They were first discovered in 1954 by the Swedish biologist J. Rhodin in the kidney cells of the mouse. They are single membrane-bound organelles containing multiple oxidases, peroxidases and catalases. Peroxisomes are found in most eukaryotic cells, including animals, plants and fungi. They are particularly abundant in cells with high metabolic activity, such as liver and kidney cells. They are heterogeneous organelles that can change their form and number to adapt to their environment. For example, in individuals who consume alcohol regularly, peroxisomes in the liver increase in number and size to more efficiently metabolize the ingested alcohol.
The peroxisomes in the cytoplasm are typically spherical or ellipsoidal vesicles with a diameter ranging from 0.2 to 1.5 micrometers, usually around 0.5 micrometers. They are surrounded by a phospholipid bilayer with proteins that selectively control the entry and exit of substances. They also contain oxidases and catalases. Catalase is the marker enzyme of peroxisomes.
Peroxisomes are involved in a range of metabolic reactions, primarily related to the breakdown of fatty acids and detoxification of harmful substances. They also participate in the synthesis certain lipids.
Detoxification: The main function of peroxisomes is to break down large molecules and detoxify harmful substances. Oxidases remove hydrogen atoms from organic compounds such as alcohols, phenols, and aldehydes, producing hydrogen peroxide as a byproduct.
RH₂ + O₂→ R + H₂O₂
Peroxisomes utilize the hydrogen peroxide to further oxidize organic compounds.
R′H₂ + H₂O₂ → R′ + 2H₂O
When there is an excess of hydrogen peroxide, the catalases within peroxisomes catalyze them into water and oxygen to prevent cellular damage.
2H₂O₂ → 2H₂O + O₂
Ultimately, these toxic organic compounds are oxidized into non-toxic substances.
Beta-Oxidation of Fatty Acids: It not only occurs in mitochondria but also in peroxisomes, especially for some long-chain fatty acids (C > 23). They hardly pass through the mitochondria inner membrane and must be broken down in peroxisomes for shorter fatty acids which are for further oxidation. This process involves the sequential removal of two-carbon units from the fatty acid chain, generating short fatty acids, acetyl-CoA molecules and high energy compounds, such as NADH and FADH₂. They are transported to mitochondria and enter the tricarboxylic acid cycle, also called the citric acid cycle, to be oxidized to water and carbon dioxide.
In detoxification and fatty acid oxidation, these oxidases and catalase form a simple electron transfer chain without ATPase, so the energy released from the oxidation of organic matter is dissipated as heat energy.
Metabolism of Lipids: Peroxisomes play a role in lipid metabolism, including the synthesis and breakdown of specific lipids. They are involved in the synthesis of plasmalogens, a type of phospholipid that is important for cellular membranes.
Biosynthesis of Bile Acids: In liver cells, peroxisomes are involved in the biosynthesis of bile acids. Bile acids are important for the digestion and absorption of dietary fats and fat-soluble vitamins.
Metabolism of Purines and Polyamines: Some peroxisomes contain enzymes involved in the metabolism of purines, which are components of DNA and RNA. They also participate in the synthesis and degradation of polyamines, which are involved in various cellular processes, including cell growth and differentiation.