112 PROTEIDS. 



chromatin fibers of the nucleus ; those which occur in the nucleoli 

 contain less nucleic acid. 



3. Pseud onucleins. These are sometimes called paranucle- 

 ins, and are obtainable from the nucleoproteids, such as caseinogen 

 and vitellin. They yield none of the bases as do the true nucleins, 

 but only proteid and phosphoric acid. 



The nucleoproteids are divided into two groups : 1. Those 

 which yield true nucleins on gastric digestion, and to which Ham- 

 marsten restricts the name "nucleoproteids;" and 2. Those which 

 yield pseudonucleins on gastric digestion, called by Hammarsten 

 " nucleo-albumins." In this latter group are caseinogen and vitellin. 

 To make this remme complete, mention should be made of the 

 phospho-glucoproteids. A glucoproteirl is a compound of proteid 

 with a carbohydrate, and includes mucins among other substances. 

 From mucins a carbohydrate may be obtained called ^animal gum, 

 which when acted upon by a dilute mineral acid is converted into 

 a reducible but non-fermentable sugar having the formula C 6 H 12 O 6 . 

 Most of the glucoproteids contain no phosphorus, but some do, 

 and these constitute the phospho-glucoproteids. There is some 

 evidence to show that from many of the proteids (acid- and alkali- 

 albumin, serum-albumin, serum-globulin) a reducing substance may 

 be obtained, which may be a carbohydrate. 



Caseinogen. This was formerly regarded as an alkali- 

 albumin, but is now placed among the nucleoproteids ; and if we 

 accept the classification of Hammarsten, it would be placed among 

 the nucleo-albumins, for the reason that on gastric digestion it 

 yields pseudonuclein. 



Caseinogen is the most abundant proteid of milk, the two other 

 proteids being lactoglobulin and lactalbumin, and may be obtained 

 from it by saturation with sodium chlorid or magnesium sulphate, 

 or by half-saturation with ammonium sulphate. It is not coagu- 

 lated by heat. Human caseinogen has the following percentage- 

 composition : C, 52.24; H, 7.31; N, 14.9; P, 0.68; S, 1.17; 

 O, 23.66 ; and yields no pseudonuclein on gastric digestion. 

 When acted upon by rennet caseinogen coagulates, becoming 

 casein, which is soluble in dilute alkalies, such as lime-water. 

 Rennet is obtained from the stomach of the calf, and owes its 

 property of coagulating caseinogen to the enzyme rennin, also 

 called chymosin. Upon the addition of rennet to cows' milk a 

 curd or clot is formed, which consists of casein and the fat of the 

 milk ; the liquid portion of the milk, after the curd is formed, is 

 whey, consisting of water holding in solution the proteids, lacto- 

 globulin and lactalbumin, lactose, and the salts. Another proteid, 

 whey-proteid, is produced from the decomposition of that portion 

 of the caseinogen which is not changed into casein. The curd of 

 human milk is of a softer and more flocculent character than that 

 of cows' milk, and to render the curd of the latter more like that 



