2 I. GENERAL CHARACTERISTICS AND CLASSIFICATION 



compared with protein, but which could not be separated into their several 

 components by the most ingenious devices then available. Even as re- 

 cently as 1917, Bull"* noted that only approximately 150 articles on lipids 

 had appeared in the literature, as estimated from the references to them in 

 Chemical Abstracts. However, with the widespread upsurge in research 

 which has followed World War II, somewhat over 783 references are in- 

 cluded in the 1947 volume of Chemical Abstracts. New methods have now 

 become available for their separation and purification, such as molecular 

 distillation. This procedure has been especially valuable for the prepara- 

 tion of several components of the non-saponifiable residue in especially pure 

 condition and in sufficient yields to facilitate further study. The appli- 

 cation of x-ray methods for the investigation of molecular structure has 

 been particularly useful. Much of the more recent work on the qualitative 

 identification and quantitative estimation of particular fats is traced to the 

 discovery of the specific absorption curves which these compounds possess, 

 in the infrared as well as in the ultraviolet region of the spectrum. 



Another stimulus which has focused interest on the fat field has been the 

 discovery of the pronounced physiological activity of a variety of sub- 

 stanceswhich maybe regarded as impurities of fat. These interesting prod- 

 ucts can be separated and concentrated in the non-saponifiable extract. 

 They presumably occur in natural fats, because the triglycerides are excel- 

 lent solvents for them, while they are completely insoluble in water. In 

 this group of substances the most interesting, undoubtedly, are the fat- 

 soluble vitamins. A, D, and E, However, e(iual importance should be 

 centered on the carotenoids, which maj^ or may not serve as provitamins A. 

 Similarly, the sterols are important not only as possible precursors of 

 vitamin D, but also as components of all plant and animal cells. Because 

 of their widespread distribution and the importance of hormones and of 

 other compounds with the steroid nucleus, studies in this field have been 

 especially active. 



2. Definitions 



The term lipid^ has gained much favor during the last few years to define 

 fats and fat-like substances. Most biochemists now consider it synony- 

 mous with the term lipoids (used by the English and Germans) or lipins, 

 although the latter term was formerly applied only to such complex lipids 

 as phospholipids and galactolipids.® 



The lipids include all those substances which are insoluble in water but 

 soluble in the so-called fat solvents (diethyl ether, petroleum ether, chloro- 



< H. B. Bull, The Biochemistry of the Lipids, Wiley, New York, 1937. 

 * In current American literature, lipid is frequently spelled lipide. However, in 

 conformity to the Hilditch usage, lipid will be the form used throughout this monograph. 

 « I. Smedley-MacLean, Ann. Rev. Biochem., 1, 135-150 (1932). 



