PROTEINS AND THEIR CLASSIFICATION. 1025 



in the spermatozoa of different animals, and are, therefore, designated accord- 

 ing to the zoological name of the fish from which they arise, as salmin, sturin, 

 clupein, scombrin, etc. They show a biuret reaction, but in most cases fail to 

 give Millon's reaction. On hydrolysis they give spht products, which consist 

 chiefly of the so-called diamino-bodies (arginin, histidin, lysin) rather than the 

 monamino-acids. Some of the latter may occur, however, such as alanin, 

 serin, aminovalerianic or «-pyrrollidin-carboxylic acid. The protamins all give 

 an alkaline reaction, form salts with acids, and are precipitated easily. Their 

 molecular structure is relatively simple. Salmin is given the formula C30H57- 

 (',7115. The molecule contains no sulphur and is characterized also by its 

 large percentage of nitrogen. Protamin must be regarded as the simplest 

 form of protein occurring normally in the animal body, a protein in which 

 many of the groupings, such as cystin, tyrosin, carbohydrates, found in the 

 usual protein molecule are entirel.y lacking and in which the basic groupinga 

 (arginin) predominate. The histons form a series of compounds intermediate 

 in many ways between the protamins and the usual proteins. The reaction 

 usually considered as characteristic of the class is that they are precipitated 

 by ammonia. They are precipitated also by the alkaloidal reagents — e. g., 

 phosphotungstic acid — in neutral solutions. Ordinary proteins give a pre- 

 cipitate with these reagents only in acid solutions, while the protamins give 

 one even in alkaline solutions. Protamins, histons, and the usual proteins 

 form a series, therefore, in which the basic reaction is less and less marked. 

 The best known of the histons is the globin obtained from hemoglobin ; an- 

 other form has been obtained from the nucleohiston in the white corpuscles, 

 from the spermatozoa of mackerel (scombron), codfish (gadushiston), sea- 

 urchin (arbacin), and frog (lotahiston). They do not occur free in the liquids 

 or tissues of the body, but in combination, as in the case of hemoglobin. 

 They give the biuret reaction, a faint Millon reaction, and also respond to the 

 tests for sulphur. The products obtained by their hydrolytic cleavage are 

 much more numerous than in the case of the protamins — a fact which would 

 indicate that their molecular structure is correspondingly more complex. 



The Conjugated Proteins. — The chromoproteins or hemoglobins may be 

 defined a,> consisting of a simple protein in combination witii a pigment 

 grouping, such as occurs in the case of hemoglobin. A number of such com- 

 pounds are known — hemoglobin, hemocyanin, hemerythrin, chlorocruorin — 

 all characterized physiologically by the fact that they serve to transport 

 oxygen from the air or water to the tissues. On boiling, heating with alkalies 

 or acids, etc., they readily decompose into their constituent parts (see Blood). 

 Glycoproteins are compounds of a carbohydrate group with a simple protein. 

 Numerous bodies have been put in this class; some of them contain phos- 

 phorus (phosphoglucoproteins). Those free from phosphorus fall into two 

 divisions: one, the mucins, which on decomposition yield the carbohydrate 

 group in the form of an amino-sugar (glucosamin), and one, the chondropro- 

 teins, found in the connective tissues and in the pathological substance known 

 as amyloid, which yield their carbohydrate group in the form of chondroitin- 

 sulphuric acid (CisHjjNSO,,). True mucin is obtained from the secretion 

 of the salivary glands and the mucous glands of the various mucous mem- 

 branes. The nudeoproteinfi constitute the most interesting of the group 

 of compound proteins. They are recognized as forming an important con- 

 stituent of the cell nuclei. They may be defined as consisting of a compound 

 of simple protein with a nucleic acid. In the nuclei (head) of spermatozoa 

 the compound, in some cases at least (fishes), contains a nucleic acid and a 

 protamin. In other cases the protein constituent is more complex. On 

 digestion with pepsin-hydrochloric acid the more complex nucleoproteins 

 split, with the formation, first, of a protein substance and a sim^pler nucleo- 

 protein, richer in phosphorus and designated as a nuclein. On further 

 decomposition this latter yields a nucleic acid. Nucleic acid is, therefore, 

 the characteristic constituent, and a number of different forms have been 

 described, all rich in phosphorus, such as thymonucleic acid, salmonnucleic 

 acid, guanyhc acid, etc. Levene and Jacobs have shown that the various 

 nucleic acids are constructed on a general type which consists of a phosphoric 

 acid group linked to a nitrogenous base by means of a carbohydrate group. 

 This latter group is d ribose, one of the pentoses. Compounds of this type 



