albumin

albumin

Any of a class of simple, water-soluble proteins that can be coagulated by heat and are found in egg white, blood serum, milk, and many other animal and plant tissues.

In chem., a substance named from the Latin for the white of an egg, in which it occurs in its purest natural state (see albumen).

The albumins are highly complex organic bodies which enter prominently into the composition of all animal and vegetable tissues and form the groundwork, so to speak, of every living cell. They are the most important food-stuffs of all classes of animal life, and can be elaborated by the chlorophyl-bearing plants from such simple substances as water, carbon dioxid, and certain nitrates or ammonium salts. All albumins contain carbon, hydrogen, oxygen, nitrogen, and sulphur in definite proportions which vary but little in the different members of the group: one albumin, which is found in the thyroid, also contains iodine. Other elements are not met with in albumins proper, but are encountered in certain compound albumins, in which an albuminous radical is united with other more or less complex groups. Thus hemoglobin contains iron, hemocyanin copper, and the nucleo-albumins and nucleoproteids phosphorus. All albumins also contain variable amounts of mineral salts in firm combination. Their molecular size is very large. For crystallized egg-albumin Hofmeister established the formula C₂₃₉H₃₈₆N₅₈S₂O₇₈, which corresponds to a molecular weight of 5,378. The, protamins and histons apparently have the smallest molecules, while the compound albumins are proportionately heavier: the weight of oxyhemoglobin has thus been determined as 14,800. The greater number of the albumins are amorphous. A few, however, can be obtained in crystalline form, such as egg-albumin, serum-albumin, oxyhemoglobin, and certain vegetable albumins, as edestin, etc. The so-called Bence Jones albumin, which has been met with in the urine in certain pathological conditions (multiple myeloma), also belongs to this order. All true albumins are levorotatory, while certain compound albumins (the nucleoproteids) turn the plane of polarization to the right. Osborne has shown that this property, in the case of the nucleoproteids, is very likely wholly referable to the nucleinic acid complex which they contain. All albumins — owing to the great size of the molecule, no doubt — are incapable of diffusing through animal membrane or vegetable parchment. Some members of the group are soluble in water, others only in dilute saline solution, and still others in dilute acids and alkalis. From their solutions they can be precipitated by mineral acids, the salts of the heavy metals, the so-called alkaloidal reagents (as tannic acid, phosphotungstic acid, iodomercuric iodide, etc.), strong alcohol, and certain neutral salts (sodium chlorid, magnesium sulphate, sodium sulphate, and notably ammonium sulphate). All albumins further give certain color-reactions, of which the biuret reaction (production of a bluish or reddish violet-color on the addition of very dilute copper-sulphate solution in the presence of an excess of strong caustic alkali) is especially characteristic. The true albumins are all coagulated by heat. As a result they lose their individual characteristics and are then said to be denaturized. After this they can be brought into solution only by means which at the same time will produce integral changes in their composition. From study of the various cleavage-products which result from the albumins on hydrolysis by boiling mineral acids and alkalis, by digestion with the proteolytic ferments, etc., a certain insight is now possible into the complex structure of the albuminous molecule. Thus it appears that various a-amido acids (as leucin, tyrosin, asparaginic acid, glutaminic acid), and the diamido acids (ornithin, lysin, etc.), exist in the albuminous complex in the form of Fischer’s polypeptides, which have the general structure represented by the formula NH₂.(CH₂.CO.NH)n.CH₂.-COOH. These in turn are combined with other groups, such as the sulphur-containing cystin complex, the glucosamin-group, etc., to form still more complex radicals, which are further combined with similar groups to even larger complexes, which last in turn are again united with correspondingly large groups to form the complete molecule. Evidence of the correctness of this supposition is furnished by a study of the products of albuminous digestion. Here we find among the primary products of cleavage three complex bodies which individually differ from one another and which in the intact molecule were manifestly in combination. These are the three primary albumoses, termed proto-albumose, heteroalbumose, and glucoalbumose. The first-mentioned on further decomposition yields diamido acids in small amount, much tyrosin, little leucin, no glyeocol, etc.; while the second contains diamido acids in large amount, much leucin, no tyrosin, and the total amount of glycocol of the original substance. Glucoal-bumose in turn contains the entire carbohydrate-group and a larger percentage of oxygen, while the amount of nitrogen and carbon is less than in the two other groups. (See also products of digestion.) The albumins maybe divided into 5 classes, namely the native albumins, the nucleo-albumins, the proteids, the albuminoids, and the derived albumins. Examples of the first group are the serum-albumin and serum-globulin of the blood-plasma, the ovalbumin of white of egg, the lactalbumin of milk, and the myosin and myogen of muscle-plasma. The same group further comprises the glucoalbumins, which are characterized by the special predominance of a carbohydrate-group, and of which the various mucins and mucoids are common representatives; further, the markedly sulphurous ceratins of the skin and related substances (hair, horn, etc.); then the histons and the closely related protamins. These latter represent albumins of simplest structure, and are fairly typical representatives of Fischer’s polypeptides. Kossel’s salmin thus apparently consists only of an ornithin complex, associated with tyrosin, serin, tryptophan, and a-pyrrolidin- carbonic acid. The second group of albumins is formed by the nucleo-albumins or phosphoglobulins. These are more complex than the members of the first group in having a special phosphorized radical in combination with an albuminous complex. They comprise many important foodstuffs, such as the casein of milk, the vitellins of the yolks of birds ‘eggs, the ichthulin of fishes’ eggs, besides the phytoglobulins or phytovitellins of the leguminous plants. The third class is represented by the proteids, which are complex albumins, containing an albuminous group united with other complex radicals. In the nucleoproteids, which are important constituents of cell-nuclei, we find nucleinic acid, from which the so-called purin or xanthin bases and uric acid are derived. In the hemoglobins we meet with pigment radicals: so in the common coloring matter of the blood, the hemoglobin, with hematin. The albuminoids, which form the fourth group, in contradistinction to those already mentioned, are notably constituents of intercellular structures and thus especially abundant in the skeletal parts of the animal body. To this group belong the collagens or glutins of fibrous tissue and cartilage, the elastin of elastic tissue, the various skeletons found in the supporting structures of the invertebrates, etc. The last class comprises substances which are albuminous derivatives, but still possess albuminous character, such as the coagulated albumins and the various intermediary digestive products, including the albuminates, albumoses, and peptone.

A thick, viscous nitrogenous substance, which is the chief and characteristic constituent of white of eggs and of the serum of blood, and is found in other animal substances, both fluid and solid, also in many plants. It is soluble in water and is coagulated by heat and by certain chemical reagents.

a modification of albumin produced by the action of dilute acids. It is not coagulated by heat.

albumin as modified by the action of alkaline substances; — called also albuminate.

A class of monomeric proteins that are soluble in water, and are coagulated by heat; they occur in egg white, milk etc; they function as carrier protein for steroids, fatty acids, and thyroid hormones and play a role in stabilizing extracellular fluid volume.

a simple water-soluble protein found in many animal tissues and liquids