158 CARBON METABOLISM III 



Other compounds of this type include the closely related poly- 

 porenic acids A, B, and C, isolated from Polyporus spp. and Poria 

 cocos (91, 92, 151, 154, 257, 260). The triterpenoids are biosyntheti- 

 cally related, as judged by the occurrence of all three polyporenic acids 

 in one species (154) and by the fact that three strains of Poria cocos 

 form polyporenic acid B and a fourth forms eburicoic acid (151). In 

 Polyporus eucalyptorum eburicoic acid constitutes at least 20 per cent 

 of the dry weight of the mycelium (220) and must therefore be con- 

 sidered a major metabolite. The chemistry of these compounds has 

 been reviewed by Jones and Halsall (306); triterpenoids occur also as 

 lichen products (24). 



The antibiotics cephalosporin P 1 and helvolic acid are tetracyclic 

 compounds and may be related to the steroids, but their complete 

 structure is not known (100). 



The Metabolism of Carotenoids. The carotenoid pigments, which 

 appear among the unsaponifiable lipids, include: (1) the true caro- 

 tenes, e.g., /3-carotene and lycopene, and (2) oxygen-containing deriva- 

 tives, e.g., rhodoviolascin. All may be considered as built up of 



C 



I 

 eight isoprene units, the carbon skeleton of which is: C=C — C=C. 



In addition to the colored carotenoids, colorless polyenes are found in 

 the fungi and will be considered with the pigments. 



The formula of ^-carotene illustrates the essential features of the 

 carotenoid pigments: 



CH3 CH3 



\ / 



C GH3 GH3 



/ \ I I 



CH 2 C— CH=CH— C=CH— CH=CH— C=GH— CH=CH— CH 



I II 



CH2 C — CH3 



\ / 



CH 2 



CH3 CH3 



\ / 



GH3 CH3 C 



I I / \ 



=C— CH=CH— CH=C— CH=CH— C CH- 



II I 



CH3 — C CH2 



\ / 

 CH 2 



