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Albumin
1911 Encyclopedia Britannica
or Albumen (Lat. albus, white), an organic substance typical of a group of bodies (albumins or albuminates) of very complicated chemical composition. They are sometimes called the histogenetic bodies or proteids, because they are essential to the building up of the animal organism. The vegetable kingdom is the original source of albuminous substances, the albumins being found in greatest quantity in the seed. They also occur in the fluids of the living organism. The chemistry of the albumins is one of the most complicated and difficult in the whole domain of organic chemistry. It has attracted the attention of many workers, and has formed the subject of a huge literature. In this field Bechamp, Cohnheim, Albrecht Kossel, and, especially, Emil Fischer and his pupils have been extremely active. The general trend of these researches lies in the study of the decomposition or " breaking down " products of the albumin molecules; once these are accurately determined, the synthesis of an albumin is but a matter of time. Already we have proceeded far in our knowledge of the decomposition products, and certain simple proteids have been synthesized.
The albumins contain in all cases the elements carbon, hydrogen, nitrogen, sulphur and oxygen; their composition, however, varies within certain limits: C= 50-55%, H = 6.9-7'.3%,N = 15-19%,S =0.32.4%7 0=1 92 4%, General char- crystallized albumin is C = 51.48%, H = 6.76%, N= acters. 18.14%, S=0.96%, O = 22.66%, which points to the formula C720H1134N218S50241, corresponding to the molecular weight 16,954. A high molecular weight characterizes these substances, but so far no definite value has been determined by either physical or chemical means; A. P. Sabanezhev obtained the value 15,000 by Raoult's method for purified egg albumin. All albumins are laevo-rotatory; and on incineration a small amount of inorganic ash is invariably left. They are usually insoluble in water, alcohol and ether; and their presence as solutes in vegetable and animal fluids is not yet perfectly understood, but it is probably to be connected with the presence of salts or other substances. A remarkable change occurs when many albumins are boiled with water, or treated with certain acids, their solubility and general characters being entirely altered, and the fluid becoming coagulated. This change is seen in the transformation of the " white " of an egg on boiling. Albumins are generally detected by taking advantage of this property, or of certain colour changes. The reagents in common use are: Millon's reagent, a solution of mercuric nitrate containing nitrous acid, this gives a violet-red coloration; nitric acid, which gives a yellow colour, turning to gold when treated with ammonia (xanthoproteic reaction); fuming sulphuric acid, which gives violet solutions; and caustic potash and copper sulphate, which, on warming, gives a red to violet coloration (biuret reaction).
Boiling with dilute mineral acids, or baryta water, decomposes albumins into carbon dioxide, ammonia and fatty aminoand other acids. These decomposition products include: glycocoll or aminoacetic acid, NH 2 CH 2 Oooh, alanine d ucts. or aminopropionic acid ,CH 3 � CH(NH 2) � COOH, a-aminobutyric acid, a-aminovalerianic acid, leucin or isobutyla-aminoacetic acid, (CH 3) 2 CH � CH 2. CH(NH 2) � Cooh, isoleucin, probably 0-aminocaproic acid, serin or a-amino-(-hydroxy propionic acid, aspartic acid or aminosuccinic acid, HOOC�CH 2 �CH(NH 2)�Cooh, glutaminic acid or a-amino-n-glutaric acid,HOOC� (CH 2) � CH(NH 2) � Cooh, diaminoacetic acid, a-O-diaminopropionic acid, lysin. or a-E-diamino-n-caproic acid, NH 2 (CH 2) 4. CH(NH 2) � Cooh, arginin or guanidine-a-amino-n-valerianic acid, (NH)(NH2)C�NH� (CH 2) 3 �CH(NH 2)�Cooh, ornithin or aa-diamino valerianic acid, � (CH 2) 3 � CH(NH 2) � Coon, -(3-imidazol propionic acid, [[Hooc� Ch(Nh 2) �Ch 2 � C: Ch�N:Ch�Nh]], proline or a-pyrrolidin carboxylic acid, [[Hooc� Ch. Nh]]�CH2�CH2�CH2, hydroxyproline, phenyl alanine or phenyl-a-aminopropionic acid, C 6 H 5 � CH 2 � CH(NH 2) � Cooh, tyrosine or p-hydroxyphenyl-aaminopropionic acid, phenyl ethylamine, p-hydroxyphenyl ethylamine, tryptophane or indol aminopropionic acid, A. cystin (protein-cystin) or a-amino-,-thioglyceric acid " disulphide," (S. CH 2. CH(NH 2)� Cooh) B. cystin (stone-cystin), or a-thio-/3-aminoglyceric acid " disulphide," (NH 2 � CH 2 � CH: S. Cooh) 2. This list is not exhaustive; other products are given in Gustav Mann, Chemistry of the Proteids (1906), to which reference should be made for a complete account of this class of compounds.
