Two antioxidants are better than one.

New evidence suggests that the cellular oxygen-sensing hypoxia-inducible factor(HIF) pathway may be protected by a double buffer of cellular antioxidant defense. Key players in the oxygen-dependent regulation of this pathway are the prolyl hydroxylase domain-containing enzymes (PHDs) that catalyze the prolyl-4-hydroxylation of HIFα, dependent on the presence of oxygen, 2-oxoglutarate, and iron in the ferrous (Fe(2+)) form. Vitamin C is also required as a cofactor, possibly to maintain the catalytic iron center in its functional Fe(2+)) state, although both the mechanism and the in vivo requirement are not absolutely clear.