520 IX. CAROTENOIDS AND VITAMINS A 



least occurred in the nuclei. When vitamin A was administered to rats in 

 sufficient amounts to increase the quantity in the liver, a shift in distribu- 

 tion occurred. Fraction "X" still contained the largest proportion of vita- 

 min A, followed by the nuclei; the mitochondria had the lowest concentra- 

 tion. On the other hand, Krinsky and Ganguly^^^ reported that \ntamin 

 A ester was present solely in the centripetally migrating fraction (''cream" 

 fraction), while vitamin A alcohol occurred not only in the centripetally- 

 migrating material but also, in appreciable amounts, in the supernatant 

 and microsomal fractions. The mitochondrial and nuclear fractions Avere 

 found to be essentially free from vitamin A. It is believed that vitamin A 

 ester and vitamin A alcohol are stored in combination with different 

 proteins within the liver cell. Several years ago, Glover and Morton^'^" 

 called attention to the fact that two types of A-itamin A storage occur in the 

 liver. It is believed that the one site in which the ester is stored contains 

 no lipase, while this enzyme is present in locations where vitamin A occurs 

 as the free alcohol. The Kupffer cells are considered to be the site of lipase- 

 free storage. In fact, Gomori^^^ demonstrated the absence of lipase from 

 these cells, by a histologic procedure. During the absorption of vitamin 

 A, two processes are believed to be operating in the liver, namely (1) the 

 hydrolysis of the incoming ester by serum esterase, which releases free vita- 

 min A to form a complex capable of diffusing into the liver storage cells, 

 and (3) phagocj^tic action by the Kupffer cells whereby the vitamin A ester 

 in the chylomicrons is trapped in the liver sinusoids, and becomes incor- 

 porated into the Kupffer cells. The vitamin A (ester) is not believed to 

 contribute to the functional Aitamin A in the blood. ^^^ However, the im- 

 plication that the level of plasma A'itamin A is a function of the quantity 

 of hepatic \dtamin A alcohol is not borne out experimentally (see page 

 468). 



(4) Factors Altering the Storage of Carotenoids and of Vitamins A 



a. The Effect of Age on the Storage of Vitamin A. Age has a marked 

 effect on the proportion of vitamin A present in the liver of rats. According 

 to Guerrant,®^^ newborn rats contain only a small amount of vitamin A. 

 The concentration of vitamin A in the livers of nursing rats doubles during 

 the first seven days, after which it remains fairly constant over the balance 

 of the lactation period. The amount and concentration of the vitamin A 



669 N. I. Krinsky and J. Ganguly, ./. Biol. Chem., 202, 227-232 (1953). 

 "0 J. Glover and R. A. Morton, Biochem. J., 43, xii-xiii (1948). 

 «7i G. Gomori, Arch. Pathol, 41, 121-129 (1946). 

 6" N. B. Guerrant, J. Nutrition, 37, 37-51 (1949). 



