CHAPTER 14: Metabolism of Vitamin E

Almost 100 years after the detection of vitamin E, its biological function is still waiting to be identified. The postulated function of an antioxidant is obviously not the only one. All forms of vitamin E have a chromanol structure and a 13-carbon-long side chain. The first degradation products to be found pointed to an oxidative opening of the chromanol structure, which supported the antioxidant theory. However, in all more recently analyzed metabolites, the chromanol ring is intact, which does not point to an oxidative action. The start of degradation is catalyzed by enzymes of the CYP system with two preferential ones: CYP3A4 and CYP4F2. CYP3A4 obviously acts preferentially on α-tocopherol, whereas CYP4F2 appears to preferentially degrade non-α-forms. Non-α-forms are metabolized fast, α-tocopherol only if present in excess. Both CYPs can be up-regulated, but differ in the response to different vitamin E forms. Detailed studies of the functions of individual metabolites are needed since they are appearing to turn out to be a new class of regulatory signaling molecules.