This work is believed to be the first report on the physiological and biochemical characterization of alpha-L-rhamnosidases in lactic acid bacteria. A total of 216 strains representing 37 species and eight genera of food-grade bacteria were screened for alpha-L-rhamnosidase activity. The majority of positive bacteria (25 out of 35) were Lactobacillus plantarum strains, and activity of the L. plantarum strain NCC245 was examined in more detail. The analysis of alpha-L-rhamnosidase activity under different growth conditions revealed dual regulation of the enzyme activity, involving carbon catabolite repression and induction: the enzyme activity was downregulated by glucose and upregulated by L-rhamnose. The expression of the two alpha-L-rhamnosidase genes rhaB1 and rhaB2 and two predicted permease genes rhaP1 and rhaP2, identified in a probable operon rhaP2B2P1B1, was repressed by glucose and induced by L-rhamnose, showing regulation at the transcriptional level. The two alpha-L-rhamnosidase genes were overexpressed and purified from Escherichia coli. RhaB1 activity was maximal at 50 degrees C and at neutral pH and RhaB2 maximal activity was detected at 60 degrees C and at pH 5, with high residual activity at 70 degrees C. Both enzymes showed a preference for the alpha-1,6 linkage of L-rhamnose to beta-D-glucose, hesperidin and rutin being their best substrates, but, surprisingly, no activity was detected towards the alpha-1,2 linkage in naringin under the tested conditions. In conclusion, we identified and characterized the strain L. plantarum NCC245 and its two alpha-L-rhamnosidase enzymes, which might be applied for improvement of bioavailability of health-beneficial polyphenols, such as hesperidin, in humans.
