744 XI. VITAMINS E (tocopherols) 



otherwise oecnirs on \'itamiii E-fi-ee diets. In 1953 Dam''^'' reviewed the 

 subject of \'itamin E as an in vivo antioxidant. 



8. The Biopotency of Various Forms of Vitamin E 



{1) Comparative Biopotency of a-Tocopherol 

 as Ester and Free Alcohol 



Although Joffe and Harris, ^^^ in 1943, reported that free a-tocopherol 

 had the same biopotency against sterihty as it did in the form of an ester 

 (a-tocopheryl acid succinate), Demole et al^^^ and Mason and Harris^^" 

 reported that, under standard conditions, the tocopherol esters, both 

 natural and synthetic, showed a consistent superiority in vitamin E 

 potency as compared Avith the respective tocopherols in the active form. 

 In a later, more thorough, evaluation, Harris and Ludwig,"*^^ basing their 

 calculations upon an experimentally determined equivalency of natural 

 D-a-tocopheryl acetate, 1 mg. = 1.36 I.U., exhibited 62% more vitamin E 

 potency than it did as the free alcohol. *^^ This relationship was observed 

 both for the natural tocopherols, i.e., D-a esterified versus D-a free, and for 

 synthetic tocopherols, DL-cc-esterified versus Dh-a free. Harris and Lud- 

 ^jg462 explain the greater biopotency of the tocopherol esters over the 

 free tocopherol on the basis that the former are more resistant to oxidative 

 destruction such as might occur prior to absorption in the intestinal tract 

 of the assay rats. Thus, when antioxidants were added to the tocopherol 

 solutions fed, free a-tocopherol was preserved sufficiently to induce a 

 response equivalent to that of an equimolecular dose of the esterified 

 a-tocopherol. 



(2) Comparative Biopotency of dl- and D-a-Tocopherols 



Since position 2 involves a center of asymmetry in the tocopherol mole- 

 cule, it is optically active. The natural form has been found to be d 

 (or c?) -a-tocopherol. Although the optical activity is very slight, Harris 

 and co-workers**^ reported that synthetic dl- a-tocopherol has only two- 

 thirds of the biopotency of natural o-a-tocopherol, while sjTithetic dl-/3- 



*" H. Dam, Experientia, Suppl. 1, 1953, 195-207. 



4« M. Joffe and P. L. Harris, /. Am. Chem. Soc, 65, 925-927 (1943). 



^*' V. Demole, O. Isler, B. H. Ringier, H. Salomon, and P. Karrer, Helv. Chim. Acta, 22, 

 65-68(1930). 



«» K. E. Mason and P. L. Harris, Biol. Symposia, 12, 459-483 (1947). 



«i P. L. Harris and M. I. Lvidwig, J. Biol. CJiem., 179, 1111-1115 (1949). 



«2 p. L. Harris and M. I. Ludwig, /. Biol. Ctiem., 180, 611-614 (1949). 



«3 P. L. Harris, J. L. Jensen, M. Joffe, and K. E. Mason, ./. Biol. Chem., 156, 491-498 

 (1944). 



