Bible Encyclopedias
Albumin

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 (7) 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 pepticpeptones; 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 2amino-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 C34H34N4FeOs; under certain conditions the iron-free " haematoporphyrin " is obtained. This last substance may be reduced to mesoporphyrin, C34H3804N4, which by further reduction gives haemopyrrol, C 8 11 13 N, 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) may be 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 N =C6 1 The pyrimidin ring is numbered 2C "C5. For the purin ring, see [[Purin. - C]] 7 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. KoXXa, glue, and root yevof yevv6.aav, to produce, yiyvEo Oat, 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 woodworker), 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 wellmarked Millon's reaction. Keratin (Gr. KEpas, 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. paas, 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.