The complexity of composition militates in a great measure against a rational classification of albumins by purely chemical considerations. Such classifications have been at- Classif1- tempted by A. Kossel and by W. Kiihne and E. P. Pick; but in the present state of our knowledge, however, p g > the older classification of E. Dreschel and F. HoppeSeyler, based primarily on solubilities and distribution, may be conveniently retained. This classification is with certain modifications as follows: I. Albumins proper: characterized by having colloidal solutions.
Albumins: serum-albumin, egg-albumin, lact - albumin.
Globulins: serum-globulin, egg-globulin, lacto - globulin, cell-globulins.
Plant-globulins and plant-vitellines.
Fibrinogen.
Myosin.
Phosphorus containing albumins (nucleo-albumins), caseins, vitellines, nucleo-albumins of the cellprotoplasm, mucoid nucleo-albumins.
Histones.
(8) Protamines.
II. Transformation products of the albumins proper. (I) Acid-albumins, alkali albuminates.
(2) Albumoses, peptones and peptides.
(3) Halogen-albumins, oxyprotein, oxyprotsulphonic acid, &c.
III. Proteids. (I) Nucleo-proteids.
(2) Haemoglobin and allied substances.
(3) Glyco-proteids, mucins, mucoids, helico-proteid.
IV. Albuminoids. (I) Collagen.
(2) Keratin.
(3) Elastin.
(4) Fibroin.
(5) Spongin, &c.
(6) Amyloid.
(7) Albumoid.
(8) Colouring matters derived from albumin.
Albumins proper
Albumins (as classified above) are soluble in water, dilute acids and alkalies, and in saturated neutral salt solutions; they are coagulated by heat. " Serumalbumin," or " blood-albumin," possibly C450H720N116S60140,, occurs in blood-serum, lymph, chyle, milk, &c.; its coagulation temperature is about 67°. It differs from egg-albumin in its specific rotation (-57° to - 64°), and in being slowly coagulated by alcohol and ether. Egg-albumin is the chief constituent of the white of egg; this fluid also contains a globulin and a mucoid. It coagulates at about 56°, and its specific rotation is - 30.70°. " Lact-albumin " occurs in all kinds of milk. The globulins are insoluble in water and in dilute acids, but soluble in alkalies and in neutral salt solutions; these solutions are coagulated on boiling. " Serum-globulin," also termed globulin or fibrino-plastic globulin, paraglobulin and paraglobin, occurs in blood serum; " cell-globulins " occur in many organs - liver,. kidneys, pancreas and the thyroid gland, also in muscle-plasma; " crystalline," a globulin occurring in two forms a and /3, is found in the lens of the eye; " egg-globulin " and " lactoglobulin " occur respectively in the white of egg and in milk. Plant albumins or phyto-albumins have been chiefly investigated in the case of those occurring in seeds; most are globulins,. insoluble in pure water, but soluble in salt solutions; " edestin," a globulin of this class, is very widely distributed. Other varieties or classes of these compounds are: plant caseins, phyto-vitellines, legumins and conglutins. Fibrinogen occurs in the blood plasma, and is changed by a ferment into fibrin, to which the clotting of blood is due. Fibrinogen is insoluble in water, but soluble in salt solutions; it has three different coagulation temperatures, 56°, 6 7°, 75°. Fibrin, produced from fibrinogen by a ferment, is a jelly-like substance, coagulable by heat, alcohol, &c. The muscle-albumins include " myosin " or paramyosinogen, a globulin, which by coagulation induces rigor mortis, and the closely related " myosinogen " or myogen; myoglobulin and myoalbumin are also found in muscles. The nucleo-albumins or phospho-globulins are insoluble in water and acids, but soluble in alkalies, and have an acid reaction. " Caseinogen " (after W. D. Halliburton) is the chief albumin of milk; its composition varies with the animal. It is insoluble in water, while its salts are readily soluble. " Eucasein " is the ammonium salt; " nutrose " and " plasmon " are sodium salts. By the rennet ferment caseinogen is converted into casein, a substance resembling caseinogen in being soluble in water, but differing in having an insoluble calcium salt. The formation of casein involves the curdling of milk. Other phosphoglobulins are vitelline, found in the yolk of hens eggs, and ichthulin, found in the eggs of fish. Histones are a class of albumins soluble in water and acids, but essentially basic in character; hence they are precipitated by alkalies. It is remarkable that many histones are soluble in an excess of alkali. They do not exist in a free state, but in combination with a " prosthetic group " (after A. Kossel) they give rise to important cell constituents - haemoglobin, nucleo-proteids, &c. " Thymus histone " occurs in the thymus gland; globin occurs in combination as haemoglobin; other histones have been extracted from the red blood corpuscles of the goose and the testes of fishes and other animals. The protamines are a wellcharacterized class of albumins found in the ripe spermatozoa of fishes.