The cell is recognized as the basic unit of life, present in all living organisms, each of which has at least one cell that performs essential functions such as nourishing itself, growing, reproducing, differentiating, and reacting to chemical stimuli (Sánchez Amador, 2021). In the case of humans, it is estimated that the body contains an average of 30 trillion cells, where red blood cells are the most abundant, with approximately 5 billion per cubic millimeter of blood, playing a crucial role in oxygen transport. Thus, every individual is composed of all their cells, from those shed in the epidermis to the neurons that accompany them throughout life, highlighting the importance of each one in forming the organism. In this context, according to Sánchez Amador (2021), peroxisomes emerge as relevant organelles, whose study offers a deeper understanding of cellular functions and their contribution to the organism's well-being.
What are Peroxisomes?
Peroxisomes are cytoplasmic organelles found in most eukaryotic cells, which are characterized by having a nucleus differentiated from the rest of the cytoplasm by a membrane, classifying them as crucial components of multicellular organisms (Sánchez Amador, 2021). According to Sánchez Amador (2021), an organelle is defined as an essential part of the cell, possessing a structural unit and fulfilling specific functions; among the various organelles are mitochondria, chloroplasts, vacuoles, and, of course, peroxisomes.
Specifically, peroxisomes are round organelles, surrounded by a membrane and measuring between 0.1 and 1 micrometer in diameter. Inside, they house enzymes critical for carrying out various metabolic reactions essential for cellular metabolism, allowing each of these organelles to obtain the energy needed to perform their functions (Sánchez Amador, 2021). It is estimated that each peroxisome contains, on average, around 50 different enzymes that catalyze various reactions, which vary depending on the type of cell that contains the organelle and its physiological state. For example, Sánchez Amador (2020) notes that these organelles possess 10% of the total activity of two enzymes involved in the pentose phosphate pathway, closely related to glycolysis, a process that involves the oxidation of glucose to obtain energy.
Differences from Other Organelles
Peroxisomes differ significantly from typical organelles such as mitochondria and chloroplasts in terms of complexity and function (Sánchez Amador, 2021). Unlike these, peroxisomes do not have their own genetic material, meaning they lack circular DNA, are surrounded by only one membrane, and do not contain mitoribosomes or chlororibosomes in their matrix. According to Sánchez Amador (2021), the endosymbiotic theory suggests that both mitochondria and chloroplasts were ancient prokaryotic bacteria and archaea that were ingested by eukaryotic cells, complicating the comparison of their physiological complexity within the cellular context.
Morphologically, peroxisomes are similar to lysosomes; however, they share with more complex organelles the characteristic that the proteins constituting them come from free ribosomes in the cytoplasm (Sánchez Amador, 2021). Without the protein synthesis activity carried out by these ribosomes, peroxisomes, as well as mitochondria and chloroplasts, could not form. However, since peroxisomes lack their own genome, all their proteins must originate from cytosolic ribosomes. In contrast, according to Sánchez Amador (2021), a small percentage of the protein molecules in mitochondria and chloroplasts are synthesized within these organelles.
Functions
Peroxisomes, named after the first enzymes discovered within them (peroxidases), can contain more than 50 different enzymes (Megías et al., 2024). Initially, these organelles were defined as structures responsible for carrying out oxidative reactions, resulting in the production of hydrogen peroxide, thanks to the discovery of peroxidase enzymes within them (Sánchez Amador, 2021). Sánchez Amador (2020) mentions that since hydrogen peroxide is a potentially harmful compound to the cell, peroxisomes also contain catalase enzymes that decompose this compound into water or use it to oxidize other substrates.
In peroxisomes, various oxidative reactions occur, notably the oxidation of uric acid, amino acids, and fatty acids (Sánchez Amador, 2021). Interestingly, although the enzyme urate oxidase, responsible for oxidizing uric acid to 5-hydroxyisourate, is present in many organisms, it is not found in humans. While humans possess the gene encoding this enzyme, it is non-functional due to a mutation. One of the most remarkable aspects of peroxisomes is their role in fatty acid oxidation, which constitutes a fundamental energy source for the functioning of living organisms, both at the micro and macroscopic levels. In animal cells, the oxidation of these lipid biomolecules occurs in peroxisomes and ribosomes. However, according to Sánchez Amador (2021), in other species, such as yeast, peroxisomes are the only organelles capable of carrying out this process.
