Zeuthen Thomas, MacAulay Nanna
Institute of Medical Physiology, The Panum Institute, University of Copenhagen, Denmark.
Int Rev Cytol. 2002;215:259-84. doi: 10.1016/s0074-7696(02)15012-1.
Molecular water pumps are membrane proteins of the cotransport type in which a flux of water is coupled to substrate fluxes by a mechanism within the protein. Free energy can be exchanged between the fluxes. Accordingly, the flux of water may be relatively independent of the external water chemical potential and can even proceed uphill. In short, water is being cotransported. The evidence for water cotransport is reviewed with particular emphasis on electrogenic cotransporters expressed in Xenopus oocytes under voltage clamped conditions. Phenomena such as uphill water transport, tight coupling between water transport and clamp current, cotransport of small hydrophilic molecules, and shifts in reversal potentials with osmolarity are discussed with examples from the Na+/glutamate and Na+/glucose cotransporters. Unstirred layers and electrode artifacts as alternative explanations for such cotransport can be ruled out for both experimental and theoretical reasons. Indeed, substrate fluxes mediated by channels or ionophores generate much smaller water fluxes than those observed with cotransporters. Theoretical models, using reasonable values for the intracellular diffusion coefficient, indicate the presence of only small unstirred layers in the membranes studied.
分子水泵是共转运类型的膜蛋白，其中水的通量通过蛋白质内部的机制与底物通量相耦合。自由能可以在通量之间交换。因此，水的通量可能相对独立于外部水的化学势，甚至可以向上流动。简而言之，水是被共转运的。本文回顾了水共转运的证据，特别强调了在电压钳制条件下非洲爪蟾卵母细胞中表达的生电共转运体。通过钠/谷氨酸和钠/葡萄糖共转运体的例子，讨论了诸如水向上运输、水运输与钳制电流之间的紧密耦合、小亲水分子的共转运以及反转电位随渗透压的变化等现象。由于实验和理论原因，可以排除未搅动层和电极伪迹作为这种共转运的替代解释。事实上，由通道或离子载体介导的底物通量产生的水通量比共转运体观察到的要小得多。使用细胞内扩散系数的合理值的理论模型表明，在所研究的膜中仅存在小的未搅动层。
