MadSci Network: Biochemistry
Query:

Re: Why don't digestive enzymes attack living cells?

Date: Sun Feb 19 20:24:12 2006
Posted By: Steve Mack, Assistant Staff Scientist, Molecular and Cell Biology
Area of science: Biochemistry
ID: 1140306063.Bc
Message:

Hi Craig,

Thanks for the great question. The brief answer to your question is that, if they had the chance, digestive enzymes would attack living cells. The trick is that our cells and tissues don't give the digestive enzymes they make a chance to digest them.

They do this in a few different ways; for example, the digestive enzyme pepsin is found in the stomach, but it does not digest the stomach, because the walls of the stomach are covered in a thick layer of mucus that serves as a physical barrier to protect the stomach tissue from pepsin digestion. This is discussed in this answer (931544157.Bc) in our archives. In addition, this answer (871783451.An) points out that pepsin is only active at low pH conditions, while the tissue covered by the mucous layer experiences much higher pH conditions, so that pepsin will not be active when it is near the stomach tissue. Finally, as described in this answer (975694774.B), when pepsin is made in the cell, it is made in an inactive form (pepsinogen) that is 44 amino acids longer than pepsin. When pepsinogen finds itself in an environment with a pH lower than 5, it cleaves off the 44 amino acids on its N-terminus, resulting in active pepsin.

So, living cells can protect themselves by physically separating themselves from digestive enzymes, by producing digestive enzymes that only work under specific pH conditions, and by producing enzymes that do not become active until they encounter specific pH conditions.

This answer (983141879.Mi) to the question of how tapeworms avoid digestion indicates that trypsin and chymotrypsin, digestive enzymes found in the small intestine, are actually inactivated by the tapeworm, so that they can't digest the creature. So, living cells can also turn off digestive enzymes. That sort of inactivation is explained in this answer (963184462.Bc).

The examples I gave here are for enzymes found in the digestive tract, but most digestive enzymes are found inside of our cells. However, the same basic mechanisms are at work protecting cells from the digestive enzymes inside of them. In most cases, the digestive enzymes are found in a special compartment of the cell (an organelle) known as the lysosome. The membrane of the lysosome is a protective barrier that keeps the enzymes out of contact with the rest of the cytoplasm. In addition, the pH inside the lysosome is lower than the pH of the cytoplasm, so that the digestive enzymes in the lysosome won't work very well outside of the lysosome, just as the pepsin in the stomach functions best at low pH conditions. Why isn't the lysosome digested itself? Well, that is explained in this answer (968681370.Cb).

You can search our archives for other answers containing the terms I've used here!

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