THE MECHANISM OF ORGANIC SYNTHESIS , ,,, 



of ammonia. In animals the power of transforming one ammo-acid into 

 another, of one group into another, is probably strictly limited. So far as 

 we know, nearly all the amino-acids utilised in the building up of the animal 

 proteins are derived directly from those contained in the food. On the 

 other hand, we have evidence in the animal body of synthesis of the purine 

 bodies, and therefore of the pyrimidine and iminazol rings. The hen's egg 

 at the beginning of incubation contains very little nuclein, nearly the whole 

 of its phosphorus being present in the form of phosphoproteins and lecithin. 

 As incubation proceeds these substances disappear, their place being taken 

 by the nucleins which form the chief constituent of the nuclei of the developing 

 chick. In the same way the ovaries and testes of the salmon are formed 

 during their sojourn in fresh water at the expense of the skeletal muscles, 

 especially those of the back. Here again there is a transformation of a tissue 

 poor in purine bases into a tissue which consists almost exclusively of nucleins 

 and protamines. Whether in this case there is a direct conversion of the 

 mono-ammo-acids of the muscle proteins into the diamino-acids and bases 

 typical of protamines, we do not know. It is more probable that only 

 diamino-acids and bases previously existing in the muscle are utilised for the 

 formation of the generative glands, the other amino-acids being oxidised 

 and utilised for the ordinary energy requirements of the animal. 



THE SYNTHESIS OF FATS 



In some plants fat globules have been stated to appear as the first products 

 of the assimilation of carbon dioxide under the influence of sunlight, but 

 there is no doubt that as a rule the formation of fats as reserve material in 

 seeds or fruits occurs at the expense of carbohydrates. In the higher animals, 

 too, although a certain amount of the fat of the body is derived from the fat 

 taken up with the food, the organism can also manufacture neutral fat out 

 of the carbohydrates presented to it in its food. The problem, therefore, 

 of the synthesis of the fats is the problem of the conversion of a sugar such 

 as glucose into glycerin and the fatty acids. Although this conversion 

 is apparently so easily effected by the living organism, it is one which from the 

 chemical standpoint involves considerable difficulties. On account of the 

 fact that the higher fatty acids consist largely of oleic and stearic acids, i.e. 

 acids containing eighteen carbon atoms in their chain, it has been thoughl 

 that the synthesis might be brought about by the linking together of thre< 

 molecules of a hexose. Such a change would involve a series of dif 

 chemical transformations. For instance, no less than sixteen out 

 eighteen oxygen atoms present in the three glucose molecules woi 

 to be dislodged in order to convert the chain into stearic acid 

 although these two acids contain a multiple of six carbon atoms, a 

 array of fats are found both in plants and animals which could not 

 by a simple aggregation of glucose molecules, and it is worthy of not 

 all the fatty acids which occur in nature, all those with more than fi 

 atoms contain an even number of carbon atoms. Thus m milk, ,r 

 to the three common fats, tristearin, tripalmitin, and tnolem, *e f 



