Hi, Bishwaranjan, catalase is only peroxisomal enzyme. NADPH oxidase is a major source of extracellular superoxide. And there are a lot of types of extracellular POD for deal with H2O2 in the apoplast.

Hellow Taras, Yes, you are correct that CAT is a peroxisomal enzyme because it evolved to be so and therefore we know it is peroxisomal. I think since ROS are basically formed in the cells a complete antioxidant cascade system including enzymes and non-enzymes developed in cytoplasm. Why such system is not developed at EC space is my basic question.

Hi Biswaranjan,

interesting question. My answer is a little speculative. However here goes:- hydrogen peroxide is very membrane permeable, whereas superoxide is not very membrane permeable hence extracellular hydrogen peroxide is only converted to oxygen and water when it diffuse into the intracellular compartment which sequesters catalase particularly in the red cells. Gaseous oxygen is buffered in red cells as there is plenty of haemoglobin to bind it and prevent it taking part in oxidation reactions. In extracellular fluid excess oxygen can result in harmful oxidation reactions particularly if there is any labile iron around. So conversion of extracellular superoxide to hydrogen peroxide in the extracellular compartment which is then diffuses to the intracellular compartment is a way of compartmentalizing the production of potentially harmful gaseous oxygen.

Hi, Richard,

Interesting hypothesis, however, one should also consider half-life of the radical. Superoxide has a half-life around 25 µsec, so, the diffusion radius is restrict to few µm only. But H2O2 half life is much longer, let's say it remain stable before meet with some antioxidants. And, of course, H2O2 can move across membranes. There is also quite high H2O2 intracellular production itself. So, I am not sure that movement of extracellular H2O2 inside cells is a the main reason of the absence of the catalase. Do not forget also that catalase require specific conditions and may not be functional effectively outside of the cells..

Hi Taras,

Certainly what you say is correct. However the point I was making is that catalase reacts with hydrogen peroxide to generate oxygen. There are many extracellular situations where oxygen is pro-oxidant, consequently since oxygen buffers are exclusively intracellular, it might be advantageous to retain as much oxygen as possible in the intracellular compartment until it is used productively in intracellular oxidative phosphorylation reactions-rather than add to the unbuffered extracellular oxygen load.

Hi, Richard, you are right, catalase may produced oxygen. However, extracellular space also have quite a lot buffering capacity like extracellular ascorbate: http://www.jbc.org/content/281/52/39852.short, which also present in human erythrocytes. However, for erythrocytes intracellular oxygen production should have a certainly positive effect!

Some have suggested that SOD is not produced for explicit transport to extracellular spaces, but exists there as a consequence of its high abundance in the cytosol and the random lysis of cells in the body (particularly erythrocytes)

Hello Biswaranjan! as many colleagues have pointed out, presence of catalase is not the only way to decompose hydrogen peroxide: there are non-enzymatic compounds capable of function as hydrogen peroxide sinks, such as ascorbate; but also other enzymes can catalyze decomposition of the peroxide, such as glutathione peroxidase. In fact, it is the latter the enzyme that catalyzes hydrogen peroxide reduction into water (and GSSG) in the cytoplasm of cells.

Now, let me pose the following thoughts (speculative) on this matter:

- why is it so surprising not to find extracellular catalase?

- is it really necessary to have this enzyme just because there is an extracellular SOD?

- it is true that plasma membrane NAD(P)H oxidases catalyze generation of superoxide anion, and that extracellular SOD catalyzes decomposition of the superoxide anion into hydrogen peroxide... but is it supposed to be further reduced into water and oxygen by a catalase?

