CAROTENOIDS 



be supplied by valine, which has this structure. Leucine is a 6-C 

 compound and must lose one carbon atom, probably by decarboxy- 

 lation ; this would yield after deamination, wovaleraldehyde, and it is 

 interesting to recall in this connection the classical work of Ehrlich in 

 1911 (quoted by Foster) ^^ which demonstrated that leucine is the 

 specific precursor of the wovaleraldehyde which occurs in the fusel oil 

 produced during yeast fermentation. The mechanism is as follows : 



CH 



•NHj 



CH, 



CH CH; CHfNH^) COOH 

 / +0 



CHj. _pn CH3 



CHj^ CH3 



:CH CH^ CHO 



A similar type of decarboxylation also occurs during leucine meta- 

 boliam in animal tissues. ^ '* At the time of writing, the results of 

 preliminary experiments ^ ^^ have indicated that these postulated 

 intermediates may stimulate carotogenesis. 



Although these experiments indicate that carotene is probably 

 synthesised via a 5 -carbon unit, the main features of the intermediary 

 steps in its biogenesis remain to be solved. Perhaps the two most 

 important problems needing solution before very much progress can 

 be made are these : (1) Do four 5-carbon units combine ** head to 

 tail " to form a C20 compound which then dimerises " tail to tail ", 



or do two 5-carbon units first react 

 ** tail to tail " and the molecule is then 

 built up by '* head to tail " condensation 

 at either end of this 10-carbon unit : 

 (2) Is the 5-carbon unit saturated 

 (? isovaleraldehyde) or unsaturated 

 (? p-methylcrotonaldehyde). 



14-0 



120 



^ 



^ 100 



I 

 r 80 



60 



40 



20 



/-/ 



I / 



7 9 



Age of culture (days) 



Fig. 19. — The % of p-carotene produced by 

 Phycomyces blakesleeanus transferred 

 to a medium containing 1% glucose 

 and either L-valine, L-leucine, or 

 L-asparagine as nitrogen source. 

 {From Goodwin, T. W., and Lijinsky, 

 W.{I95I) 6/ochem. J., 50, 268.) 



O O leucine. 



# # - - - # valine. 



#< # # asparagine. 



112 



