22 THE CHEMISTRY AND PHYSICS OF THE CELL 



Insoluble proteins, or bodies resembling the coagulated proteins in their lack of 

 solubility in various fluids, are left behind after the other proteins have been ex- 

 tracted from the cells. Their significance is not known : whether to a large extent 

 artificially produced or whether a normal structural element of the cell. 



Fats and Lipoids (Lipins)^ 



Lipoids is a term in common use but of indefinite significance; 

 most usually it comprehends the intracellular substances which are 

 soluble in ordinary fat solvents, but which are not simple fats or fatty 

 acids, lecithin and cholesterol being the most important of the lipoids. 



For the entire group of fats and lipoids the term lipins has been pro- 

 posed by Gies and Rosenbloom.^ Lipoids and ordinary fats, that is, 

 lipins, occur in all cells, but their demonstration is not always readily 

 possible. The microscopic appearance of a cell, even when special 

 stains for fat are used, gives no correct idea of the amount of lipins 

 actually present. Thus normal kidneys contain 15 to 18 per cent, 

 in their dry substance, but none of this can be detected readily with 

 the microscope. A kidney which seems microscopically the site of 

 marked fatty degeneration may show no more fat when examined chem- 

 ically than a normal kidney, which in section appears to be quite free 

 from fat. This is because some of the intracellular fat is so bound, 

 chemically or physically, with the proteins that it cannot be seen, nor 

 can it be stained by the dyes ordinarily used for that purpose; only 

 when degenerative changes of certain kinds have liberated it from com- 

 bination does it become visible and stainable bj^ ordinaiy methods 

 (Rosenfeld). By the special fixation method of Ciaccio the fatty com- 

 pounds of even normal cells may be made stainable (Bell),^° showing 

 that the so-called masked fat is really in a not altogether invisible form. 

 Whether the intracellular fat has any function other than that of 

 serving as a food-stuff is not known, but there can be no question of 

 the importance of the phosphorized fats, or phospholipins. 



Phospholipins are primary cell-constituents and are probably important both 

 in metabolism and physically. Hammarsteu regards them as concerned in the 

 ijuilding up of the nucleus. As will be shown later, manj- of the most essential 

 physical properties of the living cell depend upon the presence in it of lipoids, of 

 which phosphatids are apparently the chief. Of the ether-soluble substances in 

 the heart, for example, 09 to 70 per cent, are phosphatids, 8 per cent, of the dry 

 weight of the myocardium. Many different substances have been described as 

 phosphatids, but the chemical identity of but few is sufficiently established. Of 

 these the most imjjortant are lecithin and ccphalin, which are most intimately 

 associated. 



There are several possible varieties of lecithin, depending upon the fatty 



* Full discussion in "Lecithin and Allied Substances (The Lipins)," by Hugh 

 MacLean, Monographs on iJiociliemistry, London, 1918. 



" MacLean uses "li))in" to include "subslances of a fat-like nature yielding on 

 hydrolysis fatty acids or derivatives of fatty acids and containing in their nu)lecule 

 either N, or N and 1'. As there is need for a term covering the fats, phospliatids, 

 cholesterols and related Ixxlies, the suggestion of C.ie.s .-uid lUi.senbloom is followed 

 for tlie i)resent in tliis i)ook, and tin* word lipin used witli the broader significance. 



"> Jour. Med. lies., 1911 (1-9), SiW. 



