CARBOHYDRATES 65 



other substances may influence the optimum pH for a given 

 action. Langfelt recently has shown that the optimum pH for 

 liver amylase in the presence of chlorides is 6.8, in the presence 

 of phosphates, 6.2 and in the presence of adrenaline, 7.73. 



Reversibility. Some of the enzymes have the power of 

 causing a chemical reaction to go either way, that is, of de- 

 composing a compound, or under proper conditions of building 

 up the same compound from its decomposition products. Cer- 

 tain of the lipases are notable examples. The property is spoken 

 of as reversibility, and these enzymes are said to be reversible in 

 their action. 



This phenomenon was demonstrated by Croft Hill, working 

 with maltose and maltase, and he was able to show that the 

 enzyme caused a resynthesis of disaccharide from the glucose 

 produced. Kastle and Loevenhart on mixing butyric acid and 

 ethyl alcohol with pancreatic extract were able to detect the 

 odor of ethyl butyrate, indicating that the lipase of the pan- 

 creas had caused a recombination of these substances to form 

 the ester. The importance for the cell of the reversibility of 

 enzyme action is apparent, for much of the work of the cell 

 must be synthetic, either in building up or repairing its own 

 structures, or in producing reserve materials for storage, such 

 as glycogen, fats, lipins, or other substances. 



Active and Inactive Form. In the form in which they are 

 secreted by cells, some of the enzymes are inactive, and become 

 capable of exerting their customary activity only after they have 

 been acted on by some other substance. The enzymes are thus 

 said to exist in inactive and active forms. 



The inactive form is often called the zymogen form; the 

 activating substance, the coenzyme or kinase. The coenzyme 

 may be a salt, such as a chloride or phosphate, an acid, for 

 example the gastric hydrochloric acid which activates pepsino- 

 gen. Coenzymes are usually unaffected by heat, so they obvi- 

 ously are not themselves enzymes. They often dialyze, and 

 many enzymes are rendered inactive by dialyzing off the salts 

 or other dialyzable substances in the solution. 



