DIGESTION, ETC. OF FAT-SOLUBLE VITAMINS 311 



provitamin, carotene. In contradistinction to carotene, vitamin A is as 

 well utilized when given by the parenteral route as it is when administered 

 orally. 361 In fact, Sobel and co-workers 406 reported that, when vitamin 

 A was administered to cows intravenously as an aqueous dispersion, the 

 increased excretion of vitamin A in the milk was fifteen times as great as 

 when the same quantity of vitamin A was given orally, dissolved in oil. 

 When vitamin A in aqueous dispersion was ingested, the increased vitamin 

 A content in the milk of cows was five times that observed when the 

 vitamin was given in oil by the same route. 



(a) Hydrolysis of Vitamin A Esters as a Preliminary to Absorption. 

 On the basis of the experiments of Gray, Morgareidge, and Cawley, 407 

 it is now generally accepted that vitamin A esters must first be hydrolyzed 

 to the alcohol form before they are absorbed from the gastrointestinal 

 tract. It is believed that an esterase in the intestinal juice is responsible 

 for this hydrolysis. Gray et al. m found that, when the naturally occurring 

 ester was given to rats, a small but steady increase in the amount of free 

 alcohol in the intestinal contents resulted as absorption proceeded. How- 

 ever, the decisive finding which indicated that hydrolysis of the ester must 

 precede absorption was the proof that the vitamin A present in the in- 

 testinal wall was largely in the form of the free alcohol. In the samples 

 of gut wall obtained from rats in the three- to six-hour period after the 

 feeding of vitamin A ester, 82% of the total vitamin A in the tissues was 

 found to be in the form of the free alcohol. Eden and Sellers 408 - 409 ex- 

 tended the observation of this phenomenon to calves and sheep. When 

 the animals were slaughtered four hours after the administration of vita- 

 min A acetate, almost complete hydrolysis had obtained in the contents 

 of the intestinal lumen, in some cases, and only a partial hydrolysis in 

 other cases. When vitamin A ester was fed, the ester fraction in the 

 mucosa was 73% of the total vitamin A in the case of the calves, and 56% 

 in the case of the sheep. On the other hand, the corresponding values for 

 the ester fraction in the intestinal mucosa of the calves and sheep, re- 

 spectively, after the administration of vitamin A alcohol, were 82 and 

 77%. This indicates that esterification of the vitamin A alcohol occurs 

 after it has been absorbed into the mucosa. Further confirmation of the 

 hydrolysis of vitamin A esters prior to absorption is to be found in the 

 results of Clausen 410 and in those of Popper and Volk. 411 



406 A. E. Sobel, A. Rosenberg, and E. Engel, J. Nutrition, 48, 183-192 (1952). 



407 E. L. Grav, K. Morgareidge, and J. D. Cawley, J. Nutrition, 20, 67-74 (1940). 



408 E. Eden and K. C. Sellers, Biochcm. J., 45, xxxiii (1949). 



409 E. Eden and K. C, Sellers, Biochem. J., 46, 261-266 (1950). 



410 S. W. Clausen, Harvey Lectures, 38, 199-226 (1943). 



411 H. Popper and B. W. Volk, Arch. Pathol., 38, 71-75 (1944). 



