In an article published about a year and a half ago, Dr. Anthony Bassler1 made the following statements:
1. All proteolytic enzymes are dual in their action of conversion. This is true of the one in the stomach as well as of that in the pancreatic juice, both of which require outside activation, that of the stomach from hydrochloric acid and that of the pancreas from a product of the duodenal mucosa (enterokinase).
By this I judge he meant that some other agent is necessary to activate these enzymes. Certainly he cites hydrochloric acid as being necessary to activate pepsin. As this may cause misunderstanding, let me say that what pepsin needs for activation is a medium in which the hydrogen ion concentration is within certain limits. The same factor is necessary for trypsin. Therefore, no more than diastase and lipase are they dual in their action of conversion.
MARTIN L. THE TIME FACTOR IN THE ACTION OF PANCREATIC ENZYMES. Arch Intern Med (Chic). 1930;45(4):535-537. doi:10.1001/archinte.1930.00140100057005