1016 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



CHOLESTBBIN. 



Cholesterin, C 26 H 43 OH. This is found in all animal and vegetable cells and in the 

 milk. 1 It is especially present in nervous tissue, in blood-corpuscles, and in the bile. It may 

 be prepared by dissolving gall-stones in alcohol, from which solution the cholesterin crys- 

 tallizes on cooling in characteristic plates. It is insoluble in water or acids, but soluble 

 in the biliary salts, alcohol, and ether. It is probably excreted unabsorbed in the feces. 

 Cholesterin feels like a fat to the touch, but is in reality a monatomic alcohol. With con- 

 centrated sulphuric acid it yields a hydrocarbon, cholesterilin, C 26 H 4 j, coloring the sul- 

 phuric acid red (Salkowski's reaction). Iso-cholesterin, an isomere, is found combined as 

 an ester with fatty acid in wool-fat or lanolin. The physiological importance of cholesterin 

 is unknown. 



THE PROTEIDS. 



Consideration of the proteids from a purely chemical standpoint is impos- 

 sible, for their composition is unknown. There exist only the indices of com- 

 position furnished by the products of cleavage and disintegration. Bodies at 

 present classed as individuals may sometimes be shown to be identical, with 

 characterizing impurities. It remains for the chemist to do for the proteid 

 group what Emil Fischer with phenyl-hydrazin has accomplished for the 

 sugars. 



As a characteristic proteid, egg-albumin may be mentioned. Proteid forms 

 (after water) the largest part of the organized cell, and is found in all the 

 fluids of the body except in urine, sweat, and bile. Proteid contains carbon, 

 hydrogen, nitrogen, oxygen, sulphur, sometimes phosphorus and iron. 



General Reactions. A neutral solution of proteid (with the exception of 

 the peptones and proteoses) is partially precipitated on boiling, and is quite 

 completely precipitated on careful addition of an acid (acetic) to the boiling 

 solution. Proteids are precipitated in the cold by nitric and the other com- 

 mon mineral acids, by metaphosphoric but not by orthophosphoric acid. 

 Metallic salts, such as lead acetate, copper sulphate, and mercuric chloride, 

 precipitate proteid ; as do ferro- and ferricyanide of potassium in acetic-acid 

 solution. Further, saturation of acid solutions of proteid with neutral salts 

 (NaCl, Na 2 SO 4 , (NH 4 ) 2 SO 4 ) precipitates them, as does likewise alcohol in 

 neutral or acid solutions. Proteid is also precipitated by tannic acid in acetic- 

 acid solutions, by phospho-tungstic and phospho-molybdic acids in the presence 

 of free mineral acids, by picric acid in solutions acidified by organic acids. 2 



Of the color-reactions the action of Millon's reagent has been described 

 (see p. 992). Soluble proteids give the biuret test (see p. 1011). With concen- 

 trated sulphuric acid and a little cane-sugar a pink color is given when proteid 

 is present (see p. 988). Proteid heated with moderately concentrated nitric 

 acid gives yellow flakes, changing to orange-yellow on addition of alkalies 

 (xantho-proteid reaction). Proteid in a mixture of one part of concentrated 

 sulphuric acid and two parts of glacial acetic acid gives a reddish-violet color 

 (Adamkiewicz), a reaction accelerated by heating. Finally, proteid dissolves 



1 Schmidt-Muhlheiru : Pftiiger's Archiv, 1883, Bd. 30, p. 384. 



2 The above list is given by Hammarsten, Physiological Chemistry, translated by Mandel, 

 p. 18. 



