154 UNITY AND DIVERSITY IN BIOCHEMISTRY 



When the transferase systems (transphosphorylases, transglycosidases, 

 transpeptidases, transmethylases, transacylases, etc.) which function inside 

 the cell, cease to act and compete with the transfer to water, the hydrolases 

 take over and cause a certain amount of hydrolysis of the cellular contents. 



(a) Peptidases 



All cells contain peptidases, and in many specialized cases they secrete 

 enzymes of this type to the exterior ; this is the case, for example, with many 

 bacteria, and with certain cells of the digestive tract of animals, etc. There 

 are many special aspects of this subject which will not be discussed here. 



The peptidases are divided into endopeptidases and exopeptidases. 

 The endopeptidases (formerly proteinases) are able to attack all the peptide 

 bonds in a molecule, even those which are some distance from terminal 

 groups, whilst the exopeptidases (formerly peptidases) can only hydrolyse 

 peptide bonds at the ends of the chain. Among the exopeptidases, some 

 remove terminal residues having a free carboxyl group (carboxypeptidases) 

 whilst others remove those where the amino group is free (aminopepti- 

 dases). 



Still little is known about the system of intracellular peptidases. Some 

 studies have been made of the so-called cathepsin, which is the intracellular 

 peptidase system of mammalian kidney and spleen. The studies of Berg- 

 mann and his collaborators have revealed that the system contains endo- 

 peptidases, carboxypeptidases and aminopeptidases. 



[b) Carbohydrases 



Like the peptidases, these enzymes are universally found in the bio- 

 sphere. They are considered under two headings, glycosidases which 

 hydrolyse di- and trisaccharides and glycosides, and polysaccharases 

 which hydrolyse macromolecules such as starch or cellulose. 



The glycosides possessing a free reducing group can exist in the a form 

 or the j8 form. One can also characterize the reducing group involved in 

 the glycoside linkage by referring to a-glycosides and to ^-glycosides. Mal- 

 tose, for example, is an a-glucoside, lactose a j8-galactoside, and sucrose, 

 at the same time, is both an a-glucoside and a ^-fructoside. 



There are a certain number of glycosidases which are specific for a given 

 linkage regardless of the molecule which contains it : a-glucosidase 

 (formerly maltase), ^-glucosidase (formerly emulsin, cellobiase, gentio- 

 biase), a-galactosidase, /S-galactosidase, j8-fructosidase (formerly invertase, 

 saccharase, etc.), a-mannosidase. In addition, there are a number of gly- 

 cosidases which are specific for a given compound (for example, trehalase 

 which acts only on trehalose and not on other a-glucoside linkages). 



The glycosidases are without action on polysaccharide macromolecules, 

 although starch and glycogen, for example, consist of chains of glucose 



