282 IV. DIGESTION OF LIPIDS OTHER THAN FATS 



intravenous injection of colloidal solutions of 3,4-benzpyrene, methyl- 

 cholanthrene, or anthracene. BPX is probably a monohydroxypyrene. 

 In a later study 211 it was found that none of ten related polycyclic 

 hydrocarbons was eliminated unchanged in fowl bile in recognizable 

 amounts after their intravenous injection. However, the following gave 

 rise to fluorescent compounds in the bile: 1 ,2,5,6-dibenzanthracene, 

 cholanthrene, methylcholanthrene, anthracene, pyrene, 2,6-dimethyl-l,2- 

 benzanthracene, 2,7-dimethyl-l,2-benzanthracene, and fluoranthene. On 

 the other hand, 3,4-benzphenanthrene and phenanthrene did not give 

 rise to fluorescent compounds in the bile. Shear investigated the car- 

 cinogenic action, in mice, of isomers of cholanthrene and methylcholan- 

 threne, 212 of 2-amino-5-azotoluene, 213 and also of a number of anthracene 

 derivatives. 214 



7. The Digestion, Absorption, and Transformation of the Ca- 

 rotenoids in the Gastrointestinal Tract 



The carotenoids include both those compounds which are exclusively 

 hydrocarbons and those classed as oxycarotenoids; the latter may be 

 alcohols, oxides, aldehydes, or ketones. Some of the members of each 

 class are provitamins A. Thus, the hydrocarbons, a-, (3-, and 7-carotenes 

 (but not lycopene) belong in this category, while cryptoxanthin is the 

 principal oxygen-containing carotenoid which gives rise to vitamin A. 



Several types of criteria can be employed to study the absorption of 

 the carotenoids. In the first place, the only procedure which will yield 

 quantitative results is the direct method, in which one determines ab- 

 sorption by the rate at which the carotenoid leaves the lumen of the gut. 

 The indirect procedure, in which one observes the carotenoid content of 

 the blood following the administration of the test substance, is of some 

 value but lacks a quantitative interpretation. Moreover, this procedure 

 is impossible for the carotenoids which are broken down in the intestinal 

 wall, as obviously no appreciable concentrations of such substances are 

 found in the blood before or after feeding them. 



In the case of carotenoids which are convertible to vitamin A (a-, 13-, 

 and 7-carotenes, cryptoxanthin), another criterion of absorption is possi- 

 ble. One can determine the increase in vitamin A in the blood and in the 

 tissues such as the liver, or in such excretion products as milk and eggs. 



211 J. G. Chalmers and P. R. Peacock, Biochem. J., 85, 1276-1282 (1941). 



212 M. J. Shear, J. Biol. Chem., 114, Lxxxix-xc (1936). 



213 M. J. Shear, J. Biol. Chem., 114, xc-xci (1936). 



214 M. J. Shear, /. Biol. Chem., 123, cviii-cix (1938). 



