18 SCIENCE PROGRESS 



data that there is something in his pancreatic extracts which 

 considerably increases the destruction of sugar in muscle 

 extracts, when added to these in a certain proportion. 



ZYMOGENS 



Since all enzymes are products of cell activity, it follows 

 that, in all probability, some intermediate stage of their forma- 

 tion from cell protoplasm may be separated from the cells 

 producing them. Such a body is the mother-substance ol 

 pepsin, prepared from the gastric mucous membrane by Langley 

 and Edkins. This "pepsinogen" is itself inactive, but is trans- 

 formed into active pepsin by the action of acid. The similar 

 body formed by the pancreatic cells (trypsinogen) is secreted as 

 such, and remains inactive until it comes into contact with a 

 specific enzyme (enterokinase), found by Pawlow and his pupils 

 in the small intestine. This enterokinase is secreted by the 

 epithelium of the small intestine, and converts the trypsinogen 

 into active trypsin. 



These zymogens must be carefully distinguished from the 

 enzymes above mentioned as being inactive apart from their 

 respective co-enzymes. In this latter case the process of 

 activation is a reversible one, in the sense that an active system 

 can be rendered inactive by removing the co-enzyme and 

 restored to activity by replacing the latter. An active enzyme, 

 however, once produced from its zymogen, cannot be recon- 

 verted into its precursor; the process appears to be one of 

 hydrolysis, by which a new body is formed. The system of 

 inactive enzyme and co-enzyme is rather an association of two 

 different bodies, probably of the nature of adsorption. 



CHEMICAL AND PHYSICAL PROPERTIES 



Enzymes, as mentioned already, are colloids, and therefore 

 exhibit the characteristic properties of this state of matter. Such 

 are indiffusibility, precipitation by heat, greater or less instability, 

 and probably an electric charge of their suspended particles. 



It is impossible to make any definite statement as to their 

 chemical nature, except of a negative character. The really 

 active subtance is present in the usual preparations in so small 

 an amount that it appears almost hopeless to discover its 

 chemical constitution. As colloids they readily carry along with 



