Toxic chemical agents as probes for permeation systems of the red blood cell

Chemical agents with different capacities to penetrate into the membrane and with different chemical reactivities can be used to gain information concerning the location of transport sites in the membrane structure and the particular functional ligands. If the agents are highly specific in their interactions and if their inhibitory effects are irreversible, they can also be used as probes to identify the transport components. Several examples are cited using the human red blood cells as a model. The anion transport system in particular has been studied by the use of nonpenetrating irreversible inhibitors, and more recently with a photoaffinity probe, NAP-taurine. In the dark the latter is transported in competition with the normal inorganic anions but after exposure to light, it becomes fixed in an irreversible bond that allows identification of the sites of its transport. It is proposed that anion transport involves a transmembrane protein of about 90,000 daltons that forms a channel through the lipid bilayer. The exchange of anions occurs via a gating mechanism containing a specific anion-binding site. Transport of water, cations and sugars may also involve similar transmembrane protein channels.