The class of small molecules known as "compatible osmolytes" includes certain amino acids (notably proline), quaternary ammonium compounds (e.g. glycinebetaine, prolinebetaine, B-alaninebetaine, and choline-O-sulfate), and the tertiary sulfonium compound 3-dimethylsulfoniopropionate (DMSP). The quaternary ammonium compounds and DMSP are derived from amino acid precursors. These compounds share the property of being uncharged at neutral pH, and are of high solubility in water (Ballantyne and Chamberlin, 1994). Moreover, at high concentrations they have little or no perturbing effect on macromolecule-solvent interactions (Yancey et al, 1982; Low, 1985; Somero, 1986; Timasheff, 1993; Yancey, 1994). Unlike perturbing solutes (such as inorganic ions) which readily enter the hydration sphere of proteins, favoring unfolding, compatible osmolytes tend to be excluded from the hydration sphere of proteins and stabilize folded protein structures (Low, 1985). These compounds are thought to play a pivotal role in plant cytoplasmic osmotic adjustment in response to osmotic stresses (Wyn Jones et al, 1977).

The role of quaternary ammonium and tertiary sulfonium compounds as compatible compatible solutes has been extensively reviewed (Wyn Jones and Storey, 1981; Anthoni et al, 1991; Rhodes and Hanson, 1993; Gorham, 1995). These zwitterionic compounds include glycinebetaine, choline-O-sulfate, B-alaninebetaine, prolinebetaine and hydroxyprolinebetaine, and dimethysulfoniopropionate (DMSP). The quaternary ammonium compounds possess a fully methyl substituted nitrogen atom (creating a permanent positive charge on the N moiety) and a negatively charged carboxyl group (in the case of betaines) or sulfate group (in the case of choline-O-sulfate).

The function of these compounds as osmoprotectants is illustrated by their stimulation of the growth of Escherichia coli or Salmonella typhimurium in saline medium (Strom et al, 1983; Hanson et al, 1991; 1994). In these assays, glycinebetaine, B-alaninebetaine, prolinebetaine and choline-O-sulfate at 1 mM concentrations are more effective than 1 mM proline (Hanson et al, 1991; 1994). Choline, an alcohol, without a negatively charged group, is not an osmoprotectant per se. Although it stimulates the growth of wild-type E. coli in saline media, it must be oxidized to glycinebetaine to function as an osmoprotectant. Thus, betA mutants of E. coli defective in choline oxidation are unable to use choline as an osmoprotectant (Hanson et al, 1991). Like proline, glycinebetaine and structurally related compounds have stabilizing effects on proteins and membranes (Pollard and Wyn Jones, 1979; Paleg et al, 1981; Jolivet et al, 1982; Nash et al, 1982; Paleg et al, 1984; Incharoensakdi et al, 1986; Naidu et al, 1987; Nikolopoulus and Manetas, 1991; Homann, 1992; Williams and Gounaris, 1992; Papageorgiou et al, 1991; Papageorgiou and Murata, 1995), and are implicated in frost protection (Kishitani et al, 1994; Nomura et al, 1995).

Whereas mannitol may function to shield susceptible thiol-regulated enzymes (such as phosphoribulokinase) from inactivation by hydroxyl radicals in plants, glycinebetaine is not effective as a hydroxy radical scavenger (Shen at al, 1997; Smirnoff and Cumbes, 1989).

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