232 BIOCHEMICAL SYSTEMATICS 



and carotenoids (Wagner and Folkers, 1961). Mevalonic acid itself 

 apparently originates through acetate condensation. As was indicated 

 in a previous section (Chapter 11) mevalonic acid occupies a focal 

 position in isoprenoid synthesis somewhat analogous to that of 

 shikimic acid in aromatic synthesis (Fig. 13-1). 



CH3 



HOCH2CH2C— CH2COOH 



OH 



mevalonic acid 



Among the simplest terpenes, the relationship to isoprene is 

 evident at once. For example, note the monoterpene, myrcene, illus- 

 trated below: 



CH2 



A 



H2C CH 



H2C CH2 

 CH 



II 



H3C CH3 



myrcene 



The formula above, also of myrcene, represents the type 

 customarily used to represent terpenoid compounds. 



In addition to differences in the position of double bonds and 

 degree of hydrogenation of a given basic terpene structure, alcohol, 

 aldehyde, ketone, and acid derivatives of simple aliphatic terpenes 

 exist. Geraniol, for example, is a widely distributed alcohol of this 

 type. Furthermore, ring closure provides for simple cyclic structures. 

 A common example of such a compound is phellandrene: 



/^-phellandrene 



1 Although Stanley (1958) has reported incorporation of C^* labeled mevalonic 

 acid into a-pinene of Pinus attenuata, Battaile and Loomis (1961) have evidence that 

 mevalonic acid is not incorporated into mint terpenes. These latter investigators found 

 C^^ from mevalonic acid in carotenoids and other compounds in the plant. Therefore, one 

 cannot readily discount their evidence concerning terpenes on the grounds that it is negative. 



