Bresler V M, Nikiforov A A
Tsitologiia. 1977 Feb;19(2):172-80.
The dependence of active transport rates of fluorescein and uranin into proximal tubules was studied, firstly, on the concentration of these substrates in the incubation medium with constant Na-concentration and, secondly, on Na+-concentration in the medium with constant concentrations of fluorescein or uranin, Km, Vmax, and KmNa, VmaxNa values being determined correspondingly. With the decrease of Na+-concentration in the medium the apparent Km values for fluorescein and uranin increase, while Vmax values remain unchanged and ratios Vmax/Km decrease. The 1/Km value for uranin transport is a linear function of Na+-concentration, while it is a quadratic function of Na+-concentration for fluorescein transport. The KmNa value for fluorescein transport is twice as much as that for uranin transport. The obtained data suggest that one Na+-ion is spent from the medium for translocation of one uranin molecule across the basal membrane, whereas two Na+-ions from the medium are necessary for translocation of one fluorescein molecule. It is assumed that fluorescein and uranin transports are carried out by producing a four-fold transporting complex (a carrier+a substratum+2Na+; in the case of fluorescein 2Na+ appear from the medium, whereas in the case of uranin only one Na+ from the medium is used.
首先研究了荧光素和uranin向近端小管的主动转运速率对孵育介质中这些底物浓度（Na浓度恒定）的依赖性，其次研究了在荧光素或uranin浓度恒定的介质中Na⁺浓度的依赖性，相应地测定了Km、Vmax以及KmNa、VmaxNa值。随着介质中Na⁺浓度的降低，荧光素和uranin的表观Km值增加，而Vmax值保持不变，Vmax/Km比值降低。uranin转运的1/Km值是Na⁺浓度的线性函数，而荧光素转运的1/Km值是Na⁺浓度的二次函数。荧光素转运的KmNa值是uranin转运的KmNa值的两倍。所得数据表明，一个uranin分子穿过基底膜需要消耗介质中的一个Na⁺离子，而一个荧光素分子的转运则需要介质中的两个Na⁺离子。据推测，荧光素和uranin的转运是通过形成四重转运复合物（一个载体+一个底物+2个Na⁺；荧光素的情况下，2个Na⁺来自介质，而uranin的情况下，仅使用介质中的一个Na⁺）来进行的。