- maybe, this generates a sort of oxidizing/reducing potential across the plasma membrane that is essential for the function of extracellular proteins and molecules... I will elaborate this thought: For instance, there are reports showing that maintenance of a more oxidizing environment (redox potential of -180mV) in the ER lumen compared to a more reducing environment in the cytoplasm (redox potential of -220 to -240 mV, depending on the cell type and the metabolic state). This redox potential difference across the ER membrane appears to be essential for the adequate function of ER transmembrane proteins and the more oxidizing environment is crucial for protein disulfide formation during protein synthesis within the ER. The same redox potential difference could be proposed for the extracellular/cytoplasm compartments. This hypothesis is in agreement with the absence of an extracellular catalase, small concentration of extracellular GSH compared to cytoplasm GSH and the fact that the function of several extracellular proteins (e.g. integrins and tyrosine kinase receptors) depend on the extracellular disulfide bonds. Anyways, if you are interested in this, maybe you can add catalase to the extracellular media of fibroblasts and then evaluate if they are as efficient as untreated fibroblasts to attach to the culture plate or to make focal adhesions... just a brainstorm thought there!

Sorry if I diverge too much, but I think differences in redox potential depending on location is a very interesting subject that has yet to be tackled systematically and very little is known about it. Maybe you can contribute to this subject :)

Good luck!

This theory can also be bolstered by the low Km of Catalase for H2O2. It's not the best hydrogen peroxide scavenger in existence. If it was, peroxyredoxins would not work very well as an intracellular H2O2 sensor. Getting back to the comment about NADPH Oxidases, Nox4 is considered by some to be a H2O2 (not superoxide) generating enzyme...

Taking in to account previous comments: superoxide anions can dismutate to hydrogen peroxide spontaneously or catalysed by different superoxide dismutase isoemzymes. Whereas superoxide anions are confined to the site of their generation, hydrogen peroxide can pass membranes readily and reach the cytosol where it can be eliminate by endocellular antioxidant systems (enzymatic and not-enzymatic)..I'd like to speculate again on this matter like Paula with another question: is necessary their presence in the extracellular environment?? It well know the pivotal role of ROS in the complex cellular signaling network so if high ROS concentration is harmful for cells instead nontoxic levels of ROS like hydrogen peroxide (H2O2) could mediate the transmission of many intracellular/intercellular signals, including those involved in growth and transformation suggesting an autocrine and/or paracrine role for ROS in different cellular processes such as growth, apoptotic signaling ..might be an explanation about lack of higly efficient extracellular enzymatic antioxidant system compared to endocellular one.

Barbara

Thank you all of you for your answers. I am also not agreed to speculate that since H2O2 can pass the membrane and there are few compounds in EC space to nutralize the same, so CAT is not evolved to be in EC. One thing, passing H2O2 to cytoplasm from EC space against there concentration gradient may not be a favourable condition for the cells. Another thing is that the Kmax and proper environmental condition mostly pH might fixed by the cells first then to adapt CAT enzyme in peroxisome later. It could happen also CAT evolved first and then the protein adjusted it self to be more functional at 7 pH and so on in peroxidasomal environment. This may be a separate chapter for my question. Prof. Rechard's O2 buffering speculation seems to be more relevance to the answer. However, does any one believe that "H2O2 is more required in EC space so that CAT should not be there to scavenge it" in order minimize the cell dysfunction could contribute some thing to my question?

Dismutase enzmes have prime role in neutraliztion of free radicals than catalase

that why heart is more sensitive to free radicals than other tissues

Yes Catalase is intracellular

Hi,

more than an answer mine is an extra information. You are all clearly focusing on animal cells and not plants. Incredible amounts of ROS are produced by chloroplasts where CAT is present. Also note that SOD seem to be involved in the formation of lignin in the extra cell environment.

Finally, ROS are also important signalling molecules and are responsible for activation inactivation of the synthesis of several enzyme. It would not be surprising to me if the presence of CAT in the extracellular environment would limit the ability of the cell to "sense" its oxidative state.

Hi again,

I missed the reply made by Barbara which sort of goes in the same direction I pointed. I completely agree with her speculation: some ROS have to be present in the extracellualr environment to regulate and coordinate process at organism level.

Catalase is primarily located in peroxisomes and the purposeful targeting to this specific sub-cellular organelle (not by a signal peptide) suggests a fundamental role in the beta-oxidation of long chain fatty acids, i.e., an intracellular event.