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RESEARCH PRODUCT

α-L-rhamnosidases: Old and New Insights

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L-Rhamnose is a component of plant cell wall pectic polysaccharides (Mutter et al., 1994; Ridley et al., 2001), glycoproteins (Haruko and Haruko, 1999) and secondary metabolites such as anthocyanins (Renault et al., 1997), flavonoids (Bar-Peled et al., 1991) and triterpenoids (Friedman and McDonald, 1997). It has also been found in bacterial heteropolysaccharides (Hashimoto and Murata, 1998), rhamnolipids (Ochsner et al., 1994) and in the repeating units of the O-antigen structure of the lipopolysaccharide component of bacterial outer membranes (Chua et al., 1999). Some rhamnosides are important bioactive compounds, e.g. cytotoxic saponins (Bader et al., 1998; Yu et al., 2002), antifungal plant glycoalkaloids (Oda et al., 2002) and bacterial virulence factors (Deng et al., 2000). In plants L-rhamnose-containing terpenyl glycosides are important aroma precursors (Gunata et al., 1985) and may also play a protective role against the toxicity of free aglycons; L-rhamnose-containing flavonoid glycosides have antioxidant and antiinflammatory activities (Benavente-Garcia et al., 1997). -L-rhamnosidases (EC 3.2.1.40) and -L-rhamnosidases (EC 3.2.1.43) catalyse the hydrolysis of terminal, non-reducing L-rhamnose residues in and -L-rhamnosides respectively. In contrast, endorhamnosidases (EC 3.2.1.-) act by cleaving specific linkages between internal rhamnose residues in rhamnosides. -L-rhamnosidases have been found in many micro-organisms and in some plant and animal tissues (see below), whereas -L-rhamnosidase has only been described