Popular Science Monthly/Volume 32/February 1888/Vegetable and Animal Albumen
|VEGETABLE AND ANIMAL ALBUMEN.|
HOWEVER numerous and prolific our investigations into the composition and properties of organic bodies may have been, we yet are very imperfectly acquainted with those processes by which these manifold forms of matter originate from simple constituents of air and water, and are as yet unaware of the causes on which the changes are depending that they undergo during life. We are acquainted with the artificial preparation of many of them, and still our experience is not sufficient to explain their natural origin.
We may produce oxalic acid, a body contained in the juice of many plants and also in certain animal secretions, by heating sugar together with nitric acid, but this is not the natural process of its formation, nor does it explain it. We imitate Nature in preparing grape-sugar from starch, but our method is different from hers, although the sugar we make by treating starch with hot dilute acids is identical-with the product of the natural act of fermentation (caused in germinating grain by diastase, a product of decomposition of albuminous matter). It was considered a success of the highest scientific consequence, when Wohler found out a way of artificially preparing urea, a matter resulting from the decay of muscular fiber in living animals; but yet, about sixty years after this event, we do not understand the chemical changes leading to that result. In a thousand other questions concerning the chemistry of human, animal, and vegetable organisms, our researches for finding satisfying interpretations have been futile, and even the most important query concerning the origin of life is likely to remain unanswered for many years to come.
There is a certain class of organic nitrogenous compounds, the origin, chemical nature, and decompositions of which are particularly far from being cleared up, although they concern the most indispensable functions of our own life, and are essential to vital energy in animals. Misunderstood, as many of their properties are, the facts which we know about them suffice to justify the high interest in their study which is manifested by chemists and physicians, as well as by the educated public in general. They are comprised under the name of albuminous matter, and have a very complex constitution, containing carbon, oxygen, hydrogen, nitrogen, and a small amount of sulphur. One of the chief features of their chemical character is a remarkable liability to decompose into the most various products. The best-known representatives of the group are—
1. Albumen, chief constituent of the white of egg and of blood-serum, dissolvable in water of common temperature, the dissolved matter coagulating and becoming insoluble when heated to 70° C.
2. Casein, dissolved as cheese in milk, coagulating upon addition of acids or certain ferments and warming.
3. Fibrin, coagulating from blood upon its exposition to air.
Plants are the manufacturers of albuminoids, or proteids, as they are also called; transformation of carbonic acid, water, and nitrogen, which are permanent constituents of the atmosphere and soil, into those combinations which constitute the body of plants, and which are designed to effect their propagation, is the chief function of the roots of plants. Among these combinations albuminoids are the most important, both for the vital process of the plants and as food for animals. From this reason the question of their origin has induced numerous investigations, to which we owe the knowledge that certain other nitrogenous bodies, called amides, which in varying amounts seem to be present in the roots of all plants at the time of beginning growth, play a prominent part in the genesis of albuminoids. The amides best known are asparagin (originally found in the shoots of asparagus) and leucin. The conclusion to which these researches have led is that sugar is formed in the root partly from starch by the action of diastatic ferment, partly by direct assimilation out of carbonic acid and water. Combination of sugar with one of those amides results. in the formation of vegetable albumen, from which the rest of the proteids are derived by slight variations of chemical composition. Vegetable albumen being of a very unstable nature, is partly again decomposed into amides and sugar, and, while the latter is used in accomplishing the structure of the plant by changing into cellulose, the amide is again transformed into albuminoid, and by decomposition reduced to amides. Thus, by continually assimilating carbonic acid and water, combining with them to form albuminoids, and giving them off again as sugar, the amides act a prominent part in the development of plants. New supplies of amides, in the mean time, are continually formed, while the albuminous matter is partly transferred to remote organs, where, exposed to light and other agents, it undergoes various decompositions, by which the deposits of solid proteids, alkaloids, and many other bodies are produced. A certain quantity of proteids becomes stored up in the seeds, modified into gluten, or legumin, for the purpose of hereafter entertaining and supporting the life and growth of the offspring. Vegetable albumen, gluten, and legumin so closely resemble animal albumen, fibrin, and casein, that the same names have been given to them.
