CHEMICAL STRUCTURE 



is that of lanosterol, but in addition they possess the substituent 

 at C24 which is typical of ergosterol. The acids isolated from 

 the molds probably represent secondary oxidation products 

 rather than metabolic intermediates; yet the disclosure of this 

 structural type strongly suggests that the carbon skeleton of 

 lanosterol can be enlarged by attachment of carbon atoms to the 



CH3 



OH OH 



TATUM AND ADELBERG, 1951 



C29 STEROLS 



(STIGMASTEROL, ETC ; 



POLYPORENIC A, ETC 



Cge STEROLS (ERGOSTEROL) 



Fig. 7. Postulated mechanism for the synthesis of ergosterol and plant sterols 



from lanosterol. 



aliphatic side chain. The carbon source for the alkyl substit- 

 uents of the plant sterols (stigmasterol or sitosterol) and of 

 ergosterol is obscure, but it is not unreasonable to presume that 

 they are derived from acetate also. There exists one biochemi- 

 cal process for the branching of carbon chains which may have 

 some bearing on the synthesis of the C28 and G29 sterols. The 

 amino acids valine and isoleucine are the /3-methyl and the 



489 



