METABOLISM OF ABSORBED TOCOPHEROLS 701 



they are related and the effect of pathologic conditions, the reader is re- 

 ferred to The Lipids, \o\. II, pages 509-520. 



{2) The Excretion of Absorbed Tocopherols 



Absorbed tocopherols can be removed from the body either by excretion 

 into the bile or, to some extent, by ehmination via the kidney. As has 

 already been discussed, bile has about the same tocopherol content as that 

 of the plasma. ^■' 



There has been some question as to whether or not absorbed tocopherols 

 are excreted in the urine. McArthur and Watson ^^s were unable to find 

 anj^ e"\ddence of tocopherols in the urine of cows. Moreover, Klatskin and 

 Molander^i^'^^^ could not account for any significant amount of vitamin E 

 in the urine of healthy humans, although the fecal excretion amounted 

 to two-thirds of that ingested. Simon, Gross, and Milhorat^"^ also re- 

 ported that none of the C ^-labeled a-tocopheryl succinate was excreted 

 in the urine of the rabbit when this compound was administered orally. 

 However, Harris^^ states that imder similar conditions some urinary excre- 

 tion of C^"* occurs. 



WTien the tocopherols are administered by parenteral routes, vitamin E 

 or vitamin E-like compounds are unquestionably excreted in the urine. 

 When C^'*-labeled a-tocopheryl succinate was injected subcutaneously, 

 Simon, Gros, and Milhorat^"^ were able to recover about 14% of the 

 original dose administered from the site of the injection after eleven daj^s. 

 After intravenous injection, 90% of the radioactivity in the blood stream 

 was lost within twenty-four hours. In this case, about one-third of the 

 administered tocopherol-C^^ was excreted in the urine. All of the radio- 

 active product in the urine and about one-half of that eliminated via the 

 gut comprised a new metabolite which must be considered as an inter- 

 mediate of a-tocopherol.^- It is e^vident from these results that the 

 esterified tocopherols are hydrolyzed by the tissue fluids. 



3. The Catabolism of Tocopherols 



The first step in the catabolism of a-tocopherol is believed to be its con- 

 version to a-tocopheryl quinone, with a possible further reduction to a- 

 tocopheryl hydroquinone, as illustrated below: 



'28 C. S. McArthur and E. M. Watson, Can. Chem. Process Inds., 23, 350-352 (1939). 



