Vitamin E Regulatory Mechanisms

Dietary and supplemental vitamin E is absorbed and delivered to the liver, but of the various antioxidants with vitamin E activity, only α-tocopherol is preferentially recognized by the α-tocopherol transfer protein (α-TTP) and is transferred to plasma, while the other vitamin E forms (e.g., γ-tocopherol or tocotrienols) are removed from the circulation. Hepatic α-TTP is required to maintain plasma and tissue α-tocopherol concentrations. The liver is the master regulator of the body's vitamin E levels in that it not only controls α-tocopherol concentrations, but also appears to be the major site of vitamin E metabolism and excretion. Vitamin Es are metabolized similarly to xenobiotics; they are initially ω-oxidized by cytochrome P450s, undergo several rounds of β-oxidation, and then are conjugated and excreted. As a result of these various mechanisms, liver α-tocopherol and other vitamin E concentrations are closely regulated; thus, any potential adverse vitamin E effects are limited.