Antioxidant ascorbate is stabilized by NADH-coenzyme Q(10) reductase in the plasma membrane

Plasma membranes isolated from K562 cells contain an NADH-ascorbate free radical reductase activity and intact cells show the capacity to reduce the rate of chemical oxidation of ascorbate leading to its stabilization at the extracellular space. Both activities are stimulated by CoQ(10) and inhibited by capsaicin and dicumarol. A 34-kDa protein (p34) isolated from pig liver plasma membrane, displaying NADH-CoQ(10) reductase activity and its internal sequence being identical to cytochrome b(5) reductase, increases the NADH-ascorbate free radical reductase activity of K562 cells plasma membranes. Also, the incorporation of this protein into K562 cells by p34-reconstituted liposomes also increased the stabilization of ascorbate by these cells. TPA-induced differentiation of K562 cells increases ascorbate stabilization by whole cells and both NADH-ascorbate free radical reductase and CoQ(10) content in isolated plasma membranes. We show here the role of CoQ(10) and its NADH-dependent reductase in both plasma membrane NADH-ascorbate free radical reductase and ascorbate stabilization by K562 cells. These data support the idea that besides intracellular cytochrome bs-dependent ascorbate regeneration, the extracellular stabilization of ascorbate is mediated by CoQ(10) and its NADH-dependent reductase.