CONCLUSION 



symptoms in suitable experimental animals when these are main- 

 tained on a diet from which the factor is missing but which is other- 

 wise complete, and they should cure these deficiency symptoms when 

 added to the diet ; they should stimulate the growth of exacting 

 strains of micro-organisms when grown on a medium deficient in the 

 factor, but otherwise adequate for growth ; and their growth-pro- 

 moting action on micro-organisms, and possibly on animals too, 

 should be antagonised by substances that closely resemble them in 

 chemical structure ; most important of all, they should be present in 

 enzjmie systems responsible for specific biochemical reactions. 



It must not, of course, be assumed that all coenzymes have vitamins 

 as their prosthetic groups. Cytochrome, for instance, contains iron- 

 porphyrin, whilst peroxidase and catalase contain other iron 

 compounds, and ascorbic acid oxidase contains copper. Again, 

 glutathione is a coenzyme for glyoxalase and possibly for many other 

 enzjones, for there is a long list of enz5mie systems that appear to 

 depend for their activity on the presence of sulphhydryl groups. 

 The vitamin B complex, therefore, comprises only one special class of 

 substances essential for the activity of coenzymes ; it is characterised 

 by the fact that its members cannot, except in certain circumstances, 

 be produced by the organisms that require them, but have to be 

 supplied ready made in the diet. 



In concluding this survey, I can only express the hope that my 

 attempt to systematise the large mass of information now available 

 will have given the reader a clear picture of the chemical and bio- 

 logical behaviour of each member of the vitamin B complex, and an 

 appreciation of the important function the group as a whole performs 

 in the economy of mammals and birds, insects, plants and micro- 

 organisms. I hope, too, that I have conveyed something of the sense 

 of confusion, almost chaos, that seemed to herald the discovery of 

 each new vitamin. It was rather as though one were watching the 

 weaving of some large tapestry without knowing anything of the 

 ultimate design. At first, one saw only jimibled patches of colour, 

 formless and meaningless, but as the work progressed these patches 

 resolved themselves one by one into familiar objects that one recog- 

 nised and that became of greater an4 greater significance as the 

 picture grew, until finally one could perceive the meaning and purpose 

 of the whole picture. Even now the tapestry is incomplete, and we 

 do not fully understand its purport. Perhaps we never shall, though 

 we may comprehend its meaning more fully as we watch the weaving 

 of other tapestries related in some way to the one that has formed the 

 subject of this book. 



629 



