Ascorbate and plasma membrane electron transport--enzymes vs efflux.

Transplasma membrane electron transport (tPMET) systems transfer electrons across the plasma membrane, resulting in the net reduction of extracellular oxidants (e.g., ferricyanide) at the expense of intracellular reductants such as NADH and ascorbate. In mammalian tPMET systems, the major proximal electron donor is ascorbate. The classical description of ascorbate-dependent tPMET views ascorbate as a restrictively intracellular electron donor to a transplasma membrane enzymatic activity that transfers electrons across the plasma membrane to various physiological acceptors (e.g., ferric iron and the ascorbyl radical). Candidate proteins involved in this process include members of the cytochrome b(561) family (e.g., duodenal cytochrome b). However, mounting evidence suggests that cellular export of ascorbate (and concomitant import of its two-electron oxidation product, dehydroascorbate) may constitute a novel and physiologically relevant form of ascorbate-dependent tPMET. As with enzymatic tPMET, cellular ascorbate export results in net electron transfer from the cytoplasm to the extracellular space. The mechanisms of ascorbate release from cells are ill-defined, though volume-sensitive anion channels and exocytosis remain promising candidates. Cellular ascorbate release is implicated in various homeostatic processes including ascorbate maintenance in blood and brain, and the uptake of non-transferrin-bound iron by cells. Recent insights into the "duality" of ascorbate-dependent tPMET are discussed.