Oxygen-Sensitive Membrane Transporters in Vertebrate Red Cells

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Abstract
Oxygen is essential for all higher forms of animal life. It is required for oxidative phosphorylation, which forms the bulk of the energy supply of most animals. In many vertebrates, transport of O2 from respiratory to other tissues, and of CO2in the opposite direction, involves red cells. These are highly specialised, adapted for their respiratory function. Intracellular haemoglobin, carbonic anhydrase and the membrane anion exchanger (AE1) increase the effective O2- and CO2-carrying capacity of red cells by approximately 100-fold. O2 also has a pathological role. It is a very reactive species chemically, and oxidation, free radical generation and peroxide formation can be major hazards. Cells that come into contact with potentially damaging levels of O2have a variety of systems to protect them against oxidative damage. Those in red cells include catalase, superoxide dismutase and glutathione. In this review, we focus on a third role of O2, as a regulator of membrane transport systems, a role with important consequences for the homeostasis of the red cell and also the organism as a whole. We show that regulation of red cell transporters by O2is widespread throughout the vertebrate kingdom. The effect of O2 is selective but involves a wide range of transporters, including inorganic and organic systems, and both electroneutral and conductive pathways. Finally, we discuss what is known about the mechanism of the O2 effect and comment on its physiological and pathological roles.