Bioactivation of selenocysteine derivatives by beta-lyases present in common gastrointestinal bacterial species

Studies in cell cultures and animal models have demonstrated cancer chemopreventive effects of certain selenium compounds. Here we describe the screening of cysteine S-conjugate beta-lyase activity in bacterial species that are implicated in the bio-activation of sulfur- and selenocysteine derivatives. We screened a range of bacterial species commonly found in the human intestine for beta-lyase activity on Se-p-methoxybenzylselenocysteine and the natural occurring S-methylcysteine and Se-methylselenocysteine conjugates. A high-performance liquid chromatography (HPLC)-assisted assay was established to determine specific activities of each strain. Of the 29 tested bacterial species, 22 showed specific activities towards the test compound reaching up to 10.1 U/mg protein, thereby accounting for 75% of total fecal activity (13.3. U/mg protein). Lysates of four bacterial strains (Bacteroides distasonis, bacteroides vulgatus, Enterococcus faecalis, and Enterococcus faecium), which exhibited high specific activities towards the test compound and which are known to be present at high numbers in the human intestine, were characterized further. Our results indicate that beta-lyase activity is widely distributed in human intestinal bacteria and might play a key role in the bioactivation of selenocysteine derivatives.