Isomers of alpha‐tocopheryl acetate and their biological activity

α‐Tocopheryl acetate, an ester of the naturally occurring form of vitamin E, has a biopotency of 1.66 IU/mg as determined by bioassays based on biological function. On a similar basis, 2 l ‐α‐tocopheryl acetate, the 2‐epimer of α‐tocopheryl acetate, has a biopotency of 0.35 IU/mg. Results of hemolysis bioassays indicate relative activities of 1.53 and 0.56 for α‐ and 2 l ‐α‐tocopheryl acetates, respectively, compared with 2 dl ‐ or all‐ rac ‐α‐tocopheryl acetate. Responses to plasma and liver storage tests average 1.35 for α‐tocopheryl acetate compared with all‐ rac ‐α‐tocopheryl acetate. The configuration at C‐2 is most important but the configurations at C‐4’ and C‐8’ may also be important in determining biopotency. There is no evidence of synergism between α‐tocopheryl acetate and 2 l ‐α‐tocopheryl acetate. Since the two isomers comprising 2 dl ‐α‐tocopheryl acetate, the former International Standard, have different rates of absorption, tissue storage, metabolism and excretion, a better standard for vitamin E is needed. α‐Tocopheryl acetate has both the chemical and biological properties required for a new International Standard for vitamin E. Redefinition of an International Unit in terms of 1 IU=0.60 mg of α‐tocopheryl acetate would give a precise reference for future research and a reasonable correlation with data collected in the past.