Knoth J, Handloser K, Njus D
Biochemistry. 1980 Jun 24;19(13):2938-42. doi: 10.1021/bi00554a019.
An ATP-dependent proton pump drives epinephrine transport in chromaffin granule ghosts. When ghosts are suspended in a medium free of permeant anions, ATP addition leads to an increase in membrane potential (interior positive) and epinephrine uptake but not to a change in intravesicular pH. Since ATP does not affect the pH gradient, the energy for transport must be drawn from the membrane potential (delta psi), and epinephrine uptake must result in a net efflux of positive charge. This can be achieved by an antiport (exchange diffusion) mechanism in which each catecholamine cation is taken up in exchange for more than one H+. Measurements indicate that the stoichiometry is close to 2 H+/epinephrine cation, so the equilibrium epinephrine gradient is theoretically [E]in/[E]out = ([H+]in/[H+]out)2eFdelta psi/(RT). In deenergized ghosts, the epinephrine concentration gradient equals the [H+] gradient. This is consistent with a situation in which the H+ concentration gradient is in equilibrium with the membrane potential as described by the Nernst equation. Then, in the equation above, the membrane potential term (eFdelta psi/(RT)) will exactly cancel one power of the [H+] gradient, leaving [E]in/[E]out equal to [H+]in/[H+]out.
一种依赖ATP的质子泵驱动嗜铬粒蛋白颗粒空泡中的肾上腺素转运。当空泡悬浮于不含渗透性阴离子的介质中时，添加ATP会导致膜电位升高（内部为正）以及肾上腺素摄取增加，但泡内pH值不变。由于ATP不影响pH梯度，转运所需能量必定来自膜电位（Δψ），且肾上腺素摄取必定导致正电荷的净外流。这可以通过一种反向转运（交换扩散）机制实现，即每个儿茶酚胺阳离子的摄取会交换出不止一个H⁺。测量表明化学计量比接近2H⁺/肾上腺素阳离子，因此理论上平衡时的肾上腺素梯度为[E]in/[E]out = ([H⁺]in/[H⁺]out)²eFΔψ/(RT)。在去能化的空泡中，肾上腺素浓度梯度等于[H⁺]梯度。这与能斯特方程所描述的H⁺浓度梯度与膜电位处于平衡的情况一致。那么，在上述方程中，膜电位项（eFΔψ/(RT)）将恰好抵消[H⁺]梯度的一次幂，使得[E]in/[E]out等于[H⁺]in/[H⁺]out。
