Restrepo D, Kimmich G A
J Membr Biol. 1986;89(3):269-80. doi: 10.1007/BF01870669.
Phlorizin binding is studied in isolated intestinal epithelial cells of the chick. Cells are ATP depleted to allow extensive manipulation of ionic gradients and membrane potential (delta psi). Phlorizin binding is assayed at steady state. Carrier specific phlorizin binding is defined as D-glucose (90 mM) inhibitable binding. Specific binding displays simple Michaelian kinetics as a function of phlorizin, indicating the presence of a single homogeneous binding site. Sodium concentrations and delta psi modify the apparent binding affinity but not the maximum number of binding sites. In contrast, the activation curve as a function of sodium concentrations is sigmoid and the apparent maximum number of binding sites at saturating sodium is phlorizin dependent. The rate of phlorizin association is both delta psi and sodium-concentration dependent. Dissociation is sodium-concentration dependent but not delta psi dependent. Theoretical analysis indicates binding order of substrates is random. In addition, data suggests that the phlorizin/sodium stoichiometry is 2:1. The delta psi dependence can be explained by two models: either translocation is the delta psi-dependent step and the free carrier is anionic, or sodium binding is the delta psi-dependent step.
在雏鸡的离体肠上皮细胞中研究根皮苷结合情况。使细胞的三磷酸腺苷（ATP）耗尽，以便对离子梯度和膜电位（Δψ）进行广泛调控。在稳态下测定根皮苷结合情况。载体特异性根皮苷结合定义为可被D - 葡萄糖（90 mM）抑制的结合。特异性结合表现出作为根皮苷函数的简单米氏动力学，表明存在单一的同质结合位点。钠浓度和Δψ改变表观结合亲和力，但不改变结合位点的最大数量。相比之下，作为钠浓度函数的激活曲线呈S形，并且在饱和钠时的表观结合位点最大数量取决于根皮苷。根皮苷结合速率既取决于Δψ也取决于钠浓度。解离取决于钠浓度，但不取决于Δψ。理论分析表明底物的结合顺序是随机的。此外，数据表明根皮苷/钠化学计量比为2:1。Δψ依赖性可以用两种模型来解释：要么转运是Δψ依赖性步骤且游离载体是阴离子，要么钠结合是Δψ依赖性步骤。
