Page:The New International Encyclopædia 1st ed. v. 16.djvu/536

* PROTEINS. 468 PROTEINS. intestinal bacteria. However, too little is known of the functions of the nonprolcids to warrant very definite assumptions, l-'roni what has been said it is evident that protein is essential for building and repairing the body and that it may also be burned in the body to yield energy, the amount being very often estimated as 9.3 calories per gram digestible protein. Thus protein has two functions, whereas fat and carbohydrates have but one, namely, to serve as sources of energy. See Food. As noted above, proteids are vi<iely distrib- uted in proilucts of animal and vegetable origin. Oreen plants build up proteids from simple bodies, obtaining the necessary nitrogen from complex nitrogenous organic compounds occur- ring in the liumus of the soil, from ammonium .salts, from nitrates, and from the ilitrogcn of the air. Among the higher plants it seems to be principally the Leguminosip which can utilize free nitrogen. This tbey do by the aid of the micro-organisms present in the tubercles on their roots. (See (Jreen Manuring.) Amides and asparagin are intermediate bodies in the for- mation of proteids in plants. It may be said in general that animals derive their proteids pri- marily from vegetable sources. The changes which proteids luidergo before being added to the substance of the body and- the possibility of the forTuation of proteids from simi)le. nitrogenous substances in the body are questions which are not settled or well understood. Classification of Proteids. The following classification of proteids. which is an expansion of that given above, is largely that proposed by Halliburton, and is believed to be applicable to both animal and vegetable proteids. It is based chielly upon differences in solubility of the vari- ous bodies : .lburain8 (Serum albumin, epfp^ albumin, lact - albumin, gluten ami Kliaden of wheat, zeiu of maize, etc.) Globulins (Fibrinogen, Berum globulin or paraglobulin, egg globulin from white of egg. m.vosi- nogen frijni muscle, cr,vstallin from the crystalline lens, wheat globulins, and other vegetable vitellins. ) Proteoses (Albumoses, globuloses. vitel- loses, etc.) Peptones (Diffusible end-products of di- gestion.) Those coagulated by heat. Those coagulated by ferments (Fibrin, m.vosin, casein, etc.) Proteids ■ Simple proteids Coagulated Proteids Compound proteids Haemoglobin Gluco-proteids (Mucin and mucoids.) Nucleins and uucleo-proteids CoUagens or gelatinoids * (Collagen, casein, gelatin, elastiu, keratin, and chitin.) Protamines (Histidin, arginin, and lysin.) • Halliburton and some others call this group ' albumi- noids,* This term is otherwise applied b,v many investi- gators and its use as imlicat^d leads to considerable confusion. The t^rm ' collagens or gelatinoide ' is perhaps more commonly used in the United States. The proteids possess a number of general char- acteristics. All are insoluble in alcohol and ether. Water will dissolve some, but not all. Many of those insoluble in water are soluble in weak saline solutions, and some are solul)le and others insoluble in concentrated saline solutions. With the aid of heat all proteids are soluble in concentrated mineral acids and alkalis, but are transformed as well as dissolved. Proteids are soluble by the aid of ferments, as those in gas- tric and pancreatic juices, but in this case they also undergo a change, adiling water and breaking down into proteids of smaller molecular weight known as peptones, intermediate bodies formed in the process being called 'proteoses' or 'all)U- moses.' ilany ])roteids soluble in water or saline solutions, especiaHv the albumins and globulins, are rendered insolulile or are coagulated when heated, A familiar example of this is the coagu- lation of egg white in cooking. The temperature required for coagulation dill'ers with different proteids. With the exception of peptones, pro- teids are colloids, and pass with difficulty, if at all, through animal membranes. Some animal proteids like luenioglobins are crystallizable. A considerable number of crystallizable proteids have been prepared from seeds, nuts, etc., by Osljorne and his associates and others. As re- gards the action of |)roteids on polarized light, all the proteids are hevorotatory. the amount of rotation varying with the different kiniis. There are several color reactions characteristic of proteids. Some of the principal ones follow; With strong nitric acid, the proteids when heated to boiling give yellow flakes or a yellow solution, this being known as the xanthoi)roteic reaction. When a proteid in the solid state or in solution is boiled with a solution of mercuric nitrate in nitric acid containing some nitrous acid (Mil- Ion's reagent) a red coloration is produced. This and the former reaction depend on the pres- ence of an aromatic radicle in the jiroteid mole- cule. On treating a proteid substance with a little sugar and some strong sulphuric acid a red coloration is produced. On adding to a solution of a proteid body some caustic potash and then a few drops of a very dilute solution of copper sulphate a violet blue color is obtained (biuret reaction ) . Peptones and albumoses behave sorae- what differently, giving a rose-red color instead of violet if only a trace of co])pcr suljiliate is used. After being boiled with alcohol and washed with ether, and dissolved in strong hydro- chloric acid, proteids give a blue coloration. Proteids are precipitated by a great many re- agents, the pe])tones and albumoses being ex- cepted in a number of cases. Thus solutions of proteids are precipitated ( 1 ) by strong acids, as nitric; (2) by picric acid; (3) by acetic acid and potassium ferrocyanide; (4) by acetic acid and excess of neutral salts like sodium phos- phate; (.5) by salts of heavy metals, as copper sulphate; (fi),by tannin; (7) by alcohol; (8) and by saturation with some neutral salts, as ammonium sulphate. When proteids unite with mineral substances the resulting bodies are called albuminates ; thus the precipitate formed when copper sulphate is a'dded to a solution of albu- min is copper albuminate. Precipitation .should be distinguished from coagulation. The latter term applies when an in- soluble or coagulated proteid is formed from a