR < 7 min
corresponded to unbound cisplatin and allowed determination of second-order
rate constants (k), using the second-order rate equation. The
k value for cisplatin reacting with apo-MT was
0.14
M1 s1, Cd/Zn-MT
0.75 M1 s1,
Cd7-MT
0.53 M1
s1, and Zn7-MT
0.65
M1 s1. Thus,
cisplatin displaced Cd and Zn equally well. Leukocyte MT concentration was
1.0 mM, so that the kinetics of cisplatin binding to cellular MT is
pseudo-first order (pseudo-first-order rate constant,
0.63 x
103 s1; half-life,
18 min). With [cisplatin] = 10 µM, the rate of cisplatin reaction with
MT is
6.3 µmol s1
cm3. We conclude that cellular MT can trap
significant amounts of cisplatin and may efficiently contribute to cisplatin
resistance.
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