Se(IV) and Se(VI) are taken up from seawater by cells as inorganic ions and are biotransformed to seleno-amino acids (Amouroux et al., 2000). Photolytic reactions of released seleno-amino acids produce dimethyl selenide or dimethyl diselenide. Se methylation can occur in the environment (Ridley et al., 1977). There is considerable microbial biotransformation of Se (Stolz et al., 2002).

Various Se species are biotansformed (Diplock, 1976). There are two primary metabolic pathways; 1) reduction followed by methylation and 2) incorporation into or binding by proteins.

Inorganic Se is not stored; it is utilized in selenoprotein synthesis (Patterson and Zech, 1992). Tissue turnover was more rapid for selenomethionine than selenite (Swanson et al., 1991). Selenium associated with proteins is in the form of selenide (Se(II)).

Selenite can be taken up by erythrocytes and reduced by glutathione to selenide, then released to plasma where it is albumin bound and is then taken up by the liver (Lee et al., 1969; Sandholm, 1973; Hsieh and Ganther, 1975a; Gasiewicz and Smith, 1978; Shiobara and Suzuki, 1998; Suzuki et al., 1998).

Selenite can also be reduced to selenide in human plasma by glutathione (Mas and Sarkar, 1989).

Selenate was also shown to be reduced by liver homogenate and supernatant fractions to selenide (Shiobara et al., 1999).

Selenite may be too reactive to persist in the free form (Suzuki 2005).

In the liver selenide can be utilized in the synthesis of selenoproteins (Olson and Palmer, 1976) or be excreted into the urine as monomethylselenon and the trimethylselenonium ion (Itoh and Suzuki, 1997).

Rabbits and rats can biotransform selenite to selenocysteine which is incorporated into tissue proteins (Godwin and Fuss, 1972; Olson and Palmer, 1976).

Although ruminant animals can synthesize selenomethionine from inorganic Se species, non-ruminant animals are apparently unable to do so (Schwarz, 1972).

Selenomethionine can be directly incorporated into proteins, substituting for methionine (McConnell and Hoffman, 1972).

An example of Se reduction followed by methylation is the production of dimethylselenide from selenate and selenite, as shown in rats (Hsieh and Ganther, 1975b; Nakamuro et al., 1977). Dimethylselenide is a volatile intermediate metabolite which is exhaled during high selenium intake when its formation rate exceeds the rate of its further methylation to trimethylselenonium ion, which is excreted in the urine (McConnell and Portman, 1952; Byard, 1969; Palmer et al., 1969). It was found that significant amounts of dimethylselenide formed only at high selenite concentrations and under anaerobic conditions (Ganther, 1966). Exhalation of dimethylselenide produces a characteristic garlic breath. Selenite biotransformation has been shown to occur in rat (Gasiewicz and Smith, 1978), sheep (McMurray and Davidson, 1979) and human erythrocytes (Sandholm, 1975). The biotransformation of selenite to methylated selenides facilitates the excretion of high/potentially toxic amounts of Se. Exposure to large amounts of Se can result in replacement of sulfide with Se in internal organs.

High doses of selenate, selenite, selenomethionine, selenocysteine, methylselenocysteine, and seleniferous wheat given to rats were biotransformed, mainly to methylated selenides and the trimethylselenonium ion, a major urinary metabolite of Se which was mostly excreted within a few days (Palmer et al., 1970).

In the proposed pathway of selenite to (CH₃)₃Se⁺, selenite is reduced in 3 steps: selenite to H²Se, which is methylated to (CH₃)₂Se, which is converted to (CH₃)₃Se⁺ (Ganther and Hsieh, 1974; Hsieh and Ganther, 1976, 1977; Foster et al., 1986; Hoffman and McConnell, 1987).

The trimethylselenonium ion is also formed in man (Nahapetian et al., 1984).

Se forms a protein complex with Hg found in plasma and blood cells (Burke et al., 1974).

L- selenocysteine can be synthesized in the liver (Esaki et al., 1981).

L- selenocysteine is too reactive to persist in the free form. It is incorporated in amino acids and peptides (Suzuki, 2005).

Organic Se species, selenocysteine and selenomethionine, can be metabolized to selenide (human liver, kidney, heart, adrenal & muscle). For example, L- selenocysteine can be metabolized by L-selenocysteine β lyase to yield hydrogen selenide (Esaki et al., 1982). Selenomethionine can also be metabolized to selenide (Beilstein and Whanger, 1992).

Selenomethionine can be biotransformed to selenocysteine by a trans-sulfuration pathway (Esaki et al., 1981; Sunde, 1990) and by a methionine transamination pathway (Esaki et al., 1979).