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Laboratory Report on Succinate Dehydrogenase (SDH)


Succinate dehydrogenase or SDH is a vital component in the tricarboxylic acid cycle (TCA) or Krebs cycle (1,2,3). As an important enzyme in the Kreb's cycle, succinate dehydrogenase oxidizes succinate to fumarate (1,2). Purification of this enzyme is difficult but it has been isolated from organisms such as Bacillus subtilis, E. coli where it presents itself as one of the transmembrane proteins of these organisms (4). It has also been likewise detected and isolated from Rhodospirillum rubrum (4). However, in both mammal and animal tissues, SDH has been purified and the enzyme's properties in both tissues were seen to be identical (5).  Analysis of the structure of SDH (5,6) from those isolated in E. coli revealed that it has two large polypeptide subunits (64.3 kDa and 26 kDa) and two smaller subunits (14.2 kDa and 12.8 kDa) (4, 7).  These larger subunits act as the active sites of SDH (7) while the smaller subunits were discovered to be hydrophobic and are attached to the cell membrane of the E. coli (8,9). SDH is unique due to the fact that in the Kreb's cycle, it is the only single enzyme that is found to be bound to the mitochondria's inner membrane (8, 9, 10). Existing as a component of complex II in the electron transport chain (10, 11) it functions as a transporter of electrons to uniquinone-10 from succinate(11).

Revealing the presence of SDH in a muscle tissue indicates that these tissues have abundant mitochondria (13) compared to other tissues that do not use up as much energy. Meanwhile, quantification of SDH is also vital in determining whether there is an overproduction of superoxides that will lead to formation of tumors in humans. As Rustin, P and his colleagues expressed, "SDH plays a specific role in the handling of oxygen in mitochondria" (14). This group of researchers explained that SDH, when it is unable to perform its function in reducing the pool of ubiquinones, can lead to condition where cells and tissues are unable to resist the stress caused by oxygen toxicity. As presented in their research paper (14), SDH not only plays an important role in the Kreb's cycle and electron transport chain but also serves as an antioxidant enzyme. On the other hand, SDH can also be used in the succinate dehydrogenase inhibition (SDI) test for predicting the heat sensitivity of tumor tissues (14). SDI test will show whether malignant or tumor tissues can be effectively treated with hyperthermia (14, 15).

This experiment is integral in understanding the presence and role of SDH in tissues of animals and humans. In tissues that are highly active, SDH would be present in higher quantities as compared to tissues that are less active and need less energy to carry out their functions. Generally, quantifying SDH would also reveal if there is an abnormality in its production and abundance. For example, in tumor lesions, when SDH is unable to reduce the level of ubiquinones normally, it may lead to inability of tissues and cells to handle oxidative stress. In malignant tissues, the SDI test is also used as a reliable tool to indicate whether these lesions will respond to hyperthermia- one of the important therapies used to treat or contain malignant tissues.

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