Diffusion- and reaction rate-limited redox processes mediated by quinones through bilayer lipid membranes

Abstract

The mediation of redox reactions through bilayer lipid membranes was studied. With an appropriate choice of electron acceptors the redox process can be limited either by the chemical reaction rate between the mediator and the reactants or by the shuttle frequency of the mediator through the membrane. Both modes were demonstrated for redox reactions mediated by 2,6 dichlorobenzoquinone (DCBQ) and by alpha-tocopherol with ascorbate entrapped inside vesicles using ferricyanide (a mild oxidant) or hexachloroiridate (a strong oxidant) in the external solution. The redox processes were reaction rate-limited and diffusion-limited for ferricyanide and hexachloroiridate, respectively. The kinetics of the redox processes in the diffusion- and the reaction rate-limited modes allows the determination of the shuttle frequencies and of the interfacial reaction rates of the mediators, respectively. The shuttle frequencies of DCBQ and alpha-tocopherol were approximately 8 and 0.08 s-1, respectively, in L-alpha-dipalmitoyl phosphatidylcholine (DPPC) cholesterol vesicles at 25 degrees C. Interfacial reaction rates between the mediators and ferricyanide were about two- and tenfold lower compared with bulk reaction rates for DCBQ (water) and tocopherol (50% ethanol solution), respectively, i.e., tocopherol is relatively less accessible to aqueous oxidants at the membrane interface. Tocopherol and oxidized tocopherol are reversible hydrophobic redox couples that interact very rapidly with strong oxidants. In both modes of mediation DCBQ was more effective than alpha-tocopherol.

Ascorbate Stimulates Ferricyanide Reduction in HL-60 Cells through a Mechanism Distinct from the NADH-dependent Plasma Membrane Reductase

“…There have been reports suggesting a role for ␣-tocopherol in assisting the transport of electrons over the membrane (2,28). Under standard culture conditions, cells have a very low ␣-tocopherol content due to the low level of this compound in serum (29).…”

“…It could involve either direct chemical reactions or a membrane-based enzyme (2,28). There have been reports in the literature suggesting a nonenzymatic route of electron transport.…”

Interaction of Ascorbate and α-Tocopherol in Resealed Human Erythrocyte Ghosts

“…It has been shown that ␣-tocopherol can directly transfer ascorbate-derived electrons to ferricyanide across artificial liposomal bilayers (48), although transbilayer movement of ␣-tocopherol is slow relative to quinones with short hydrophobic tails (49). To determine whether ascorbate-dependent ferricyanide reduction requires ␣-tocopherol, two types of experiment were carried out.…”

Is ascorbic acid an antioxidant for the plasma membrane?

“…5, mechanism 4), especially since ␣-tocopherol is much more abundant than ubiquinol/one-10 in membranes (68). It has been shown that ␣-tocopherol can transfer electrons from ascorbate trapped within liposomes to extravesicular ferricyanide (74). We recently confirmed this reaction and found that it was associated with sparing of ␣-tocopherol and protection against lipid hydroperoxide formation (77).…”

“…Initial studies using dithionite reduction of ferricyanide trapped within liposomes showed that long-chain quinones were more effective electron transfer agents than were short-chain forms (72,73). However, when less hydrophobic reducing agents such as ascorbate (73,74) and sodium borohydride (75) are used, there is very little tendency for transbilayer electron transfer by long-chain quinones or semiquinones. Long-chain quinones are viewed as restricted to the center of the bilayer and incapable of movement to either membrane face (75).…”