NORMAL FAT METABOLISM 185 



interest from a biochemical point of view because of the potential optical 

 activity of many of these mixed esters, since optical activity is recognized 

 as a property closely connected with life processes. Thus 



H H 



O-R! HO O-Ro (Rl Ra R: < ^S Afferent 



fatty acid radicals) 



HC O R 2 HC O R 3 



H H 



should from the structure be optically active. Up to the present time no 

 optically active fats have been found in nature or been prepared syn- 

 thetically, which may mean merely that present day methods of prepara- 

 tion and separation of isomers are not adequate for the purpose. On the 

 other hand many of the phospholipoids are optically active and contain 

 different fatty acids in combination, and since there is good reason to 

 believe that the phospholipoids are stages in the metabolism of the fats 

 and are known to be constituents of living tissues, the inference is that 

 while the fats themselves may not take part in life processes they are 

 readily changed into substances which do. 



Waxes. Distinguished from the fats by the fact that the alcohol in 

 combination is not glycerol. These are substances widely distributed in 

 nature but in amounts much smaller than the fats. They are characterized 

 in general by great chemical inertness; they are much more difficult to 

 oxidize or to hydrolyze either by enzymes or other agents. The con- 

 stituents of the waxes have been completely worked out in but few cases, 

 so that our knowledge of the chemistry of these substances is very frag- 

 mentary. The alcohols found in combination in the waxes are mostly of 

 largo molecule (see under Sterols), and the fatty acids are also generally 

 large molecular and either saturated or containing hydroxyl groups. Com- 

 mon examples of the waxes are : 



Beeswax. A mixture of many substances of which the best-known 

 ones are esters of myricyl (C 3 oH G1 OH) and ceryl (C 2C H 53 OH) alcohols 

 wirh palmitic (C 1G H 32 O 2 ), cerotic (C 2G H 52 O 2 ) and melissic (C 30 H CO O 2 ) 

 acids and much free cerotic acid. 



Cetin. The ester of cetyl alcohol (C 1G H 33 OH) and palmitic acid. 



Wool Wax (Lanolin). Contains esters of cholesterol derivatives with 

 various fatty acids. 



Cholesterol esters of palmitic and oleic acids are present in blood. 



Compound Lipoids. Phospholipoids. Compounds of the fatty acids 

 and glycerol containing phosphoric acid and nitrogen. They are widely 

 distributed in nature, being constant constituents of living cells. They 



