138 



TERPENOIDS AND STEROIDS 



CH2OH 



betulin (bark of Betula alba) 



STEROLS 



The fundamental steroid nucleus is the same as that of lanosterol and other tetra- 

 cyclic triterpenoids, but only two methyl groups are attached to the ring system, at posi- 

 tions 10 and 13. The eight-carbon side chain found in lanosterol is also present in many 

 steroids, especially from animal sources; but most plant steroids have one or two addi- 

 tional carbon atoms. The name "sterol" applies specifically to steroid alcohols; but since 

 practically all plant steroids are alcohols with a hydroxy 1 group at C-3, they are frequently 

 all called sterols. The steroid numbering system is as follows: 



Steroids occur throughout the plant kingdom as free sterols and their esters in many 

 lipids and as more complex derivatives to be discussed in sections to follow. In plants 

 they have no known function although they have profound importance in animal metabolism 

 as hormones, coenzymes, bile acids and provitamin D. Certain animal steroids have 

 been shown to influence plant growth strongly, but whether presently unknown plant ster- 

 oids may act similarly is an open question (17). The steroid alkaloid tomatine (see below) 

 may be somehow related to flowering since its concentration may be photoperiodically con- 

 trolled in the same way as the flowering response (18). Cholesterol, the most common 

 animal steroid, has not been found in plants; but there is no sharp distinction between 

 plant and animal steroids, since other members of the group are found in both kingdoms. 

 Pregnane type steroids have recently been found in plants (19). 



Classification of sterols is done on the basis of their optical rotations. This is not 

 purely arbitrary but reflects important structural differences as summarized in Table 10. 



