Four widely documented mechanisms of chloride transport across plasma membranes are: anion-coupled antiport, sodium and hydrogen-coupled symport and an electrochemical coupling process. No direct genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl--stimulated ATPases coexisting, in the same tissue, with uphill chloride transport that could not be accounted for by the four common chloride transport processes. Cl--stimulated ATPases are a common property of practically all biological cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl--stimulated ATPase activity. Recent studies of Cl--stimulated ATPase activity and chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl--stimulated ATPases.
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