Nakamura Y, Ohtaki S, Yamazaki I
Central Laboratory of Clinical Investigation, Miyazaki Medical College Hospital.
J Biochem. 1988 Oct;104(4):544-9. doi: 10.1093/oxfordjournals.jbchem.a122507.
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.
采用密理博过滤法，以2 mM高氯酸盐作为反应终止剂，测定猪甲状腺质膜囊泡对¹²⁵I的摄取。在高浓度细胞外Na⁺（Na⁺o）存在的情况下发生有效摄取。在莫能菌素和尼日利亚菌素等Na⁺离子载体存在时，未观察到摄取，且积累的I⁻被释放。I⁻摄取的初始速率根据简单的饱和动力学随细胞外I⁻（I⁻o）浓度增加，使摄取速率达到最大值一半时的[I⁻o]约为5 μM。摄取速率对[Na⁺o]的依赖性表现出协同性，希尔系数为1.8，KNa值为0.0064 M²，这表明至少2个Na⁺离子与载体分子结合才能转运1个I⁻离子。进一步的动力学数据与一种机制一致，即离子结合迅速，且Na⁺结合先于I⁻结合。囊泡内的Na⁺抑制I⁻摄取，抑制常数（KiNa）约为4 mM，与[I⁻o]和[Na⁺o]无关。囊泡内的I⁻抑制I⁻摄取，表观KiI值约为100 μM。结果表明，囊泡内外Na⁺和I⁻结合模式的差异是导致I⁻逆浓度梯度摄取的重要因素。
