122 Comparatwe Animal Physiology 



utilization of urea and ammonia may occur in many metatrophic animals when 

 they are supplied with minimal amounts of the essential amino acids. 



SPECIFIC FACTORS IN NUTRITION 



A vitamin is an organic compound which must be supplied to an organism 

 in small amounts and which is used in specific ways other than as an energy 

 source or as a basic structural unit, although it may be incorporated as a 

 prosthetic group into a structural protein. Essential amino acids are usually 

 needed in slightly greater amounts than vitamins and are used primarily as 

 structural units in the building of protein; hence they are not vitamins. Some 

 vitamins are synthesized and distributed like hormones, but the vitamins 

 which are synthesized by animals are formed only from closely related sub- 

 stances (sometimes called provitamins), whereas hormones can be formed 

 from the basic foodstuffs. More vitamins are required for growth than for 

 adult maintenance, and what is a vitamin for one animal may not serve for 

 another, or it may be synthesized by another animal. Vitamins are discovered 

 by omitting various fractions from an animal's dietary and observing effects 

 of the deficiencies on growth, blood cell count, and skin appearance, and 

 sometimes pathology at the time of death. These criteria of deficiency are 

 many steps removed from the sites of cellular reaction, and recently a few 

 vitamins of the B complex have been shown to act as coenzymes in certain 

 metabolic reactions. Also the assay of foods for specific vitamins is facilitated 

 by the need of certain microorganisms, bacteria and yeasts particularly, for 

 vitamins. Yeasts and bacteria have a remarkable ability to synthesize most of 

 the water-soluble vitamins, yet certain species and strains are unable to syn- 

 thesize specific single ones, and thus they can be used for assay of these sub- 

 stances. For purposes of classification, vitamins can be divided into two groups: 

 those which are water soluble, and those which are fat soluble. 



Water-Soluble Vitamins: Thiamine (Vitamin BJ. The first B vitamin to 

 be discovered was thiamine, active in preventing beriberi in man and poly- 

 neuritis in birds. Thiamine as diphosphothiamine occupies a key position in 

 cellular metabolism as cocarboxylase, the coenzyme in pyruvate oxidation. In 

 all animals thiamine promotes growth, and at least in mammals its lack reduces 

 appetite. 



Thiamine is widely distributed in plant and animal tissues, and probably all 

 cells, whether aerobic or anaerobic, require it. Unlike other B vitamins it is 

 readily destroyed by heat. The thiamine molecule consists of two parts, a 

 pyrimidine and a thiazole moiety: 



+ 

 NH3 CH3 



/ I 



N = C C = C - CHo - CH.OH 



/ \ +/ 



CH3 - C C - CH2 - N 



X / \ 



N - C CH-S 



H 



pyrimidine + thiazole = thiamine 



Thiamine stimulates the growth of many unicellular organisms, even where 