In thus tracing the origin of albuminoids in plants, we see them partly dissolved as vegetable albumen in the juice (upon the heating of which coagulated albumen gathers as foam on the surface), partly as "plasma" forming the contents of cells, and partly as solids in various organs, but chiefly in the seed, which by their presence acquires more or less valuable properties as food for animals, the nutritive value of grain and leguminous products being due merely to the high percentage of gluten and legumin contained in them.
Plants, indeed, are the sole source on which most animals depend for their food, for they are incapable of assimilating the constituents of air and water, as vegetables do. Only in the most simple forms of animals, such as moneres and amœbæ, the question is undecided whether such an assimilation takes place or not; but our knowledge of the limits between these low forms of vegetable and animal beings is very imperfect, and, in view of the numerous parasitic plants which draw their food from other organisms, we can not declare the source and process of nourishment to be a correct and pervading mark of difference between the two classes. If, however, some one should raise the objection that animals of prey do not depend upon plants for food, he might easily be corrected by showing that, in feeding on vegetable-eating creatures, they "indirectly" live on the plants themselves.
The changes which vegetable proteids undergo by being taken up into the animal organism are insignificant at first; having been transformed into soluble bodies (peptones) by pepsin, the digestive agent of the stomach, we see them appearing again in the circulation of blood as albumen, globulin, fibrinogen, combinations of very similar character to the proteids of plants. There is, indeed, no practical difference existing between albumens and legumins. The proteid of beans, peas, lentils, etc., is identical with casein, the proteid of cheese, as to composition and properties.
Blood, upon leaving the vein, is separated into a liquid and a solid part by fibrinogen, one of the proteids dissolved in it, undergoing a change, by which it becomes insoluble fibrin; muscular plasma, the semi-solid constituent of muscular fiber, by solidifying after death, and changing into fibrin, affords the well-known phenomenon of rigor mortis. It is a process closely resembling these, by which one of the soluble proteids of gluten, by accession of air and water, is transformed into insoluble gluten fibrin.
The conformity existing between the nitrogenous compounds of vegetables and animals is not limited to the proteids mentioned, but extends to various products of their decomposition. Diastase, the fermentative agent originating from proteids in the seeds of many plants during germination, is also present in the saliva of animals, where it exerts upon amylaceous foods the same action of forming sugar from starch. Agents very similar to pepsin, the digesting ferment of the stomach, are to be found dissolved in the juices of various plants; upon contact with muscular fiber, or coagulated albumen, or cheese, they will dissolve these bodies, and transform them into peptones as well as pepsin does. Thus papayotin, a substance extracted from the juice of Carica papayn—a kind of fig-tree—is therapeutically applied for dissolving morbid membranes and tumors; not less are the well-known insect-devouring properties of the leaves of Nepenthes, Drosera, and Utricularia, due to the presence of such ferments in their viscid secretions. Leucin and tyrosin, amides occurring in the roots of plants, which are both products of decomposition and regenerators of albuminoids, are also found in animal organs. The inflammation of skin, caused by touching a nettle, has been ascertained to be due to a kind of decomposed, or changed, proteid; and the virulent properties of the secretions contained in the venom glands of serpents having become known to depend on proteids, the conclusion appears well founded that the virus of insects and other animals also owes its pernicious effect to metamorphosed albuminoids, too, rather than to formic acid, as had been hitherto believed.
