jgo TERPENOIDS AND STEROIDS 



The K vitamins and ubiquinones are two naturally occurring groups of quinones 

 which are assuming great importance in the oxidation-reduction reactions of respiration 

 and photosynthesis. Presumably they act as hydrogen carriers through reduction to hy- 

 droquinones (47,48). The different forms of ubiquinone (or coenzyme Q) are designated 

 by subscripts indicating the number of isoprene residues in the side chain (e. g. UQg). 

 They are found in mitochondria and may be characterized by their ability to restore suc- 

 cinic-cytochrome c reductase activity which has been lost by extraction with acetone. 



A benzoquinone isolated from chloroplasts has been named plastoquinone and is pre- 

 sumed to have a function in photosynthesis (49). Recently two new quinones have been 

 found from the same source, and they are apparently homologues of the original plasto- 

 quinone (50). For several years there has been controversy regarding the presence or 

 absence of vitamin Kj in chloroplasts. Problems of analysis have recently been solved 

 and Kj definitely shown to be present (51). There are many other naturally occurring 

 quinones of unknown function and without isoprenoid side chains. They are discussed in 

 Chapters 4 and 6. 



^"3 CHo CH3 



CH2CH=CCH2(CH2CH2CHCH2)3H 



vitamin Kj 



CH3 



ubiquinoneg 



CHq 

 ■ ■ II I 



CH30^^^^^(CH2CH=CCH2)9H 

 



CHq 



(CH2CH=CCH2)9H 



plastoquinone 



The pyrethrins are a very valuable group of insecticides found only in the flowers 

 of some members of the genus Chrysanthemum (52). The most important commercial 

 source is C. cinerariaefolium. Four substances of very similar structure have been 

 isolated. They are all esters; and while the acid portion of the molecule is clearly an 

 isoprenoid structure, the keto-alcohol part is not derived either from mevalonic acid or 

 acetate (53). 



