Rudnick G, Kaback H R
J Biol Chem. 1975 Sep 10;250(17):6847-51.
2'-N-(2-Nitro-4-azidophenyl) aminoethyl-1-thio-beta-D-galactopyranoside (APG2) is a competitive inhibitor of lactose transport in membrane vesicles isolated from Escherichia coli ML 308-225, exhibiting an apparent Ki of 30 to 40 muM, but is not transported. When irradiated with visible light in the presence of D-lactate, APG2 irreversibly inactivates the lac transport system. Imposition of a membrane potential (positive outside) by valinomycin-induced potassium efflux also causes APG2 photoinactivation. Strikingly, photoinactivation is not observed in the absence of D-lactate or a potassium diffusion gradient. Kinetic studies of the inactivation process yield a KD of 35 muM. Since lactose protects against the inactivation, it is apparent that these effects are specific for the lac transport system. The results show that APG2 inactivates from the outer surface of the vesicle membrane and support the previous hypothesis that the lac carrier protein is unable to bind external substrate in the absence of energy coupling.
2'-N-(2-硝基-4-叠氮苯基)氨基乙基-1-硫代-β-D-吡喃半乳糖苷（APG2）是从大肠杆菌ML 308-225分离出的膜囊泡中乳糖转运的竞争性抑制剂，其表观抑制常数Ki为30至40 μM，但它本身不被转运。当在D-乳酸存在下用可见光照射时，APG2会使乳糖转运系统不可逆地失活。缬氨霉素诱导的钾外流产生膜电位（外侧为正）也会导致APG2光失活。引人注目的是，在没有D-乳酸或钾扩散梯度的情况下未观察到光失活现象。失活过程的动力学研究得出解离常数KD为35 μM。由于乳糖可防止失活，显然这些效应对于乳糖转运系统具有特异性。结果表明，APG2从囊泡膜的外表面使转运系统失活，并支持先前的假设，即在没有能量偶联的情况下，乳糖载体蛋白无法结合外部底物。
