Selenium enzyme system

Fig. 7. The glutathione peroxidase (a selenium enzyme) system where GSH = A -(A -L-7-giutamyi -L-cysteinyi )giycine and G—S—S—G, the disulfide.

Heavy metals stimulate or inhibit a wide variety of enzyme systems (16, 71, 72), sometimes for protracted periods (71, 73). These effects may be so sensitive as to precede overt toxicity as in the case of lead-induced inhibition of 8 ALA dehydrase activity with consequential interference of heme and porphyrin synthesis (15, 16). Urinary excretion of 8 ALA is also a sensitive indicator of lead absorption (74). Another erythrocytic enzyme, glucose-6-phosphatase, when present in abnormally low amounts, may increase susceptibility to lead intoxication (75), and for this reason, screens to detect such affected persons in lead-related injuries have been suggested (76). Biochemical bases for trace element toxicity have been described for the heavy metals (16), selenium (77), fluoride (78), and cobalt (79). Heavy metal metabolic injury, in addition to producing primary toxicity, can adversely alter drug detoxification mechanisms (80, 81), with possible secondary consequences for that portion of the population on medication. 

Plant intolerance to metals may be associated with absorption in root cells (151). Chromium concentrates in root cells, and that is where it exerts its toxicity (138). Intolerance to selenium in certain crop plants high in sulfur, such as cabbage, may be related to an indiscriminate sulfur-selenium substitution in enzyme systems (77). 

Stadtman TC. 1974. Selenium biochemistry. Proteins containing selenium are essential components of certain bacterial and mammalian enzyme systems. Science 183 915-921. 

Calcium, chromium, copper, selenium, magnesium, manganese, potassium, sodiinn and zinc are present in Aloe vera. Magnesium lactate is responsible for the anti- allergic attribute of Aloe vera (Javed and Rahman, 2014). Most of the minerals are essential for the proper functioning of various enzyme systems in different metabolic pathways while others act as antioxidants (Surjushe et al., 2008). 

The specificity of human 5 -methylthioadenosine phosphorylase is rather strict if compared with that of the enzyme purified from E. coli The replacement of the sulfur atom of 5 -methylthioade-nosine by selenium and the replacement of the methyl group by an ethyl one are the only substrate modifications compatible with enzymic activity. The rate of breakdown of 5 -methylselenoadenosine equals that of 5 -methylthioadenosine (see Fig. 8). This finding agrees with the generally accepted view that the enzyme systems that normally utilize sulfur metabolites also convert their selenium analogues, i.e. the interchangeability of methionine and selenomethionine has been demonstrated in protein synthesisas well as that of S-adenosylmethionine and Se-adenosylselenomethionine in polyamine biosynthesis. 

Mammalian thioredoxin reductases are a family of selenium-containing pyridine nucleotide-disulfide oxidoreductases. These enzymes catalyze NADPH-dependent reduction of the redox protein thioredoxin (Trx), which contains a redox-active disulfide and dithiol group and by itself may function as an efficient cytosolic antioxidant [77]. One of the functions of Trx/ thioredoxin reductase system is the NADPH-catalyzed reduction of protein disulfide [78] ... 

Figure 1 Overview of specific use of seienium in bioiogical systems. Selenium can be incorporated into macromolecules in at least three separate pathways. From the reduced form of selenide, selenium is activated to selenophosphate by the action of the enzyme selenophosphate synthetase (SPS or SelD). This activated form is then used as a substrate for pathway-specific enzymes that lead to (1) insertion as selenocysteine into proteins during translation (selenoproteins), (2) incorporation into tRNA molecules as mnm Se U or Se U, and (3) insertion into a unique class of molybdoenzymes as a labile, but required, cofactor. The need for activation to selenophosphate has been demonstrated in all cases at the genetic and biochemical level, with the exception of the labile selenoenzymes, where activation of selenium has only been proposed based on proximity of genes within an operon encoding SPS and a molybdoenzyme. ...

Once identified as a selenoprotein in this model (C. sticklandii), the need for selenium was also shown for C. sporogenes The addition of selenium to the culture medium was reported to improve the level of D-proline reductase activity as early as 1976, ° yet the first identification of the selenoprotein component of this enzyme did not occur until more recentiy in 1999 by Andreesen s group. It is quite clear now from data from these model systems, as well as from DNA sequence analysis of the grd aiiAprd operons, ° ° that Stickland reactions are common to many amino acid-fermenting clostridia. Those that are capable of proline reduction all... 

The critical discovery that acetyl phosphate is generated and the information gained from several studies of each of the components of GR allowed an enzyme mechanism to be proposed (Arkowitz and Abeles 1991). However, with the current knowledge that one of the subunits of protein B also contains selenium, further work is needed to characterize the intermediates of the reaction and to explain the role of an additional selenocysteine residue. Whether this additional selenocysteine residue in protein B might serve as a direct reductant of the postulated thioselenide derivative of selenoprotein A and possibly serve as a link to the Trx-TrxR system is unknown. It should also be noted that the selenium-limited cultures that were initially studied during analysis of selenoprotein A (Turner and Stadtman 1973) apparently contained active fractions of proteins B and C, suggesting the role for selenium in protein B may not prove to be absolutely necessary for enzyme catalysis. 

Selenium is a significant component of the enzymatic system of the glutafion for antioxidant protection. It is included in the composition of 200 enzymes engaged in different biochemical reactions, demonstrates the immune-tropic, antiteratogenic and anticancer properties, betters the functional state of muscles, especially myocarditis, and takes part in hormones synthesis of the thyroid gland. Selenium s deficit in soil is the cause of hearth deficiency in endemic zones. 

A second dietary trace element, selenium, is also essential for normal thyroid hormone metabohsm. Selenium in the form of selenocysteine is a required component for three enzymes that remove iodide from thyroid hormones. Deiodination is the major metabohc pathway by which T4 and T3 are cleared from the system. After secretion by the thyroid gland, T4 may be deiodinated to yield either T3 or the physiologically inactive reverse Tj (3,3, 5 -triiodothyronine, or rX3). T3 and rTj are further deiodinated to form less active metabolites. Selenium, like iodine, is deficient in many areas of the world. 

Selenium is an essential trace element in the human body. This nutrient is an important part of antioxidant enzymes that protect cells against the effects of free radicals that are produced during normal oxygen metabolism. Selenium is also essential for normal functioning of immune system and thyroid gland. 

In some mixed acid fermentations (e.g., that of Shigella) formic acid accumulates, but in other cases (e.g., with E. coli at pH 6) it is converted to C02 and H2 (Eq. 17-25). The equilibration of formic acid with C02 and hydrogen is catalyzed by the formic hydrogen-lyase system which consists of two iron-sulfur enzymes. The selenium-containing formate dehydrogenase (Eq. 16-63) catalyzes oxidation of... 

Phosphorus, vitamin D, and calcium are all needed for bone health (Huth et al., 2006) and selenium has a role in the immune and antioxidant systems and in DNA synthesis and repair. Zinc is necessary for DNA repair, cell growth, gene expression, and is an essential part of some enzymes and metalloproteins. Magnesium has been implicated in the prevention of CVDs (Haug et al., 2007). For example, it is known that calcium, phosphorus, and magnesium may mediate beneficial effects on... 

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