## 3 Organosehenium Compounds
### Organosehenium Natural and Synthetic Compounds  
Natural organosehenium compounds are products obtained from the metabolism of bacteria, fungi, plants, and animals [160]. In the environment (soil, rocks, water), selenium is present in the inorganic forms, as selenate (SeO\({}_{4}\)\({}^{2-}\)), selenite (SeO\({}_{3}\)\({}^{2-}\)), elemental (Se), and selenide (Se\({}^{2-}\)) [161]. During the bioaccumulation in plants, fungi, and bacteria, the inorganic forms of selenium are incorporated into amino acids like selenomethine, selenocysteine, methylselenocystine, selenocystathionine, \(\gamma\)-glutamyl-Se-methylselenocysteine, and Se-adenosyl-selenohomocysteine [161]. These forms of selenium can also be methylated and form dimethylselenide, dimethyldiselenide, dimethyl selenone, methylselenol, and dimethyl selenyl-sulfide [162; 163]. Several plant and mushroom species are known to be good Se-accumulators [164]; however, selenium is not considered an essential micronutrient for those organisms as it is for humans. Their ability to accumulate large amounts of organosehenium compounds may be the result of the detoxification process [165]. In humans, the inorganic forms of selenium are converted to selenocysteine and incorporated into selenoproteins or transformed into methylated metabolites, which are excreted through exhalation, urine, and faeces [166]. Mammals have no efficient mechanism of methionine synthesis. Therefore, they are also unable to synthesize selenomethine, which is produced along with methionine in fungi and plants in quantities dependent on the amount of Se available. Consequently, selenomethine, which can be incorporated nonspecifically in body proteins, constituting the main selenium store, must be supplied in the human diet. Less known products of selenium metabolism are selenosugars. The major metabolite in humans (hepatic metabolite) is the selenosugar 1\(\upmu\)-methylseleno-\(N\)-acetyl-D-galactosamine [167; 168]. According to [PERSON] et al., two other selenocarbohydrates may be present in human urine after ingestion of selenium supplements, and they are 1\(\beta\)-methylseleno-D-galactosamine and 1\(\beta\)-methylseleno-\(N\)-acetyl-D-glucosamine [169]. Several fungi and plant species, grown on the selenium-enriched substrates mainly as inorganic forms, such as sodium selenite, are able to incorporate selenium into the cell wall polysaccharides [170; 171]. The knowledge about their structures is still fragmentary, and its current status will be presented in more detail in Section 5. The number of known synthetic organoselenium compounds is incomparably greater than that of known natural compounds. In the past three decades, an exponential growth of organoselenium chemical studies was observed, as well as of the number of reactions and the variety of selenium compounds [172]. For example, organoselenium compounds were already used in organic chemistry as an electrophile, nucleophile, or even as a radical in chemo-, regio-, and stereo-selective reactions [173; 174]. The most important types of synthetic organoselenium compounds are diorganodise-lenides (R-Se-Se-R) [175; 176], selenols (R-Se-H) [177], diorganoselenides (R-Se-R\({}^{\prime}\)) [178; 179], diorganoselenoxides (R\({}_{2}\)Se = O) [180; 181; 182], selenenyl sulfides (R-Se-S-R\({}^{\prime}\)) [183], organoselenium halides (R-Se-X, R\({}_{2}\)SeX\({}_{2}\) and R\({}_{3}\)SeX) [184], selenenic and selenionic acids (R-Se-OH, R-Se(O)-OH, R-Se(O)\({}_{2}\))-OH) [185], and selenic and selenic and selenious acid aliphatic and aromatic and cyclic esters (alkyl and aryl selenates and selenites). In the last 20 years a great effort has been directed toward the synthesis of biologically active organoselenium compounds as potential antitumor agents [20; 21; 25; 186; 187; 188; 189; 190; 191], enzyme modulators [192], antioxidants [193; 194; 195], antimicrobials [196; 197; 198; 199; 200; 201; 202], antivirals [203; 204; 205; 206; 207; 208; 209], and cytokine inducers [210].