3 FUNCTION IN THE ANIMAL ORGANISM 13 



vitamin A, the conversion of pro-vitamins into vitamin A takes place rapidly 

 and fairly completely (up to 70 or 80 %). If the organism is saturated with 

 vitamin A, however, or if high doses of pro-vitamins are given, then only a 

 small proportion is converted into the vitamin-^. For this reason, carotene and 

 other carotenoids are always found in the faeces. 



The form in which the pro-vitamins A are supplied to the organism is of 

 decisive importance for their absorption and conversion into vitamin A. If the 

 carotenoids are dissolved in animal or vegetable fats, they are easily taken up ; 

 if, on the other hand, solutions in paraffin oil or ethyl oleate are used, hardly 

 any absorption takes place^*. Ignorance of these facts is partly responsible for 

 the contradictory results recorded in the early literature concerning the activity 

 of carotene^*. 



Very recently a number of attemps have been made to convert /3-carotene 

 into vitamin A in vitro. Although some of these attempts are claimed to have 

 been successful, this problem cannot yet be regarded as finally solved, as it 

 has not been possible to isolate the vitamin A formed in a pure state and 

 to establish its identity with certainty. Willstaedt^^ reports a transformation 

 of this type, using liver preparations, while Hunter and Williams^^ obtained 

 traces of vitamin A by the action of hydrogen peroxide on ^-carotene and sub- 

 sequent reduction of the aldehyde produced. 



It is an interesting fact that not all mammals have the same capacity for 

 converting pro-vitamins A into vitamin A. The most suitable experimental 

 animal appears to be the rat^^. Guinea pigs^^, rabbits-^, pigs^° and cattle^^ 

 possess the capacity to a reduced extent, dogs only to a very small extent^^, 

 whereas in cats it is completely absent^^. Chicken also appear capable of trans- 

 forming /3-carotene into vitamin A^*. The facts regarding fresh water and salt 

 water fish are not yet completely known, but it appears that fish are capable 

 of converting pro-vitamins A into vitamin Aj^^ (and vitamin Ag)^®. 



After recognition of the fact that carotene consists of several isomers 

 (cf. p. 125), and that a-carotene also possesses vitamin A activity, though to 

 a reduced extent, a number of different investigations were begun with the 

 view to elucidating the relationships between the structure of a compound and 

 its vitamin A activity. In the course of these investigations several naturally 

 occurring carotenoids were recognised as pro-vitamins A and a number of partly 

 synthetic carotenoids were also shown to possess growth-promoting properties. 

 Before dealing with the theoretical aspects of these results, a summary is given 

 here of the compounds which possess vitamin A activity (see table i, p. 14). 

 As all these compounds (with the exception of vitamin A methyl ether and 

 vitamin A acid) will be dealt with in detail in later sections, the reader is 

 merely referred to the alphabetical index at this stage. 



The relationships which exist between the vitamin A activity of a compound 

 References p. ly-ig. 



