32 Inside the Living Cell 



solved when it was discovered that the microflora of the animal's 

 digestive tract were manufacturing vitamins and so contributing to 

 the nutrition of their host. 



The wide occurrence of vitamins in all kinds of living things is 

 another proof of the essential unity of life. From micro-organisms to 

 man, they are all buUt on much the same plan. They all have 

 extremely similar enzyme systems, and require the same auxiliary 

 substances. But some organisms are dependent on others for some 

 of these vital substances, as they have lost the power of making their 

 own. Broadly speaking, the higher the organism is in the scale of 

 living things the less is its synthetic ability and its dependence on 

 other forms of life for the compounds it needs is greater. 



As I said before, many of the vitamins can now be constructed in 

 the laboratory. Chemists can do more than this. Not only can they 

 construct the actual vitamin, the molecule which has been selected in 

 Nature to do a particular job; they can also make a variety of 

 similar molecules, which may differ from the vitamin in a slightly 

 different arrangement of atoms or in the slight additions here and 

 there, which do not spoil the general plan of the molecule. What 

 happens when an organism is presented with these pseudo vitamins? 

 Does it reject them or can it make use of them? Actually it tries to 

 make use of them, but often unsuccessfully. The 'unnatural' vitamin 

 combines with its enzyme. The protein part of the enzyme does not 

 notice that the vitamin is not quite right. It accepts it as a co-enzyme 

 but, alas, the combination does not function properly. As we have 

 seen, enzymes act by achieving a very exact fit with the substances 

 they act on — like the fit of a lock and key. If a part of the enzyme, 

 the co-enzyme for example, is changed slightly, the key will not go 

 into the lock. Moreover, even if the natural vitamin is also present, 

 it may be partly prevented from doing its work, for the unnatural 

 vitamin competes for the available enzyme molecules and put a 

 proportion of them, at least, out of action. 



So the result is that 'unnatural' vitamin molecules act as 

 antagonists to the real vitamins. They often prevent the proper 

 functioning of the real vitamin and produce the same symptoms as a 

 severe vitamin deficiency. 



This seems to provide a wonderful way of preventing the growth of 

 harmful organisms, like bacteria. All you need do is to introduce a 

 suitable antagonist which will put out of action a vital enzyme in 

 the bacteria, which will then be unable to grow. 



Unfortunately, the hosts and the parasites are usually very much 

 alike and possess very similar enzymes, and it is frequently found 

 that they require the same vitamins and respond similarly to their 



