ANIMAL METABOLISM 341 



In general, it must be concluded that acetic acid, or some closely related 

 Co substance, is a very active material metabolically and enters into 

 many of the catabolic and anabolic activities of the living animal 

 cell.' 



METABOLISM OF PROTEINS 



Synthesis and interconversion of amino acids in animal tissues 



Essential Amino Acids. The metabolism of proteins in the animal 

 body is largely a matter of the transformations of amino acids. At least 

 20 of these "building blocks" are present in animal proteins and must 

 therefore be supplied to the animal, either directly from food proteins 

 or indirectly by synthesis from other food constituents. Those acids which 

 cannot be synthesized by the animal at measurable rates are called nutri- 

 tionally essential amino acids. Those which can be synthesized, but at 

 rates which are sometimes inadequate {e.g., during rapid growth) may be 

 called semiessential. Presumably all other amino acids present in body 

 proteins must be capable of being synthesized in adequate amounts (and 

 fast enough) to meet all requirements. Of course, these "nonessential" 

 amino acids also may be present in the food, and, in fact, the main supply 

 normally comes from this source. 



Lists of essential and semiessential amino acids are given in Table 13-1 

 for several species. These lists are based mainly on studies of the growth 

 of young animals and on nitrogen balance studies with adults. The 

 latter method, which is the one used for experiments with human beings, 

 involves a comparison of the total intake and output of nitrogen when 

 only certain amino acids are given the subjects. If the lack of a par- 

 ticular amino acid results in a negative nitrogen balance (output larger 

 than intake), this is evidence that the acid is needed and cannot be syn- 

 thesized in the body. For the best nutrition the diet should supply not 

 only the essential and semiessential amino acids, but a good selection 

 of the nonessential ones as well. Although the latter can be produced 

 in the tissues from other materials (see below), it is probably more 

 efficient to consume them ready-made in the form in which they are 

 needed. 



It will be noted in Table 13-1 that some of the essential amino acids 

 can be replaced by certain closely related substances, namely, the 

 D-isomers, or the alpha-keto or alpha-hydroxy analogs. This situation 

 probably results from the fact that these amino acids can enter into 

 the process of transamination (see below) . In these cases it is the carbon 

 chain of the amino acid which is the essential feature, and not the alpha- 

 amino group. It has also been shown that a-amino adipic acid can 

 replace lysine for the rat, that is, it acts as a physiological precursor of 

 this essential amino acid. 



