Catalases, or hemes containing redox enzymes, are mostly found in the peroxisome of most cells. Peroxisomes are about the size of lysosomes and are bound by a single membrane. They are filled with enzymes and its functions in the human liver include the oxidation of excess fatty acids and the making of cholesterol and bile acids. However, lethal hydrogen peroxide results from such fatty acid oxidation. Fortunately, peroxisomes also prevent the hydrogen peroxide from harming the cell itself by housing life-saving catalases, which may be released whenever necessary to destroy meandering free radicals.
For those organisms that lack a nuclear membrane and any membrane bound organelles such as peroxisomes, catalases are located in the space between the inner and outer membranes of the cell wall (periplasmic space). The periplasmic space also stores numerous toxic enzymes that would kill the cell itself, but catalases at the site are able to break them down or the enzymes are simply released to the environment to attack other toxic molecules. Sometimes, catalases are released by the bacteria to shield them from being eaten by white blood cells during an infection.