42 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



absence of double bonds, position of methyl and ethyl groups, etc.). Simi- 

 larly, it is the character of the side-chain which differentiates the sterols of 

 group II (cholesterol, camposterol, clionasterol, etc.). 



2. Bile Acids 



These substances, so named because they were isolated from the bile of 

 vertebrate animals, are cousins to the sterols of group IV and they possess 

 two methyl groups and at least one hydroxyl in the ring system. Their C-5 

 side-chain ends in a carboxyl group. Cholic acid is an example : 



CH:, 



CH, — COOH 



OH 



CH3 c— CH, 



HO 



Cholic acid 



3. Other Natural Steroids 



There exist in the biosphere steroids having a chain of only two carbon 

 atoms attached at position 17. These are the 21 -carbon steroids. They are 

 derivatives of pregnane. Progesterone and the mammalian adrenocorti- 

 costeroids fall into this category. 



Other steroids do not have a side-chain attached to carbon 17. They 

 usually carry two methyl groups and are C-19 steroids derived from aetio- 

 cholane. Among them are testosterone and those steroids which have a 

 keto group in position 17 and are consequently called 17-ketosteroids; they 

 are often present in mammalian urines. 



CH, 



Aetiocholane 