In addition to providing a specialized compartment for oxidative reactions, peroxisomes are involved in lipid biosynthesis (Sánchez Amador, 2021). In animals, both cholesterol and dolichol, a fundamental lipid in the membrane bilayer, are synthesized in peroxisomes and the endoplasmic reticulum. In liver cells, these versatile organelles are also responsible for producing bile acids, derived from cholesterol. Furthermore, peroxisomes contain the enzymes necessary for the synthesis of plasmalogens, phospholipids that are especially important in the structure of heart and brain tissue. In summary, according to Sánchez Amador (2021), peroxisomes are essential centers for oxygen utilization and play multiple crucial roles at both tissue and cellular levels.
Peroxisome Plasticity and Multiplication
Peroxisomes exhibit remarkable plasticity in the world of organelles (Sánchez Amador, 2021). These small circular bodies can multiply in both number and size in response to certain physiological stimuli and can subsequently return to their original state once the external trigger has ceased. Additionally, peroxisomes can modify their enzymatic repertoire based on the organism’s physiological condition. According to Sánchez Amador (2021), this adaptability is due to a highly efficient multiplication process known as budding.
To initiate this process, it is crucial that the peroxisome’s membrane contacts that of the endoplasmic reticulum, an event that allows the transfer of membrane lipids from the endoplasmic reticulum to the peroxisome, increasing its useful surface area (Sánchez Amador, 2021). Once it has received this “donation” of lipids, it can divide into two new organelles, which will mature their protein content, both internally and in the membrane, as free ribosomes produce the proteins necessary for their function. Additionally, it is worth noting that the living organism's cell has the ability to generate peroxisomes from scratch when all pre-existing peroxisomes have disappeared from the cytosol. According to Sánchez Amador (2021), this process, though biochemically complex, can be understood in a simplified manner as the result of vesicle synthesis in the endoplasmic reticulum and mitochondria of the cell.
Peroxisomal Disorders
Peroxisomal disorders are a group of inherited metabolic disorders that occur when peroxisomes are either absent or not functioning properly (Demczko, 2024). In the context of inherited disorders, these develop when parents pass on defective genes responsible for the appearance of these conditions. It is important to note that there are different classifications of hereditary disorders, and in most cases of peroxisomal disorders, both parents of an affected child carry one copy of the abnormal gene. Typically, two copies of the defective gene are required for the disorder to manifest, which means that in most situations, neither parent shows symptoms of the disorder. Additionally, according to Demczko (2024), some peroxisomal disorders are linked to the X chromosome, meaning that a single copy of the abnormal gene may be enough for the disorder to appear in male children.
Refsum Disease
This disorder is characterized by the accumulation of phytanic acid in the tissues, a byproduct of lipid metabolism (Demczko, 2024). This buildup can cause various injuries to the nerves and retina, as well as hearing loss, anosmia (loss of smell), spastic movements, and both bone and skin abnormalities. While symptoms usually appear in the twenties, there is a possibility that onset may be delayed until a later age. To diagnose the condition, doctors perform blood tests to check for elevated levels of phytanic acid. Treatment focuses on avoiding foods that contain phytanic acid, such as dairy products, beef, lamb, and fatty fish like tuna, cod, and haddock. Additionally, as noted by Demczko (2024), plasmapheresis, a procedure that removes phytanic acid from the blood, can also be helpful.
Rhizomelic Chondrodysplasia Punctata
Symptoms appear in infancy and include distinctive features such as a sunken appearance in the center of the face, notably short limbs, a prominent forehead, small nasal passages, as well as cataracts and scaly skin that may lead to peeling (Demczko, 2024). Additionally, a significant slowdown in physical activity, affecting both movement and speech, is observed. Spinal defects are common, further aggravating the condition. Diagnosis is made through X-rays and blood tests, and genetic testing is performed to confirm the presence of this disease. Although there is currently no specific treatment, infants with elevated phytanic acid levels in their blood are advised to avoid foods containing this acid. According to Demczko (2024), these foods include dairy products, beef, lamb, and fatty fish such as tuna, cod, and haddock.
