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Fig. 4. Plasma membrane redox functions. Two types of transplasma membrane electron transfer are known. One type uses coenzyme Q as a transmembrane electron carrier [72]; the other uses a low redox potential cytochrome b558 complex. This enzyme is analogous to the peroxide generating GP91 phox of neutrophils (n [40, 41]) and may be characteristic of transformed cells (t [73]). In addition, cytosolic ascorbate can reduce external semidehydroascorbate through a cytochrome b561 in some cells. Three different NAD(P)H dehydrogenases (reductases) on the plasma membrane can reduce coenzyme Q [17, 18]. Two different enzymes are indicated for oxidation of the Q quinol. One is a coenzyme Q oxidase at the outer surface which can oxidize quinol at the outer surface with production of superoxide [37]. The other is an external site sensitive to iron chelators [39]. It can be expected that autooxidation of the iron in neutral pH will produce superoxide [74]. It is not known which system is responsible for diferric transferrin reduction at the cell surface [75]. The recycling of the iron on the tranferrin through reoxidation by oxygen could also produce superoxide since diferric transferrin stimulates NADH oxidation by plasma membrane. The peroxide produced by either of these oxidase systems can then feed back into the cell to activate gene transcription [27], SH-S-S controlled calcium channels [28, 76] or inhibit phosphotyrosine phosphatase [77]. The mechanism of control of the Na+/H+ antiport is not known. Tf is transferrin. The mechanism for redox control of the Na+/H+ antiport [12] or Ca++ activated K+ channels is not known [78]. Gene transcription regulated by the hemopexin system is controlled by surface copper reduction dependent on coenzyme Q [79,80].
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