PROTEINS (from Gk. irpilmos. preitos, first). A name applied to an exceedingly important group of chemical substances. occurring abun dantly in the organisms of animals and plants. The principal component elements of the pro teins are carbon- hydrogen, oxygen. and nitrogen. When burned. however, the proteins generally leave behind certain amount of mineral matter. and it is as yet unknown whether such mineral constituents form with the organic portion of the proteins true chemical compounds, or merely physical mixtures. The classification of the pro teins is based, not on their chemical constitution. as in the case of most other groups of carbon compounds, hut on a knowledge of some of their physical properties and of their behavior toward certain reagents. Such a classification is from a scientific point of view- very imperfect. and so it must remain until light shall be thrown upon the arrangement of the atoms within their molecules_ This, however, is a very difficult problem. In the first place, the proteins are as a rule quite unstable and are strongly affected by ordinary ehemical reagents and by relatively slight changes iu the physical conditions. Further, any one familiar with the methods of determin ing the constitutional formula of an organic com pound (see CARBON Comrouxns) will readily see the difficulty of ascertaining the constitution of a compound, for instance, like the oxylnemoglobin of horses' blood, whose molecular formula is, according to Hunter. Yet this is a crystalline compound. and so it is probable that it has been isolated. In the case of amorphous substances, as most proteins are, even this often remains uncertain, and then our knowledge is from the point of view of the theo retical chemist very imperfect indeed. It must also be remembered that the protein substances as they exist in the organism of a living animal or plant are probably more or less different from what they are when examined in the test-tube of a chemist. What the difference consists in is not known, for we are entirely ignorant of the nature and character of living matter. And so all statements concerning the proteins refer to dead protein alone.
In discussions of the food and nutrition of man and animals the term 'protein' is commonly ap plied to the total nitrogenous material present in the food or feeding stuffs of animal and vege table origin. and includes a number of different groups of compounds with correspondingly differ ent nutritive values. The group in its relation to nutrition is commonly subdivided as follows: I Albuminoids.
Proteids collagens or gelatinoids.
Protein Non-proteids cids, etc.
The albuminoids, sometimes called 'true pro teids,' which include albumin, globulin. etc.. are the most important of the nitrogenous constitu ents of foods, examples being the albumin of milk and eggs, the casein of milk, myosin of meat, gluten of wheat, etc. The gelatinoids are
mostly of animal origin and include collagen, gelatin, elastin. keratin. etc.. bodies which are characteristic of connective tissues as tendons. ligaments, horns. etc. Distinguished from the above are the non-proteids, which include the kreatin, kreatinin, and other extractives of meat. and the amides, amido-acids, etc., of vegetable foods.
In general the protein of animal foods and cereal ,rains is very largely composed of proteids. especially those designated above as albuminoids or true proteids. The true proteids can be trans formed into nitrogenous body material and thus go to form the blood. muscle, tendon, nerve. etc., id the animal body. They are also sources of energy. Gelatinoids, such as gelatin. when burned in the body. yield energy, but cannot go to form nitrogenous tissue. Like carbohydrates and fat, gelatinoids undoubtedly protect protein front cleavage. The non-proteids. though useful to give flavor. etc., appear to have hut little nutritive value except in so far as they serve as fuel for the body. It may be that some of them have an especial use as sparers of protein. Thus it has been suggested that asparagin may serve as a nitrogenous nutrient in the place of proteids for intestinal bacteria. However, too little is known of the functions of the non ii to warrant cry definite assumptions. From what has been said it is (vident that protein is essential for building and repairing the body and that it may :filso be burned in the body to yield energy. the amount being very often estimated as 9.3 calories pi r grain digestible protein. Thus protein has two functions, whereas fat and carbohydrates have but one. namely, to serve: as sources of energy. See Fool).
As noted above, proteids are widely distrib uted in products of animal and vegetable origin. Breen plants build up proteids from simple bodies. obtaining the necessary nitrogen from voniplex nitrogenous organic compounds occur ring in the humus of the soil, from ammonium salts, from nitrates, and from the nitrogen of the air. Among the higher plants it seems to be principally the Leginninosx which can utilize free nitrogen. This they do by the aid of the micro-organisms present in the tubercles on their roots. ( See GREEN 11\ IANURING.) 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 undergo before being added to the substance of the body the possibility of the formation of proteids from simide, nitrogenous substances in the body arc questions which are not settled or well understood.