Se Assimilation

Higher plants metabolise Se via the sulphur assimilation pathway. Most of the sulphur assimilation pathway is well characterised and described for Se non-accumulator plants. The various biochemical steps in this pathway are described below (Zayed et al. 1999; Sors et al. 2005b).

(a) ATP sulphurylase - Selenate is absorbed by roots via the sulphate transporters (Fig. 10.1) and is usually transported through the xylem without modification to the leaves. Once selenate is inside leaves it enters the chloroplasts where it is metabolised by the enzymes of sulphate assimilation. The first, most critical and rate limiting step is the reduction of selenate to APSe by ATP sulphurylase (Burnell 1981), which is accumulated in the chloroplasts. However, if the same plants are supplied with selenite, organo-Se compounds similar to SeMet are assimilated. De-topped plants supplied with selenate accumulated only selenate in the roots; strongly supporting that the chloroplasts are the sites for ATP sulphurylase activity and selenate reduction (Shaw and Anderson 1972; Pilon-Smits et al. 1999).

(b) Reduction of adenosine 5'-phosphoselenate (APSe) to selenide (Se2-) - The next series of metabolic steps where evidence is available is that APSe can further be reduced to selenide (Se2-) via two pathways; one enzymatically and the other non-enzymatically (Fig. 10.2a):

Fig. 10.2 a Pathway for selenate activation and reduction to selenide, which can be either enzymatic or non-enzymatic. b Pathway of selenide conversion to selenocystein (SeCys) and/or selenomethionine (SeMet) and incorporation of both into proteins

1. Non-enzymatically - with the aid of GSH, NADPH and FADH; however GSH reductase (i.e., glutathione reductase) may be necessary as a side reaction (Anderson 1993; Ng and Anderson 1979); and

2. Enzymatically - via APS reductase and sulphite reductase; although one non-enzymatic step may also be required (Arvy 1997; Terry et al. 2000).

The intermediate compound selenite (SeO32-) can also undergo other transformations besides its final assimilation and reduction to selenide, and enter alternate pathways. This is achieved non-enzymatically by reduction to GS-Se-SG, which is reduced to the selenol (SeGSH). SeGSH is glutathione conjugated selenide. For example plants supplied with selenite can oxidise Se to selenate (Ng and Anderson 1979); a sort of reverse reaction to normal Se assimilation.

3.3 Incorporation of Se into Protein

It is proposed that plants like bacteria incorporate and assimilate SeCys specifically into protein, or after it is metabolised to SeMet. It is likely that this process also occurs in the chloroplasts. In both cases Cys synthase converts Se2-to SeCys, which can be a reverse reaction if the enzyme SeCys lyase is present. SeCys is converted to Secysth by the enzyme cystathionine-y-synthase, then to Sehocys by another enzyme cystathionines-lyase, and finally to SeMet by what is as yet an unknown mechanism (Fig. 10.2b). Finally, either a direct or an indirect pathway of incorporation into proteins takes place for both SeCys and/or SeMet (Foyer and Halliwell 1976; Goutierrey-Marcos et al. 1996):

(a) Direct - SeCys is incorporated via a specific SeCys t-RNA into the selenopro-teins.

(b) Indirect - SeCys is converted to SeMet as above (Fig. 10.2b), and a specific SeMet t-RNA incorporates SeMet into selenoproteins.

3.4 Localisation of Se Pathways

A summary of the cellular and sub-cellular localisation of the enzymes and metabolites in the selenium assimilation pathway are given below:

(a) Chloroplasts - for the selenate reduction pathway all enzymes and metabolites have been localised in chloroplasts, wether the reactions are enzymatic or non-enzymatic. Cys synthase and maybe also cystathionine-y-synthase and cystathionines-lyase are localised in the chloroplast. At least until the synthesis of Sehocys most reactions occur in chloroplasts (Kim and Leustek 1996; Setya et al. 1996; Ravanel et al. 1998; Turner et al. 1998).

(b) Cytoplasm - SeMet production from Sehocys and methylation of SeMet to SeMMet, DMSeP and DMSe are thought to occur within the cytoplasm (Fig. 10.3a) (James et al. 1995; Terry et al. 2000).

SeMet

SeMet methyltransferase

SeMMet

MMet hydrolase i

DMSeP (volatile)

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