Selenomethionine, oxidation

AdoSe-Hcy, adenosylselenohomocysteine AdoSe-Met, adenosylselenomethionine DmeSeP, dimethyl-Se propionate ion MeSe-Cys, methylselenocysteine MeSe-Met, methylselenomethionine Se-Et, selenoethionine Se-Cys, selenocysteine Se-Cis, oxidized selenocysteine, Se-Met, selenomethionine SeO-Met, selenomethionine oxide. 

Figure 8.4. Cation exchange HPLC-ICP-MS analysis of 77Se-enriched yeast using two enzymatic sample preparation strategies (a) sequential use of /3-glucosidase and a protease mixture (b) protease XIV and (c) mass balance of selenium and selenium species fractionated during the sample preparation and HPLC separation procedure. SeMet = selenomethionine SeOMet = selenomethionine Se-oxide. Reproduced from [133] by permission of The Royal Society of Chemistry.

Maher and coworkers [25, 30, 32, 37] found that selenomethionine and selenocysteine are stable during acid hydrolysis as long as a reducing agent such as cysteine is added and oxygen is rigorously excluded. A selenocysteine-cysteine (Cyst-SeCyst) complex is formed and some selenomethionine can be oxidized, probably to selenomethionine-selenoxide (Fig. 20.3). 

Mono- and dimethylselenide (MMSe DMSe) Mono- and diethylselenide (MESe, DESe) Trimethylselonium ion (TMSe) Selenomethionine (Se-Met) Methylselenomethionine (Me-Se-Met) Selenocysteine (Se-Cys or oxidized Se-Cis) Methylselenocysteine (Me-Se-Cys) Selenohomocystine (Se-Hcy)... 

Amino acid 78 and its y-glutamyl derivative are other components of the major pool of seleno compounds in accumulator plants, while selenomethionine (79) is the major compound in microorganisms like yeast, which is not specialized in selenium utilization. Se-Methylselenocysteine 5e-oxide (80), which is found in marine algae, tends to spontaneously decompose with the formation of pyruvate and ammonia via aminoacrylic acid and methaneselenic acid (81). The latter reacts with sulfhydryls or selenols to selenodisulfldes and diselenides. 

A batch of Se-labeled and enriched yeast was characterized with regard to isotopic composition and content of selenium species for later use in a human absorption study based on the method of enriched stable isotopes. SeMet constituted 53% of the total Se content in the yeast. Oxidation of SeMet to selenomethionine-Se-oxide (SeOMet) occurred during sample preparation... 

Conditions such as pH, oxidation-reduction potential, and the presence of metal oxides affect the partitioning of the various compounds of selenium in the environment. In general, elemental selenium is stable in soils and is found at low levels in water because of its ability to coprecipitate with sediments. The soluble selenates are readily taken up by plants and converted to organic compounds such as selenomethionine, selenocysteine, dimethyl selenide, and dimethyl diselenide. Selenium is bioaccumulated by aquatic organisms. Very low levels of selenium are found in ambient air. 

Elemental selenium (selenium[0]) is rarely found naturally, but it is stable in soils. Selenates (selenium[+6]) and selenites (selenium[+4]) are water soluble and can be found in water. Sodium selenate is among the most mobile forms of selenium because of its high solubility and inability to adsorb to soil particles. More insoluble forms, such as elemental selenium, are less mobile therefore, there is less risk for exposure. Because of greater bioavailability, water-soluble selenium compounds are probably more lethal than elemental selenium by any route. Selenium is found in nature complexed with multiple compounds, and although various forms are discussed in the profile, many others exist. Plants can contain organic selenium in the form of the amino acids selenomethionine and selenocysteine, along with the dimethyl selenides. Elemental selenium can be oxidized to form selenium dioxide. While the products of oxidation might be expected at the soil surface, elemental selenium would be the expected predominant form in deep soils where anaerobic conditions exist. Selenium sulfides, used in some antidandruff shampoos, are not very water soluble and, therefore, like elemental selenium are relatively immobile in the environment. 

It is also important that the various chemical species of the analyte be converted to a common form. Some of the organoforms of these elements commonly found in biological fluids and tissues are inert toward hydride formation (e.g., selenomethionine and arsenobetaine) and care must be taken to ensure that the sample has been completely oxidized by,... 

D,L-Selenomethionine is oxidised by peroxynitrite by two competing mechanisms, a one-electron oxidation that leads to ethylene, and a two-electron oxidation that gives methionine selenoxide (Pad-MAjA et al. 1997). Kinetic modelling of the experimental data suggests that both peroxynitrous acid and the peroxynitrite anion react with d,l-selenomethionine to form methionine selenoxide with rate constants of 20,4601440 M" s" and 2001170 M" s", respectively at 25 °C. In the presence of added bicarbonate, the yield of ethylene obtained from the reaction of 0.4 mM peroxynitrite with 1.0 mM selenomethionine, decreased by 35% with an increase in the concentration of bicarbonate from 0 to 25 mM. Kinetic simulations showed that the decrease in the yield of methionine selen-... 

Selenium compounds are more active electron donors compared to analogous sulphur compounds and play a wide role in biochemical systems. They can decompose peroxides (Caldwell and Tappel 1964,1965) and can act as antioxidants (Dillard et al. 1978) and free radical scavengers (Ursini and Bindoli 1987). Masumoto and Siess (1996) and Masumoto et al. (1996) have shown that peroxy-nitrite rapidly oxidises ebselen and its main metabolite to the corresponding selenoxides. The results of Padmaja et al. (1997) suggest that CO2 partially protects methionine selenoxide from peroxynitrite-mediated oxidation and that 0=N-00-C02 or its derivatives do not mediate the oxidation of d,l-selenomethionine or methionine selenoxide. 

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