Many facts have been produced to show that, as the theory of evolution supposes, there is a degree of consanguinity existing between plants and animals: sexual differentiation, for instance, is common to the higher forms of both of them. Sexes in both cases are sometimes united in the same individual, sometimes separated. The lowest species of both of them, consisting of single cells, propagate by simple division. Scarcely a characteristic has been discovered in these living cells of plasma, which might justify the making of a distinction between plants and animals. The voluntary movement ascribed to the latter class is not plainly discernible in many cases, while many low plants, such as diatomaceæ and certain bacteria are eminently and continually engaged in lively motion. No striking difference is to be observed in the sources of food and in the way in which it is taken. If to these facts the results of chemical investigations be added, showing that many constituents of the greatest vital importance in both are identical, and that a close relationship of others (vegetable and animal fats) is beyond doubt, we may justly claim that chemistry has contributed and will further contribute considerable support to the doctrine of evolution, if identity of matter may be regarded as evidence of consanguinity, or as a proof of common descent.
Considered from another point of view the mutual relations existing between both classes of beings are of a decidedly hostile character. The fierce "struggle for life," which causes animals to kill and to devour their fellow-creatures, is in a not less merciless degree extended to plants; and victory is not, as we might fancy, always on the side of the more perfect creature. Even man, the most accomplished being, and at the same time the most relentless despot on the earth, though without much personal trouble he may fell the strongest trees and eradicate whole forests, is yet liable to succumb to the attacks of a few micro-organisms invisible to the human eye. A theory, at first pointed out by Pasteur, accepted by Virchow, and of late experimentally confirmed by Metschnikoff, teaches, that certain low plants, fungi, called microbia, or bacteria, said to be the primary causes of infectious diseases, when entering the circulation of blood or one of the important organs of the body, become at once engaged in a struggle with the living cells of the organism, both adversaries endeavoring to kill and to devour each other; the result of this fight, if the microbia are victorious, is said to be the death of the animal.
In a lively and perspicuous representation Metschnikoff has described one of these destructive combats. Daphnia, a sweet water crustacean, served to him as the first object on which he could observe the attack by monospora, a fungus of the lowest order. As soon as the latter began to invade the body of Daphnia it became surrounded and entangled by numerous cells (leucocytes) engaged in lively motion, which gradually from all sides attached themselves to the fungus and destroyed it by some kind of intercellular digestion, or absorption. It is evident, that a single cell could not afford to give out so much dissolving matter as was required for this purpose; it would even probably have succumbed to the enterprising enemy; but by the assistance of its confederates it succeeded in overwhelming the intruder. In eighty cases out of a hundred, according to Metschnikoff, the cells would be victorious, but in twenty cases the fungus would gain the battle, with the consequent death of the Daphnia.
Concerning this view and interpretation of the origin and progress of infectious diseases, we are inclined to believe that, since bacteria have not been found in all of them, and since, where they are present, also certain products of chemical decomposition of proteids, ptomaines, occur with them—the dangerous phenomena and the lethal end of such diseases should rather be ascribed to the well-known virulent properties of these products, as is done by many eminent physicians. Bacteria may frequently be the bearers and transporters of disease, as flies are accused of communicating the virus of splenic fever to healthy individuals.
The germ theory, which declares micro-organisms to be the cause and originators of infectious diseases, although it seems to be at present recognized by many physicians, perhaps by the majority of them, is as yet far from being thoroughly established. The action and influence of bacteria have evidently often been exaggerated. Pneumonia was ascribed to them, until it was found that in some pneumonitic cases bacteria are present, whereas in many others none could be found. In hydrophobia a particular micro-organism, although most eagerly sought for, has not yet been discovered. The possibility, however, of transferring this disease from one to another animal by inoculation indicates that the virus may consist of some kind of decomposed proteid, acting as a chemical ferment upon certain constituents of blood, or nervous substance. We may, by the existence of such ferments as diastase, pepsin, or as the virus of serpents and insects in healthy individuals, conclude that other not organized ferments exist in and are the cause of morbid conditions; and although most of the fermentative processes, on which epidemical diseases depend, seem to be induced and to increase by the agency and propagation of bacteria, there is no reason for making them accountable for other troubles to the extent that has hitherto been done. There are organized and unorganized ferments existing, both of which are known to produce decomposition of organic matter. We hope and expect that the future will decide what effects in animal and human diseases belong to each of them.