Nutrition 131 



isolated three unkown essential factors. One of these proved to be a purine, 

 and guanine is specific, although others are additive; another of these factors 

 is a pyrimidine, and in cultures cytidylic acid or uracil is used; the third is still 

 unknown (see p. 135). 



A malaria organism, Plasmodiuvi knowlesii, needs purines and pyrimidines 

 for best survival in vitro.^-^'' Drosophila larvae have been grown on a syn- 

 thetic medium which includes ribonucleic acid, '-"* and it is possible to substi- 

 tute nucleotide or the purine adenine for this requirement. '^-^ Other insects 

 seem not to require dietary purines. In no mammals have purines and pyrimi- 

 dins been shown to be dietary essentials, but since all cells contain them in 

 nucleic acids, the enzymes for purine and pyrimidine synthesis must be very 

 widespread. Numerous animals, such as Tetrahyniena, lack the enzymes 

 present in mammals for synthesis of guanine and uracil. 



Hematin. Probably all aerobic organisms have oxidative enzymes and 

 carriers (cytochromes) which contain iron in a porphyrin ring similar to that 

 in hematin. The hemochromogen of cytochrome is more primitive in an 

 evolutionary sense than hemoglobin and is synthesized by many animals and 

 plants which do not make hemoglobin. A few blood parasites, the hemo- 

 flagellates, have lost the ability to synthesize the hematin which they need 

 and therefore rely on their host. Closely related flagellates which are parasitic 

 on insects and plants (e.g., Strigomonas^ do not require hematin when cul- 

 tured in vitro but presumably synthesize it. Several species of Leishviania and 

 Trypanosovia which parasitize birds and mammals require hematin, proto- 

 hematin, or protoporphyrin (but cannot use other related substances) when 

 cultured in vitro. ^^ This is an interesting instance of loss of a synthetic 

 function by a parasite with increasing dependence on the host. Hematin is, 

 then, a vitamin for these trypanosomes and for J^eishmania, but not for other 

 kinds of animals. 



Fat-Soluble Essential Substances. The fat-soluble vitamins differ from the 

 water-soluble ones in being much more restricted as dietary requirements. 

 Vitamins A, D, E, and K are known only for vertebrates. 



Vitatnin A. One form of this is: 



CH3 CH3 



^C CH3 CH3 



/\ H h| hhhIh 



HC C-C=C-C=C-C=C-C-C-CH,OH 



"I II 



H,C C-CH3 



\/ 



Vitamin A is synthesized by animals from plant carotenes and is a part of 

 several pigment systems. The best known of these is the visual purple pigment 

 of the vertebrate eye. This visual pigment is formed from vitamin A, which 

 occurs in two forms— Ai found in the retina of higher vertebrates and marine 

 fish, and A^ found predominantly in fresh-water Hsh (see Fig. 120, Ch. 11). 

 Vitamin A is stored in the Hver, and a deficiency may result in xerophthalmia. 

 Vitamin A2 of fresh-water fish can be used by mammals, but less well than A^. 



