ADAPTATION OF PANCREATIC SECRETION 45 



field of tryptic action upon definite proteins, particularly in 

 relation to chemically pure polypeptids ; the situation is com- 

 parable to one in which of two unknowns in an equation, if 

 one may somehow be eliminated, the theoretical possibility of 

 a solution attained is probable. What are the structural 

 peculiarities upon which the possibility of a given polypeptid 

 being acted upon by trypsin depends? "The (molecular) 

 structure of the individual compounds comes into considera- 

 tion at once, ' ' says Abderhalden. 26 ' ' An instructive example 

 is found in the relation of alanyl-glycin (CH 3 .CH(NH 2 ).CO. 

 NH.CH 2 .COOH) and its isomer glycyl-alanin (NH 2 .CH 2 . 

 CO.NH.CH(CH 3 ).COOH). The former is split, the latter is 

 not. The particular kind of amino acid is also of importance. 

 In the dipeptids, for example, hydrolysis is favored if alanin 

 acts as acyl. The resistance of dipeptids containing 

 a-aminobutyric acid, a-aminovalerianic acid and leucin as 

 acyl is very notable. The number of aminoacids concerned 

 in the structure of a polypeptid is also of importance. The 

 glycin group illustrates this nicely. Gylcyl-glycin, diglycyl- 

 glycin and triglycyl-glycin are not split; while hydrolysis 

 takes place in case of tetraglycyl-glycin. ' ' It is of special 

 interest, however, that pancreatic fluid splits only those poly- 

 peptids in whose molecular structure the natural optically- 

 active aminoacids take part; and that the economy, if 

 raceme bodies are at its disposition, may often be able to 

 utilize only one of the two optically opposite components. 

 This is true both for the mammalian body and for the low- 

 est forms of plants (cf. Vol. I of this series, Chemistry of the 

 Tissues, p. 15). 



Adaptation of Pancreatic Secretion to the Food. The 

 Pawlow school has assumed a general adaptability of the 

 secretion of the pancreas to the particular type of food 

 which may obtain at any given time; in other words, that 



88 E. Abderhalden, Lehrb. d. physiol. Chem., 2d ed., 626-628, 1909; cf. 

 Literature, thereto appended. 



