Amylase is secreted in active form. Pepsin and trypsin are first secreted as zymogens, inactive precursors that must be activated in order to function. Zymogens are produced with peptides blocking their active sites that must first be cleaved to function. Angiotensin is also secreted as a zymogen, angiotensinogen. Zymogens are useful in allowing the body to quickly activate an enzyme/hormone rather than synthesizing it from scratch.

The body compartmentalizes each hormone to help ensure it is it its optimum pH. Try to remember where each enzyme functions to help remember each enzyme's optimum pH. Pancreatic amylase is secreted by the pancreas into the small intestine. Amylase also occurs in the mouth, which is neutral. It's optimum is around 7. Pancreatic lipase is secreted by the pancreas into the small intestine, which is slightly basic because of the release of bicarbonate. Pancreatic lipase functions best at a pH of about 8. Trypsin is also secreted by the pancreas into the small intestine, and has an optimal pH of about 8. Pepsin functions in the stomach, which is extremely acidic. Pepsin's optimum pH is about 2.

Just after eating, the body is busily converting carbon-based life forms to glucose. If the body is about to have an influx of glucose, the the body must produce more insulin to allow for the passage of glucose into its cells. Glycogen, the stored form of glucose, would naturally increase since the person just replenished lost glucose or energy with food and will store it for later use. Glucagon would decrease because this breaks down glycogen. If the body is receiving fresh glucose, it doesn't need to break down its stores of glycogen into glucose. And finally, if the body is breaking down food into glucose, the liver doesn't need to release more freshly broken down (glucose that was previously stored as glycogen) glucose.

Salivary amylase is produced by the salivary glands and functions to hydrolyze starch to maltose. Pepsin is produced by the stomach to hydrolyze peptide bonds. Lipase is produced by the pancreas to hydrolyze lipids. Lactase is produced in intestinal glands to hydrolyze lactose to glucose and galactose. 

Vitamin B12 absorption requires intrinsic factor (IF), which is a protein secreted by the parietal cells of the stomach. When vitamin B12 binds to IF, it is able to be absorbed by the body in the terminal ileum. Therefore, removal of the ileum would likely result in a vitamin B12 deficiency, since the body cannot absorb the B12-IF complex.

The stomach has the lowest pH, roughly around 2.0 A low pH indicates a high acidic content. The stomach is the most acidic environment in the digestive system as it is the home to parietal cells. Parietal cells secrete hydrochloric acid  in response to eating in order to stimulate the acidic breakdown of food.

Pepsinogen is the inactivated form of pepsin. Pepsinogen gets activated by an increase in  ions (acid), which is released by the parietal cells. Pepsin helps with protein digestion and is released from the chief cells in the stomach. 

The tissues in the stomach are also made up of primarily protein. If it was produced in the stomach as its active form, then pepsin could also digest the stomach as well.