KONRAD BLOCK 



tions. Whether or not the details or even the principle of the 

 suggested scheme are correct, it seems probable nevertheless that 

 in the conversion of lanosterol to cholesterol carboxylic acids will 

 be found as intermediates. If steroid carboxylic acids — other 

 than the bile acids — have not hitherto been detected in animal 

 tissues, it can be pointed out that a thorough search for com- 

 pounds of this type has not so far been made and that in partic- 

 ular those tissues remain to be examined which are most active 

 in steroid biogenesis. 



The Biosynthesis of Myco- and Phytosterols 



It is an obvious defect of current biogenetic schemes that 

 they neglect to provide for the formation of those numerous 

 sterols which carry a methyl or ethyl substituent in the aliphatic 

 side chain, and which are principally found in plants or micro- 

 organisms. Experimental work to date has been limited to the 

 demonstration that acetic acid is the carbon source of ergosterol 

 (17), and that the distribution of acetate in a portion of the 

 aliphatic side chain follows the pattern found in cholesterol 

 (12). If the biogenesis of ergosterol and of the plant sterols were 

 to take the same general course as that of the sterols of animal 

 tissues, identical pathways and intermediates up to the lanos- 

 terol stage would be anticipated. The occurrence of squalene 

 in plants and microorganisms has been claimed but has not been 

 adequately documented. Lanosterol, on the other hand, is 

 invariably found to be associated with ergosterol in yeast. The 

 structural features of ergosterol and of the typical plant sterols 

 which so far lack a rational biogenetic explanation are the ad- 

 ditional branchings at the 24 position of the aliphatic side chain. 

 In this regard the structures of several mold products are of ut- 

 most interest. These are the polyporenic and eburicoic acids 

 (Figure 7) from molds such as Polyporus betulinus, identified by 

 the investigations of Jones and co-workers and in the Zurich 

 laboratories as steroid carboxylic acids with 31 carbon atoms 

 (reviewed in ref. 13). Their carbon skeleton and configuration 



488 



