MadSci Network: Biochemistry

Re: Where does Hydrogen Peroxide come from in the Body?

Date: Mon Sep 27 16:21:44 1999
Posted By: Andrew Cross, Faculty Molecular and Experimental Medicine, Scripps Clinic and Research Foundation, LaJolla CA
Area of science: Biochemistry
ID: 938367084.Bc

Dear Shadi,

Hydrogen peroxide is made by quite a few enzymes in the body. In 
particular, some enzymes breaking down certain amino acids and fatty acids 
(D-amino acid oxidase and acyl-CoA oxidase) make significant amounts of 
hydrogen peroxide. Since hydrogen peroxide can be damaging to normal 
tissue, these enzymes are kept inside specialized organelles inside cells 
called peroxisomes. The peroxisomes also contain large amounts of catalase 
to break down the hydrogen peroxide before it can escape. Other enzymes 
that make significant amounts of hydrogen peroxide are plasma amine oxidase 
and xanthine oxidase. In addition to enzymes that produce hydrogen peroxide 
as part of their normal catalytic cycle, many enzymes that undergo 
oxidation and reduction make hydrogen peroxide and other reactive oxygen 
species by autoxidation (a kind of side-reaction that is not part of their 
catalytic cycle). This happens quite a bit in the mitochondria.

Many of these autoxidation reactions do not produce hydrogen peroxide 
directly, but rather superoxide (the product of adding one electron to an 
oxygen molecule). In order to get rid of superoxide (which is more toxic 
than hydrogen peroxide) the body also contains lots of superoxide 
dismutase, that converts the superoxide into water and hydrogen peroxide.

One of the most interesting sources of superoxide in the body is that 
produced by white blood cells when they encounter harmful microorganisms. 
The white blood cells produce very large amounts of superoxide, hydrogen 
peroxide, and even hypochlorous acid (household bleach!) to kill the germs, 
just like in the TV ads.

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