CAROTENOIDS IN PLANTS 



rather than when the carboxyl group of the acetate is labelled, because 

 if acetate alone were required for the synthesis condensations would 

 probably eliminate some carboxyl atoms. Recent experiments have 

 shown that the addition of 2- ^ *C-acetate to Phycomyces metabolizing 

 glucose, results in rapid evolution of active CO 2, suggesting randomiza- 

 tion of the label between the methyl and carboxyl of acetate. 



Effect of altering the Nitrogen Source in the Medium. Schopfer * ° 

 reported that glycine and asparagine were equally effective in promot- 

 ing carotene synthesis in Phycomyces, but that ammonium nitrate was 

 better than either. Garton et al. 1 », 2 reinvestigated these compounds 

 in detail and also tested valine, leucine, woleucine and alanine as well 

 as ammonium acetate. In media containing one of these substances 

 (at a level of 0-034 per cent, of nitrogen) and 3 per cent, of glucose, 

 carotenogenesis was essentially the same in all cases except in the 

 medium containing glycine : this amino acid stimulated carotene 

 synthesis, producing mycelia containing 200 mg./lOOg. dry wt. of 

 carotene compared with the usual level of 120-140 mg./ 100 g. dry wt. 

 It is interesting to note that valine under these conditions which could, 

 theoretically, give rise to ^-methylcrotonic acid, a possible repeating 

 unit in carotenoid synthesis {see p. 64) in this manner : 



;CHCHNH,.COOH —^ ^"^^h c COOH ^^1^ "^ CH CHOH.COOH iM^ ^"';CH.CH COOH 

 CH| CH3 CH3 CHi' 



was inactive. Leucine, which could also be considered to be able to 

 provide an active residue, was also without effect. 



Recent experiments by Goodwin and Lijinsky^^ have shown that 

 in media containing insufficient glucose for optimal carotene produc- 

 tion, but sufficient for reasonable mycelial growth, leucine and valine 

 stimulated carotenogenesis by as much as 400 per cent, compared with 

 asparagine or glycine (Fig. 19). Leucine is more effective than valine. 

 It appears from these experiments that normally (on a medium con- 

 taining adequate glucose, 3 per cent.) Phycomyces utilizes the products 

 of glucose fermentation for carotene synthesis (for the pigment first 

 appears in quantity only after the mycelium is fully grown and in the 

 mycelial layer in contact with the medium where the conditions must 

 be nearly completely anaerobic) ^ ^ and that on a low glucose medium 

 (1 per cent.), these being in short supply, the fungus utilizes the 

 deaminated products of leucine and valine as the building unit. This 

 repeating unit must thus have a 5-carbon skeleton with the following 



configuration : — )C-C-C- and it is easy to visualize how this can 



c/ 



111 



