1911 Encyclopædia Britannica/Albumin

ALBUMIN, 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 Béchamp, 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＝0·3-2·4%, O＝19-24%, crystallized albumin is C＝51·48%, H＝6·76%, N＝18·14%, S＝0·96%, O＝22·66%, which points to the General characters.formula C₇₂₀H₁₁₃₄N₂₁₈S₅O₂₄₈, 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 amino- and other acids. These decomposition products include: glycocoll or aminoacetic acid, NH₂CH₂COOH, alanine or aminopropionic Decomposition products.acid, CH₃.CH(NH₂).COOH, α-aminobutyric acid, α-aminovalerianic acid, leucin or isobutyl-α-aminoacetic acid, (CH₃)₂CH.CH₂.CH(NH₂).COOH, isoleucin, probably β-aminocaproic acid, serin or α-amino-β-hydroxypropionic acid, HO.CH₂.CH(NH₂).COOH, aspartic acid or aminosuccinic acid, HOOC.CH₂.CH(NH₂).COOH, glutaminic acid or α-amino-η-glutaric acid, HOOC.(CH₂)₂.CH(NH₂).COOH, diaminoacetic acid, α-β-diaminopropionic acid, lysin. or α-ε-diamino-n-caproic acid, NH₂(CH₂)₄.CH(NH₂).COOH, arginin or guanidine-α-amino-n-valerianic acid, (NH)(NH₂)C.NH.(CH₂)₃.CH(NH₂).COOH, ornithin or αδ-diamino valerianic acid, NH₂.(CH₂)₃.CH(NH₂).COOH, histidin or α-amino-β-imidazol-propionic acid , proline or α-pyrrolidin carboxylic acid, , hydroxyproline, phenyl alanine or phenyl-α-aminopropionic acid, C₆H₅.CH₂.CH(NH₂).COOH, tyrosine or p-hydroxyphenyl-α-aminopropionic acid, phenyl ethylamine, p-hydroxyphenyl ethylamine, tryptophane or indol aminopropionic acid, A. cystin (protein-cystin) or α-amino-β-thioglyceric acid “disulphide,” (S.CH₂.CH(NH₂).COOH)₂, B. cystin (stone-cystin), or α-thio-β-aminoglyceric acid “disulphide,” (NH₂.CH₂.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 attempted by A. Kossel and by W. Kuhne and E. P. Pick; but in the present state of our knowledge, however, Classification
of albumins.the older classification of E. Dreschel and F. Hoppe-Seyler, based primarily on solubilities and distribution, may be conveniently retained. This classification is with certain modifications as follows:—

1. Albumins proper: characterized by having colloidal solutions.
   1. Albumins: serum-albumin, egg-albumin, lact-albumin.
   2. Globulins: serum-globulin, egg-globulin, lacto-globulin, cell-globulins.
   3. Plant-globulins and plant-vitellines.
   4. Fibrinogen.
   5. Myosin.
   6. Phosphorus containing albumins (nucleo-albumins), caseins, vitellines, nucleo-albumins of the cell-protoplasm, mucoid nucleo-albumins.
   7. Histones.
   8. Protamines.
2. Transformation products of the albumins proper.
   1. Acid-albumins, alkali albuminates.
   2. Albumoses, peptones and peptides.
   3. Halogen-albumins, oxyprotein, oxyprotsulphonic acid, &c.
3. Proteids.
   1. Nucleo-proteids.
   2. Haemoglobin and allied substances.
   3. Glyco-proteids, mucins, mucoids, helico-proteid.
4. Albuminoids.
   1. 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. “Serum- albumin,” or “blood-albumin,” possibly C₄₅₀H₇₂₀N₁₁₆S₆O₁₄₀, 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 α and β, is found in the lens of the eye; “egg-globulin” and “lacto-globulin” 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°, 67°, 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 well-characterized class of albumins found in the ripe spermatozoa of fishes.

Albumoses and Peptones.—The primary products of the dissociation of albumins are the albumoses, characterized by not being coagulable by heat, more soluble than the albumins, having a far less complex composition, and capable of being “salted out” by certain salts, and the peptones, similar to albumoses but not capable of being “salted out”; moreover, peptones are less complex than albumoses. By further decomposition peptones yield peptides, a certain number of which have been synthesized by Emil Fischer and his collaborators. Albumoses and peptones are white powders, readily soluble in water, with the exception of the hetero-albumoses—a subdivision of primary albumoses. They give the biuret and xanthoproteic reactions, and form salts with both acids and bases. Albumoses and peptones are obtained by peptic digestion, the latter being termed peptic-peptones; tryptic digestion also produces peptones. Acids and moist heat induce similar changes.

Proteids.—These substances are combinations of one or more albumins with a radical of an essentially different nature, termed by Kossel a “prosthetic group.” It is convenient to classify proteids by those groups. “Nucleo-proteids,” constituents of the cell-nucleus, are combinations of albumins and nucleic acid; they always contain iron. They are loose, white, non-hygroscopic powders, soluble in water and salt solutions, and have an acid reaction; they give the colour reactions of albumins. Nucleic acid is at present of unknown constitution; decomposition products are: phosphoric acid, uracil or 2.6-dioxy-pyrimidin,^[1] cytosin or 2-oxy-6-amino-pyrimidin, thymin (nucleosin) or 2.6-dioxy-5-methyl pyrimidin hypoxanthin^[1] or 6-oxypurin, xanthin or 2.6-dioxypurin, adenine or 6 amino-purin, guanine or 2-amino-6-oxypurin, pentoses (l-xylose), laevulinic acid, ammonia, etc. The nucleic acids vary with the source of the proteids, there being considerable differences in chemical composition. In general they are white, loose powders, slightly soluble in cold water, more soluble in hot water; they are precipitated by mineral acids, but dissolve in an excess. They are dextrorotatory, and the specific rotation is numerically greater than that of albumin; hence the proteids are, in general, dextrorotatory.

