Enzymes are proteins that help start and/or speed up a chemical reaction. Enzymes are sensitive to many factors the biggest being pH and temperature. An enzyme that works in the stomach can tolerate extremely acidic conditions. Gastric juices have a pH of 1 or 2, very acidic. An enzyme that works in the bloodstream would require a more neutral pH and would be deactivated if put in the stomach. The temperature is the same throughout the body so that would not be an issue.
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