Part 75

Allen; Commercial Organic Analysis, Vol. 2, pp. 224-236.

Analyse Chimique des Matiéres Grasses, p. 13.

Chemiker-Zeitung, Band 19, S. 451.

Annali del Laboratorio Chimico, 1891-92, p. 197: Bulletin No. 13, Division of Chemistry, U. S. Department of Agriculture, p. 465: Journal of Analytical and Applied Chemistry, Vol. 1, p. 449; Vol. 2, pp. 119 and 275; vid. op. cit. 311.

Rapport presenté a l’Academie Sciences le 20 fevrier, 1883: Analyse des Matiéres Grasses, p. 17: Bulletin No. 13, Division of Chemistry, U. S. Department of Agriculture, p. 446.

Analyse des Matiéres Grasses, p. 15.

Chemical News, 1888, p. 113: Bulletin No. 13, Division of Chemistry, U. S. Department of Agriculture, p. 468.

Vid. op. cit. 69, 1895, S. 535.

Justus Liebig’s Annalen der Chemie, Band 192, S. 178: vid. op. cit. 67, Band 26, S. 575: vid. op. cit. 7, p. 514.

Journal de Pharmacie et de Chimie, 1889, p. 447.

Moniteur Scientifique, Tome 13, p. 263: vid. op. cit. 69, 1884, S. 262.

Vid. op. cit. 7, p. 515.

PART FIFTH.

SEPARATION AND ESTIMATION OF BODIES CONTAINING NITROGEN.

=370. Nature of Nitrogenous Bodies.=—The nitrogenous bodies, valuable as foods, belong to the general class of proteids and albuminoids. They are composed chiefly of carbon, hydrogen, oxygen, sulfur and nitrogen. Some of them, as lecithin and nuclein, contain phosphorus instead of sulfur, but these resemble the fats rather than the proteids.

Nitrogenous organic bodies of the class mentioned above are designated by the general name proteids. The term albumin is restricted in a physiological sense to a certain class of proteids. The term albuminoid is often used synonymously, as above, for proteids, but, more strictly speaking, it should be reserved for that class of bodies such as gelatin, mucin, keratin and the like, not really proteids, but, nevertheless, closely resembling them. In chemical composition the proteids are characterized by the relative constancy of their nitrogen content, the mean percentage of this element being about sixteen, but varying in some instances more than two units from that number.

=371. Classification of Proteids.=—Many classifications of the proteids have been given based on physical, chemical and physiological characteristics. In respect of origin, they are divided into two great classes, viz., vegetable and animal. In respect of their physical and chemical properties the following classification of the proteids may be made.

Albumins.—These are proteids soluble in water and not precipitated from their aqueous solutions by sodium chlorid or magnesium sulfate. They are easily coagulated by heat and are represented by three great classes, viz., egg-, serum-, and lactalbumin.

Egg albumin occurs in the white of egg; serum albumin is found in the serum of the blood. Vegetable albumins have been prepared from wheat, rye, potatoes, and papaws. (Carica Papaya). These vegetable albumins are coagulated by heat at about 70° and are not precipitated by the salt solutions named above, nor by acetic acid. The myrosin of mustard seeds also resembles vegetable albumin.

Globulins.—These bodies are insoluble in water, soluble in dilute solutions of neutral salts, but precipitated therefrom by saturation with sodium chlorid or magnesium sulfate. They are coagulated by heat. Among others belonging to this group are serum globulin, fibrinogen, myosin, crystalin, and globin.

Serum globulin is found in the serum of blood; cell globulin is found in lymph cells; fibrinogen occurs in the blood plasma; plasmin, in blood plasma; myosin, in dead muscles; vitellin, in the yolk of eggs; crystalin, in the lens of the eye; haemoglobin, in the red pigment of the blood; haemocyanin, in the blood of certain low grade animals.

Vegetable globulins are found in the cereals, leguminous plants, papaws and other vegetables, and are divided into two groups, myosins and paraglobulins. The vegetable myosins coagulate at from 55° to 60° and are precipitated from a saline solution by removing the salt by dialysis. In this form, however, they lose their true nature as globulins, becoming insoluble in weak saline solutions.

The vegetable paraglobulins are coagulated at from 70° to 75°. Vegetable vitellin, which is not included in this classification, can be obtained in a crystalline form and of remarkable purity.

Albuminates.—This name is given to the compounds of the proteids with metallic oxids or bases, and also to acid and alkali albumins. They are insoluble in water or dilute neutral salts, but easily soluble in strong acids or alkalies. Casein is a type of this group.

Acid albumin is made from egg albumin by treatment with hydrochloric acid; alkali albumin is formed in egg albumin by the action of a dilute alkali; trinitroalbumin is formed from dry albumin by treatment with nitric acid; casein or caseinogen is the chief proteid in milk.

The chief vegetable albuminates are legumin and conglutin. Legumin is a vegetable casein and occurs chiefly in peas, beans and other leguminous seeds. It is prepared by extracting the meal of the seeds mentioned with dilute alkali, filtering the extract, precipitating with acetic acid, washing the precipitate with alcohol, and drying over sulfuric acid. Treated with sulfuric acid it yields leucin, tyrosin and glutamic and aspartic acids. Conglutin is prepared in a similar manner from almonds.

It is probable that these bodies do not exist as such in the fresh seeds in question but are produced therein from the other proteids by the alkali used in extraction. A further description of vegetable proteids will be found in the special paragraphs devoted to the study of these bodies in the principal cereals.

Proteoses.—This name is applied to proteids which are not coagulated by heat, but most of them are precipitated by saturated solutions of neutral salts. They are also precipitated by nitric acid. They are formed from other proteids by the action of proteolytic ferments. The albumoses represent this group.