Abstract
The present study describes the biotransformation of a commercially available crude extract of soy isoflavones, which contained significant amounts of the soy isoflavone glycosides daidzin and genistin, by recombinant Escherichia coli expressing tyrosinase from Bacillus megaterium. Two major products were isolated from the biotransformation and identified as 3′-hydroxydaidzin and 3′-hydroxygenistin, respectively, based on their mass and nuclear magnetic resonance spectral data. The two 3′-hydroxyisoflavone glycosides showed potent 2,2-diphenyl-1-picrylhydrazyl free radical scavenging activity with IC50 values of 7.4 and 9.8 μM for 3′-hydroxydaidzin and 3′-hydroxygenistin, respectively. The free radical scavenging activities of the two 3′-hydroxyisoflavone glycosides were, respectively, 120 and 72 times higher than the activity of their precursors, daidzin and genistin, and were also stronger than the activity of ascorbic acid, which showed an IC50 value of 15.1 μM. This is the first report of the bio-production and potential antioxidant applications of both 3′-hydroxydaidzin and 3′-hydroxygenistin.
Highlights
Isoflavones are dietary phytoestrogens occurring naturally in some plants, in legumes, such as soybeans [1]
The tyrosinase gene from B. megaterium was amplified by polymerase chain reaction (PCR)
The results showed that 1 mg of the lyophilized cell mass of the recombinant E. coli contained 0.25 units of tyrosinase activity (Figure 1c)
Summary
Isoflavones are dietary phytoestrogens occurring naturally in some plants, in legumes, such as soybeans [1]. Two major isoflavones found in soybean are daidzin and genistin, which are the glycoside conjugates of daidzein and genistein, respectively. They account for more than 0.1% (w/w) of the dry weight of soybeans. Ortho-Hydroxylation of the soy isoflavone aglycones daidzein and genistein is one of the major biotransformation reactions during the preparation of some fermented soybean foods using wild microorganisms such as Bacillus, Aspergillus, and Rhizopus. Many ortho-hydroxylated soy isoflavones, including 6-hydroxydaidzein, 6-hydroxygenistein, 8-hydroxydaidzein, 8-hydroxygenistein, 30 -hydroxydaidzein, and 30 -hydoxygenistein, have been isolated from fermented soybean foods and identified with multiple bioactivities [3]. Some genetically engineered microorganisms were developed to catalyze ortho-hydroxylation of daidzein and/or genistein [3,4,5,6,7]. Most researchers used the cytochrome P450 oxidation system to develop the genetically engineered microorganisms and Molecules 2016, 21, 1723; doi:10.3390/molecules21121723 www.mdpi.com/journal/molecules perform the biotransformation
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