308 UNITY AND DIVERSITY IN BIOCHEMISTRY 



These latter synthesize ptyalin, an endoamylase which is also an a-amy- 

 lase. When we come to the gastric mucosa, besides mucous cells we find 

 present parietal cells specialized for the secretion of hydrochloric acid by a 

 mechanism of active transport, and peptic cells which can synthesize 

 pepsinogen, from which an endopeptidase, pepsin, is formed. 



The cells of the exocrine pancreas are remarkably specialized for the 

 production of a whole series of enzyme proteins. Among these proteins we 

 find a lipase, an amylase, a maltase, a lactase, invertases, an exopeptidase 

 (carboxypeptidase) and various proenzymes, notably trypsinogen and 

 chymotrypsinogen. These two latter substances are transformed in the 

 intestinal lumen into two endopeptidases, trypsin and chymotrypsin. The 

 secretory cells of the intestinal mucosa produce a series of exopeptidases : 

 leucine aminopeptidase, glycyl-glycine dipeptidase, aminotripeptidase, 

 glycyl-leucine dipeptidase, prolidase, etc. In addition they synthesize a 

 maltase, an invertase, an enterokinase, nucleotidases and nucleosidases. 



One of the biochemical characteristics of the cells of the hepatic paren- 

 chyma, in the domain of carbohydrate metabolism, is the presence of 

 glucose-6-phosphatase. Its presence enables the liver cell to liberate glucose 

 from G-6-P. The liver cell is also able to set free glucose from G-l-P 

 and liver glycogen. The cells of the hepatic parenchyma are much more 

 active than other cells in utilizing CO2 for the synthesis of oxaloacetic acid 

 from pyruvic acid and of malic acid from pyruvic acid. The carbohydrate 

 metabolism of the liver cell is much more complex than that of other types 

 of cell for a multiplicity of operations is involved : conversion of glucose 

 into various hexoses, glycogenesis, glucose oxidation, synthesis of amino 

 and fatty acids, glycogenolysis and gluconeogenesis. In the degradation of 

 fatty acids, the liver cell is found to possess a special biochemical pecu- 

 liarity; there is a check on the speed of entry of acetyl- Co A into the tri- 

 carboxylic acid cycle. Even under normal conditions, the stationary state 

 of the cell in the hepatic parenchyma is characterized by a certain accumu- 

 lation of acetyl-CoA due to the fact that its entry into the tricarboxylic acid 

 cycle is less rapid than in other tissues. Associated with this peculiarity is 

 another, which is the existence of a side reaction in which two molecules of 

 acetyl-CoA condense together and lose a molecule of CoA to form aceto- 

 acetyl-CoA which is hydrolysed by a deacylase into acetoacetic acid and 

 CoA. This reaction only occurs to a slight degree in most cells and the 

 small amount of acetoacetic acid formed is reconverted into acetoacetyl- 

 CoA, but in the liver cell this reconversion only occurs to a slight extent. 

 The slowness of oxidation of acetyl-CoA, the concentration of which is 

 maintained at a higher level than in other cells, leads to a greater production 

 of acetoacetic acid which is only slightly reconverted to acetyl- CoA, and 

 hence ketone bodies are formed. In fact, in the liver cell, the greater part 

 of the acetoacetic acid formed is reduced to j8-hydroxybutyric acid by 



