J. Biochem, 1988, Vol. 104, No. 4 544-549
© 1988 Japanese Biochemical Society
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Molecular Mechanism of Iodide Transport by Thyroid Plasmalemmal Vesicles: Cooperative Sodium Activation and Asymmetrical Affinities for the Ions on the Outside and Inside of the Vesicles1
*Central Laboratory of Clinical Investigation, Miyazaki Medical College Hospital Kiyotake, Miyazaki 889-16
**Biophysics Division, Institute of Applied Electricity, Hokkaido University Kita-ku, Sapporo, Hokkaido 060
The 125I– uptake by plasmalemmal vesicles from porcine thyroid was measured by a Millipore filtration method using 2 mM C104– as a reaction stopper. Effective uptake occurred in the presence of high concentrations of extravesicular Na+ (Na+o In the presence of Na-ionophores such as monensin and nigericin, no uptake was observed and the accumulated I– was released. The initial rate of I– uptake increased with the concentration of extravesicular I– (I–o according to simple saturation kinetics and [I–o] giving a half-maximum rate of about 5 µM. The dependence of the rate on [Na+o showed cooper ativity with a Hill coefficient of 1.8, and a KNa value of 0.0064 M2, suggesting that the binding of at least 2 Na+ ions to a carrier molecule was required to transport an I– ion. Further kinetic data were consistent with a mechanism in which bindings of the ions were rapid and the Na+ binding occurred prior to the I– binding. Intravesicular Na+ inhibited the I– uptake and the inhibition constant (K1Na) was about 4mM, independently of [I–o] and [Na+o Intravesicular I– inhibited the I– uptake with an apparent K1I, value of about 100 µM. The results suggest that the differences in the Na+ - and I– -binding modes between outside and inside of the vesicles are important factors causing the I– uptake against its concentration gradient.
1This research was supported in part by a Grant-in-Aid for Encouragement of Young Scientists (62770856) from the Ministry of Education, Science and Culture of Japan (Y.N.).
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