Читать книгу Possible Worlds and Other Essays - J. B. S. Haldane - Страница 8

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THE chemical processes which take place in a living animal or plant are just as characteristic as its form or behaviour. Yet, taken one at a time, they can often be imitated by artificial means. Rhumbler made an artificial ‘cell’ which would absorb a glass thread covered with sealing-wax, remove the wax and spit out the thread. Hammond made a motor ‘dog’ with selenium ‘retinae’ which would follow a light or a white object. And similarly we can imitate many of the chemical reactions which take place in the cell, though often we require rather violent means, such as heat or the application of strong acids. What is characteristic of life is not the individual details of structure or behaviour, but the way in which they cohere to form a self-regulating and self-preserving whole.

When we succeed in investigating the details of a chemical process in the cell we generally, if not always, find that it is determined by the presence of an enzyme or ferment, which can be more or less completely separated from the rest of the cell and is not alive. In favourable cases we can break up the cell, and, by a series of processes not utterly unlike those employed in extracting the rarer metals from their ores, except that no heat is used, obtain one of the enzymes in a fairly pure state. A solution of cane sugar in water is stable, but if we warm it with a strong acid it breaks up into the mixture of sugars found in honey. This can also be done by enzymes found in plant cells, the saliva of bees, and our own intestines. The most active enzyme preparations, when dissolved in water, will break up ten times their weight of sugar per second and, as far as we know, will continue to do so indefinitely. Certainly they can break up more than a million times their own weight without wearing out.

The early workers on enzymes believed that a little ‘vital force’ resided in these particles, a belief analogous to that of primitive men in the magical nature of their own tools. But enzymes are certainly not alive. They do not reproduce themselves nor adapt themselves to changes in their environment. They are simply the tools of the cell. Their action is similar to that of inorganic catalysts, such as finely divided nickel or platinum, which are used in industry to speed up many chemical reactions, or to allow them to occur at a lower temperature than would be possible in their absence. And although we have not yet been able to make an enzyme artificially, and shall not be able to do so for many years, we are gradually elucidating their chemical composition. Many of them would seem to be proteins, and it has been suggested that the protein of certain cells consists almost wholly of enzymes.

One of the most characteristic things about an enzyme is its specificity. The enzyme which digests cane sugar will not touch milk sugar or malt sugar, and conversely. Enzymes have been compared to keys which will only open certain locks. One might go further and say that they are Yale keys. Many molecules which are attacked by them are asymmetrical, as is shown by the asymmetry of their crystals, and the fact that their solutions rotate the plane of polarized light passed through them. We can often make the mirror images of these molecules, and we then find that the corresponding enzymes will only attack them slowly if at all. On going through the looking-glass, Alice would have found her digestive enzymes of no more use on the looking-glass sugars than her Yale key on the looking-glass locks. This asymmetrical behaviour is not, however, peculiar to enzymes. It has been found to hold good for the action of other catalysts, including some synthetic substances.

Our knowledge of enzymes has so far had rather little direct application. Those first studied were the ones found in digestive juices, which break up our food into readily absorbable substances, and are unique in that their action normally takes place outside the cell where they are formed. Preparations of these can readily be made, and one of them, rennet, has long been employed in dairies to clot milk. But no very great success has attended the treatment of various forms of digestive trouble by administering enzymes. The main reason for this is that indigestion is seldom due to a shortage of them. So at the present day, apart from rennet and an extract of the pancreas used in the partial digestion of hides to furnish certain kinds of leather, preparations of the digestive enzymes are of more benefit to their sellers than their buyers. For example, a mixture containing pepsin is widely advertised as a tooth-paste, although pepsin is inactive in such fluids as saliva, a fairly strong acid being needed to make proteins susceptible to its action.

Several diseases and abnormalities are due to the absence of an enzyme, but we cannot cure albinism, for example, by injecting tyrosinase as we cure diabetes with insulin. For insulin appears to have a small enough molecule to allow it to get through the walls of the cells, whilst enzyme molecules are too large for this to be at all easy, and they seem generally to be manufactured on the spot where they will be used. So far, therefore, our efforts in medicine are directed rather to enabling such enzymes as exist in the cell to act more efficiently than to supplying them in their absence. We are now, however, beginning to study the enzymes not only of bacteria but of cancer cells, which seem to be slightly but significantly different from those of normal tissues.

In industry we generally find it better to use enzymes in the cell than out of it. Yeast makes alcohol from sugar much more rapidly than does any extract containing the three or four enzymes concerned in the process. But it does a good deal more. The yeast cell breaks up proteins as well as sugar, and from these it forms the fusel oil which not only gives an alcoholic liquor much of its characteristic taste and smell, but also in many cases makes it a great deal more poisonous than its alcohol content would lead one to suppose. It is quite possible, therefore, that if the liquor trade has a future it may be on the lines of utilizing enzyme preparations rather than living yeast cells. When yeast cells are given cane sugar, they break it up into simpler sugars before making these latter into alcohol. The enzyme which they employ for this purpose may easily be separated from those concerned in fermentation. Now the new sugars formed are both more tenacious when wet, and more retentive of water, than cane sugar, and therefore better material for making sweets. So the use of yeast invertase is spreading in the American candy trade, and if prohibition has raised the consumption of candy, it has not been wholly disadvantageous to the yeast plant.

Perhaps the biggest field for the commercial application of enzymes lies elsewhere. Plants make sugar from carbon dioxide, water, and sunlight. A few, like the potato, store most of this in starch, or some other form that we can digest. But the majority of them convert it into cellulose, the main constituent of wood. They commonly make a little sugar to attract bees or birds, but man is the first mammal who has seriously befriended plants, and agriculture is so recent that only a few of the more enterprising among them have had time to vary so as to bribe him with food. The cow and horse can no more digest cellulose than ourselves. No animal nearer to us than a snail can make the enzymes requisite for even a partial digestion of it. But an ungulate is a co-operative society. It consists of the mammal which forms the façade, and some millions of millions of bacteria engaged in breaking down cellulose. The products which they form from it are largely digestible by the horse or cow, but would be unpalatable, if not harmful, to man. However, one of the intermediate stages in their production from cellulose is an easily digestible sugar. When—not if—we can separate the cellulose-splitting enzymes from those which break up the sugar further, we shall be in a position to convert wood pulp or hay quantitatively into human food.

This is one of the facts which render dubious all prophecies as to over-population. The upper limit to human numbers is not set by any facts of nature, but by human ignorance and inadaptability.