Permeation as an Anion

The classic example of permeation as an anion is the ability of oxyanions of toxic metals to traverse cell membranes on either the phosphate or sulfate carriers (reviewed by Wetterhahn-Jennette 1981 Clarkson 1993). Vanadate and arsenate are structurally similar to phosphate and compete with phosphate for transport, as well as intracellular binding sites. Indeed, their toxicity is thought to be directly related to this competition (Clarkson 1993). Similarly, chromate, selenate, and molybdate are structurally similar to sulfate, and are substrates for sulfate transporters. 

The uptake of cadmium by red blood cells was found to be enhanced by a variety of experimental maneuvers that would be expected to increase the 

Lead (Simons 1986) and zinc (Alda Torrubia and Garay 1989 Kalfakakou and Simons 1990) also enter red blood cells as anionic complexes. In the case of lead, a modest sensitivity of the DIDS-sensitive uptake to membrane potential prompted the speculation that the neutral species, PbC03, might be exchanged for a monovalent anion but the transport of the 

In a study of the effects of organic mercurials on the cation permeability of human red blood cells, Knauf and Rothstein (1971) found that the rate of uptake of PCMBS was diminished by 50% in cells exposed to SITS (4-acetamido-4 -isothiocyano-stilbene 2,2 -disulfonic acid), so that the total uptake could be divided into roughly equal SITS-sensitive and SITS-insensitive components. If the chloride in the bathing solution was replaced with either sulfate or phosphate, the SITS-sensitive component of PCMBS uptake was virtually unchanged whereas the SITS-insensitive component was substantially reduced. One interpretation of these results is that PCMBS enters by two routes, one the anion transporter that serves as a route for anionic complexes, and the other the lipid layer that permits the permeation of a neutral Cl complex of PCMBS, the latter perhaps being less abundant in the anion substituted media. Alternatively, a cationic form of the mercurial could permeate via some other membrane protein. 

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