An important nucleo-proteid is haemoglobulin or haemoglobin, the colouring matter of the red blood corpuscles of vertebrates; a related substance, haemocyanin, in which the iron of haemoglobin is replaced by copper, occurs in the blood of cephalopods and crayfish. Haemoglobin is composed of a basic albumin and an acid substance haematin; it combines readily with oxygen, carbon dioxide and carbon monoxide to form loose compounds (see Nutrition). It coagulates at 64°. By a dilute acid haemoglobin is decomposed into globin, and “haematin,” a ferri-pyrrol derivative of the probable formula C₃₄H₃₄N₄FeO₅; under certain conditions the iron-free “haematoporphyrin” is obtained. This last substance may be reduced to mesoporphyrin, C₃₄H₃₈O₄N₄, which by further reduction gives haemo-pyrrol, C₈H₁₃N, possibly methyl-propyl-pyrrol or butyl-pyrrol. Other derivatives are haemin, haemochromogen and the haematinic acids.

“Glyco-proteids” differ from nucleo-proteids in containing a carbohydrate radical, which is liberated only by boiling with mineral acids or alkalies. The mucins and mucoids belong to this group; they are acid and contain no phosphorus; they give the albumin colour reactions but are not coagulated by heat. Mucins occur in most of the slimy fluids of the body; they vary in composition with their source. Mucoids resemble mucins in their composition and reactions, but differ, in general, in their physical properties. They occur in tendons, bones and cartilage. The “phospho-glyco-proteids” resemble the mucins and mucoids in containing a carbohydrate residue, but differ in containing phosphorus. Ichthulin (see above) maybe placed in this group; “helico-proteid,” found in the serous gland of Helix pomatia, the vineyard snail, also belongs here.

Albuminoids is the anatomical name given to albuminous substances forming the connective tissues. Chemically they resemble the albumins, being split up by acids or ferments into albumoses, peptones and amino-acids, forming salts, and giving the same colour reactions. They are quite insoluble in water and in salt solutions, and difficultly soluble in dilute acids and alkalies. Typical albuminoids are gelatin, keratin, elastin, fibroin, spongin and conchiolin.

“Collagen” (Gr. κόλλα, glue, and root γεν- of γεννάειν, to produce, γίγνεσθαι, to become), the ground-substance of bones and tissues, is decomposed by boiling water or on warming with acids into substances named gelatin, glutin or glue. Gelatin forms a white amorphous powder; the commercial product, however, generally forms glassy plates. The decomposition products are generally the same as with the general albumin; it gives the biuret reaction; forms salts with acids and alkalies, but is essentially acid in nature. Immersed in cold water gelatin does not dissolve but swells up; it dissolves readily in hot water, forming, according to the quantity present, a thick jelly which solidifies to a hard mass on cooling (the “glue” of the wood-worker), or a thin jelly (used in cookery). Gelatin occurs also in the cornea and the sclerotic coat of the eye; and in fish scales, the latter containing 80% of collagen, and 20% of ichthylepidin, a substance differing from gelatin in giving a well-marked Millon’s reaction. Keratin (Gr. κέρας, a horn), the chief constituent of horny material, occurs in hair, nails, hoofs and feathers. It is quite insoluble in water, dilute acids and alkalies. Related to this substance are “neuro-keratin,” found in the medullary sheath of nerves, and “gorgonin,” the matrix of the axial skeleton of the coral Gorgonia Cavolinii. Elastin occurs either as thick strands or as membranes; it constitutes the “elastic tissue” of the anatomist. Its insolubility is much the same as keratin. “Fibroin” and silk-glue or sericin occur in natural silk fibres. Fibroin is insoluble in water, acids and alkalies; silk-glue resembles gelatin in its solubility, but it is less readily gelatinized. “Spongin,” the matrix of bath-sponge, is insoluble in water and dilute acids, but soluble in concentrated mineral acids. “Conchiolin,” the matrix of shells of the mollusca, is only slightly soluble in acids. “Cornein” forms the framework of corals. “Amyloid” occurs as a pathological product, and also in the healthy aorta and in old cartilage. It is an albumin, and not a carbohydrate as was formerly held; and gives most of the colour reactions of albumins. It forms shiny, homogeneous masses, quite insoluble in cold water and in salt solutions, but soluble in alkalies. The albumoids include, according to Cohnheim, substances which possess certain properties in common, but differ from the preceding groups. In general they resemble coagulated albumin, and also the gelatin-yielding tissues, but they themselves do not yield gelatin.

Colouring matters derived from albumins include the “melanins” (Gr. μέλας, black), substances which differ very considerably in composition, the sulphur and iron content being by no means constant; they do not give the reactions of albumins. The black colouring matter of hair, the skin of negroes, and of the ink bag of Sepia have been examined. Melanins obtained from tumours form black, shiny masses; they are insoluble in water, neutral salt solutions, dilute acids and in the common organic solvents.

1. The pyrimidin ring is numbered . For the purin ring, see Purin.