316 DIGESTION 



The proteinases are therefore complementary in action and can break 

 down the large protein molecules to smaller and smaller units. Carboxy- 

 peptidases attack polypeptides at the carboxyl end, liberating the ter- 

 minal amino acid of the chain. In contrast, the aminopeptidases (mainly 

 secreted in the intestines) attack the peptide linkage at the free amino 

 end of the chain. To illustrate: 



HOlH HOjH 



NH,CHC04nH-CHC0 NH-CHCO-r-NHCHCOOH 



Ill 1 i ' 



R' j R^ R' I R^ 



Aminopeptidase Carboxypeptidase \ 



Neither trj^psin, chymotrypsin, nor carboxypeptidase are secreted as 

 the active enzyme by the pancreas, but rather as an inactive precursor, 

 called a zymogen. Trypsinogen, the precursor of trypsin, is converted 

 to the latter by the action of enterokinase (an enzyme present in intestinal 

 juice) or by trypsin itself. Chymotrypsinogen goes to chymotrypsin in 

 the presence of trypsin. Trypsin is also instrumental in the conversion 

 of procarboxypeptidase to the active enzyme. Trypsin, trypsinogen, 

 chymotrypsin, chymotrypsinogen, and carboxypeptidase have been iso- 

 lated in crystalline form. 



The pancreatic juice also contains lipases, enzymes that catalyze the 

 hydrolysis of fat, and amylases, enzymes that catalyze the hydrolysis of 



CH^O-CO-CCHOigCHs HOCH2 



CHO-CO-CCHOieCHs + HoO -^^^^ + HOCH 3CH3(CH2)i6COOH 



CH20-CO-(CHo)i6CH, HOCH2 



Tristearin Glycerine Stearic acid 



starch to lower molecular weight polysaccharides (dextrins) and maltose. 

 Pancreatic amylase has been crystallized and appears to be identical 

 in chemical, physical, and enzymatic properties with ptyalin. 



The flow of pancreatic juice is regulated in part by the nervous system; 

 however, it has also been found that the injection into the blood stream 

 of an extract of duodenal mucosa results in copious secretion of pancreatic 

 juice. This extract is a hormone called secretin. Another hormone from 

 duodenal mucosa, pancreozymin, has no effect on the volume of pancreatic 

 secretion, but it does effect an increase in the trypsin, amylase, and lipase 

 content of the juice (p. 308). About 600 ml. of pancreatic juice are 

 secreted daily by an adult man. 



