VITAMINS, ANTIBIOTICS, AND GROWTH 337 



Still unanswered is the question of why our cells lost the ability 

 to manufacture vitamins. There are experimental observations that bear 

 on this. \Vhen the mold Neurospora crassa is treated with X-rays or 

 with ultraviolet light, mutants are produced which have lost the ability 

 to synthesize various nutritional essentials. One of these mutants, No. 

 34486, cannot make choline from methionine. Now a rat can readily 

 make choline from methionine, while a baby chick on a purified diet, 

 without choline but with ample methionine, develops choline de- 

 ficiency. The chicken under these conditions fails to grow, and its bones 

 become deformed. Perhaps at some time during its evolutionary history, 

 the Archaeopteryx or the Eoornis received a dose of ionizing radiation 

 which damaged the genetic material that makes the enzymes which are 

 responsible for producing choline from methionine, and in this respect 

 the chicken resembles Neurospora mutant 34486. 



From these speculations let us turn to a review of the most active 

 field of vitamin research today— the biological synthesis of the vitamins. 

 Radioactive tracers have enabled rapid progress to be made in study- 

 ing the way in which small units such as ammonia, acetic acid, formate, 

 carbon dioxide, and phosphate are assembled into the complex forms 

 of life. 



Biological synthesis of certain vitamins 



Vitamin A. Animals depend, directly or indirectly, almost entirely 

 upon green plants as the natural source of vitamin A, which is made, 

 of course, from carotene. Vitamin A apparently arises from four iso- 

 prenoid units via acetic acid and mevalonic acid (Braithwaite and 

 Goodwin, 1957); see Figure 1. 



Although animals can manufacture squalene and cholesterol by 

 making and condensing isoprenoid units, only plants can put these 

 units together to make carotenoid pigments. These pigments have a 

 phototropic function in plants. In animals, vitamin A is involved in the 

 visual process as retinene, a retinal pigment which undergoes a light- 

 induced cis-frans shift. Thus both carotene and vitamin A are con- 

 cerned in photoreception. Vitamin A has other functions which are 



Qtl3^/^CH3 CH3 CH3 



HoC^ '"^C — CH=i=CH-C = CH-CH=|=0H-i-CH-CH20H 



I II • 



^aC-x' /C — CH3 

 ' c 

 H2 Vitamin A 



Figure 1. Vitamin A, showing method of origin from four isoprenoid units. 



