CHANGES IN THE INTESTINAL WALL 451 



arose from any one of the three sources noted a])ove. In later studies,^^^ 

 a figure of 75% was indicated as the approximate proportion of vitamin A 

 ester in the total vitamin A of the intestinal wall. Glover and co-workers*^^ 

 found evidence of active estorification of vitamin A alcohol, both in the 

 intestinal mucosa and also in the subcutaneous tissue. In sharp contrast 

 to the occurrence of vitamin A in the intestinal wall largely in the form of 

 the ester, that in the plasma of fasting animals is almost completely in the 

 form of the free alcohol. ^^^■--'^ Vitamin A in the liver occurs mainly in the 

 esterified form.-'^ However, following the administration of large doses of 

 vitamin A, the major proportion of the elevated plasma vitamin A level is 

 in the form of the ester. ^'^ Krause and Alberghini'--^^ demonstrated the 

 presence of a vitamin A esterase in the serum and plasma, but not in the 

 cellular elements of man, rats, and rabbits. 



(3) The Reduction of the Retinenes to Vitamins A 



An important reaction which occurs in the wall of the intestine involves 

 the reduction of retinene, and retinenei to the corresponding vitamins A. 

 Ball, Goodwin, and Morton--^ were the first to prove that vitamin A alde- 

 hyde and the purple-colored pigment, retinene, present in the retina, are 

 similar compounds. Although it is realized that the reduction of retinenes 

 to the vitamins A may occur in places other than the intestinal wall (includ- 

 ing the retina itself), this reaction may serve an important purpose in the 

 gut wall, particularly if vitamin A aldehyde is an intermediate product in 

 the conversion of carotene to vitamin A. 



Most of the available information concerning the vitamin A alcohol ^ 

 vitamin A aldehyde (retmene) reaction was developed in relation to the 

 visual systems. Wald--*-^^^ is responsible for the clarification of our under- 

 standing of the nature of the light-sensitive carotenoid proteins, which are 

 proteins bearing prosthetic groups to which they owe their color and their 

 sensitivity to light. Three types of compounds have been described, 

 namely rhodopsin and porphyropsin which occur in the rods, and iodopsin 

 which is present in the cones. When exposed to light, all three carotenoid- 



^•^ S. Y. Thompson, R. Braude, M. E. Coates, A. T. Covvie, J. Ganguly, and S. K. 

 Kon, Brit. J. Nutrition, J,, 398-421 (1950). 



220 J. Glover, T. W. Goodwin, and R. A. Morton, Biochem. J., 41, 94-9G (1947). 



"1 E. LeB. Gray, K. C. D. Hickman, and E. F. Brown, J. Nutrition, 19, 39-46 

 (1940). 



2" R. F. Krause and C. Alberghini, Arch. Biochem., 25, 396-400 (1950). 



"3 S. Ball, T. W. Goodwin, and R. A. Morton, Biochem. J., 1,2. 516-523 (1948). 



22< G. Wald, Vitamins and Hormones, 1, 195-227 (1943). 



226 G. Wald. Harvey Lectures, 41, 117-160 (1945-1946). 