X-Linked Adrenoleukodystrophy
This is recognized as the most prevalent peroxisomal disorder, primarily affecting the brain, spinal cord, and adrenal glands (Demczko, 2024). Due to the location of the defective gene on the X chromosome, this disorder mostly affects boys. The cerebral form of X-linked adrenoleukodystrophy typically manifests between the ages of 4 and 8. During this period, children display symptoms related to attention problems, which over time evolve into severe behavioral difficulties, dementia, and impairments in vision, hearing, and mobility. According to Demczko (2024), this form of the disease can lead to total disability and, unfortunately, death several years after diagnosis, though milder forms affecting teenagers and adults also exist.
On the other hand, adrenomyeloneuropathy (AMN) is a less severe variant of X-linked adrenoleukodystrophy, affecting individuals in their twenties or thirties (Demczko, 2024). Those affected may experience symptoms of stiffness, weakness, and leg pain, which tend to worsen progressively over time. Neurological problems associated with this form of the disease can lead to dysfunction of the urinary sphincter and sexual organs. In some cases, according to Demczko (2024), patients also develop symptoms related to the cerebral form of the disease.
It is important to mention that individuals with any form of X-linked adrenoleukodystrophy may experience reduced adrenal gland activity, which can result in adrenal insufficiency, known as Addison's disease, without necessarily having problems in the brain and spinal cord (Demczko, 2024). For diagnosis, doctors typically use a brain MRI and blood tests to detect specific fatty acids. Diagnosis is confirmed through genetic sequencing. Additionally, genetic tests are available to assess whether a couple is at risk of having a child with this disease. In some cases, bone marrow or stem cell transplants can be effective treatments. Finally, according to Demczko (2024), those with adrenal gland problems often receive corticosteroid treatment.
Zellweger Syndrome, Neonatal Adrenoleukodystrophy, and Infantile Refsum Disease
The three disorders grouped under the term "Zellweger spectrum disorders" present overlapping symptoms and affect various parts of the body (Demczko, 2024). Among them, Zellweger syndrome is considered the most severe form, while infantile Refsum disease is classified as the least severe. According to Demczko (2024), both Zellweger syndrome and neonatal adrenoleukodystrophy typically manifest during infancy, in contrast to Refsum disease, which may appear later, sometimes even in adulthood.
Symptoms associated with these disorders are varied and include distinctive facial features as well as defects in the brain and spinal cord (Demczko, 2024). Nerve sheath destruction, known as demyelination, seizures in newborns, and weak muscle tone, called hypotonia, are also present. In certain cases, affected children may experience liver enlargement and kidney cysts. Additionally, they may have short limbs and a specific bone abnormality called chondrodysplasia punctata, which affects the growth of long bones. According to Demczko (2024), other symptoms can include cataracts, abnormal growth of blood vessels in the eyes known as retinopathy, hearing loss, as well as weakness, numbness, and pain in the hands and feet.
Physical development, including movement and speech, tends to be slowed (Demczko, 2024). Doctors often suspect these disorders when elevated levels of certain fatty acids are detected in the blood, and genetic testing is performed to confirm the diagnosis. Although there is currently no specific treatment for these disorders, Demczko (2024) notes that various medications and treatments are used to manage symptoms.
References
Demczko, M. (2024). Trastornos Peroxisomales. Manual MSD. https://www.msdmanuals.com/es/hogar/salud-infantil/trastornos-metabólicos-hereditarios/trastornos-peroxisomales
Megías, M., Molist, P., & Pombal, M. Á. (2024). Peroxisomas. Atlas de Histología Vegetal y Animal. https://mmegias.webs.uvigo.es/5-celulas/6-peroxisomas.php
Sánchez Amador, S. A. (2021, abril 15). Peroxisomas: Qué son, Características y Funciones. Psicología y Mente. https://psicologiaymente.com/salud/peroxisomas
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