MadSci Network: Biochemistry

Re: Catalase?????

Date: Mon Sep 28 23:13:55 1998
Posted By: kumar chandraskharan, Grad student, Cellular and Molecular Cardiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology
Area of science: Biochemistry
ID: 906830424.Bc

In 1965 De Duve et al isolated a group of intracellular particles from 
liver cells and other tissues by using cell fractionation methods. These 
particles were enriched with some oxidative enzymes, such as peroxidase, 
catalase, D-amino oxidase, and urate oxidase. The name peroxisome was 
applied because this organelle is specifically involved in the formation 
and decomposition of hydrogen peroxide (H2O2). H2O2 and free radicals are 
produced as a result of certain oxidative reactions involved in the 
break down of amino acids and fats. These are very reactive chemical 
species that could damage cellular machinery. To protect the cell from 
these destructive byproducts, such reactions are segregated within these 
small membrane-bounded peroxisomes. Peroxisomes contain four enzymes 
related to the metabolism of H2O2. Three of them - urate oxidase, D-amino 
oxidase, and alpha-hydroxylic acid oxidase - produce peroxide (H2O2), and 
catalase destroys it (2H2O2 - 2H2O + O2).

Catalase is in the matrix of liver peroxisomes and represents up to 40% of 
the total protein. The enzyme urate oxidase and two other enzymes present 
in amphibian and avian peroxisomes are related to the catabolism of 
purines. catalase is synthesized on free ribosomes, and they are packaged 
into peroxisomes without undergoing a proteolytic processing. 
Free radicals are molecules that, because they contain an unpaired 
electron, are extremely reactive. These include the superoxide radical O2.- 
(the dot identifies the unpaired electron) and the reactive hydroxyl 
radical (OH.). H2O2 although not itself a free radical, forms reactive 
compounds in reactions that are catalyzed by Fe++ and Fe+++. H2O2 also 
reacts with the superoxide radical to produce even more reactive free 

H2O2, as well as superoxide anion (O2.-) are normal metabolites in aerobic 
cells. The peroxisome is the main source of H2O2; however, only a fraction 
of this H2O2 diffuses to the cytosol, and most of it is degraded by the 
intraorganellar catalase. Most Cytoplasmic H2O2 is produced by mitochondria 
and membranes of the endoplasmic reticulum, although there are also 
H2O2-producing enzymes in the cytoplasmic matrix. Catalase acts as a 
"safety valve" for dealing with the large amounts of H2O2 generated by 
peroxisomes, however, other enzymes such as glutathione peroxidase, are 
capable of metabolizing hydroperoxides and also H2O2, in the cytosol and 
mitochondria. The production of superoxide anion in mitochondria and 
cytosol is regulated mailnly by the enzyme superoxide dismutase. All of 
these protective enzymes are present in high levels in aerobic tissues.

In recent years the possible relationship between peroxides and free 
radicals (such as O2.-), with the process of aging is being emphasized. 
Such radicals acting on DNA could produce mutations, altering the 
transcrition into mRNA and the translation into proteins. In addition free 
radicals can affect the membranes by causing peroxidation of lipids and 
proteins. For these reasons reducing compouns such as vitamin E or enzymes 
like superoxide dismutase couls also play a role in keeping the healthy 
state of a cell.

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