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 vesicles.
The 125I- uptake by plasmalemmal vesicles from porcine thyroid was measured by a Millipore filtration method using 2 mM ClO4- 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 microM. The dependence of the rate on [Na+o] showed cooperativity 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 (KiNa) was about 4 mM, independently of [I-o] and [Na+o]. Intravesicular I- inhibited the I- uptake with an apparent KiI value of about 100 microM. 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.
Nakamura
Y
Y
Central Laboratory of Clinical Investigation, Miyazaki Medical College Hospital.
Ohtaki
S
S
Yamazaki
I
I
D000818:Animals; D001693:Biological Transport, Active; D007454:Iodides / Q000378:metabolism*; D007700:Kinetics; D008985:Monensin / Q000494:pharmacology; D009550:Nigericin / Q000494:pharmacology; D012965:Sodium Chloride / Q000378:metabolism*; D012974:Sodium Iodide / Q000378:metabolism*; D013552:Swine; D013961:Thyroid Gland / Q000378:metabolism*
D007454:Iodides; D012965:Sodium Chloride; D008985:Monensin; D012974:Sodium Iodide; D009550:Nigericin