464 



PROTOPLASM 



Biologically, they may be regarded as of two groups — the plant 

 sterols, or phytosterols, and the animal sterols. Cholesterol is 

 the chief sterol of animals and until recently was thought to be 

 the only one, but others have since been found (e.g., ostreosterol 

 in oysters; agnosterol and lanosterol in wool fat). As cholesterol 

 does not occur in plants and as the several plant sterols are not 

 common in animals, there appears to be a distinction between 

 the plant and animal kingdoms in their sterol content. There 

 is one sterol that is found in both kingdoms, viz., ergosterol; it is 

 essentially a plant sterol but always occurs with cholesterol in 

 animals. Cholesterol may apparently be synthesized by animals, 

 but ergosterol must be obtained by animals from plants. 



Cholesterol was one of the earliest known sterols. It consists 

 of three rings of six carbons each, one of five carbons, a side 

 chain of eight carbons, two methyl groups, and one molecule of 

 water, thus: 



which yields the formula C27H46O. Among the 1,024 possible 

 isomeres of cholesterol, 2 are known with certainty, and several 

 others postulated. (A number of the supposed isomeres of 

 cholesterol are probably distinct sterols, as is true of "iso- 

 cholesterol," now known as lanosterol.) 



Ergosterol is second in importance among the sterols so 

 far as our knowledge goes. It appears always to accompany 

 cholesterol. Its formula C2gH440 presents an interesting prob- 

 lem, viz., how to fit an extra carbon into the structural formula 

 for cholesterol (as given above). Ergosterol was discovered by 

 the French apothecary Tanret (about 1888). He isolated it 

 from ergot of rye, Claviceps purpurea. It is of common occur- 

 rence in fungi; its present commercial source, however, is yeast. 

 (Three or more sterols occur in yeast.) 



Ergosterol has come into prominence of late because of its 

 antirachitic properties when irradiated with ultraviolet light. 



