SIGNIFICANCE OF A TAURINE:PYRUVATE AMINOTRANSFERASE IN TAURINE DESULFONATION BY ANAEROBIC BACTERIA

Sulfonates are organic compounds in which the carbon-linked sulfur atom has an oxidation state of +5. These compounds are widely synthesized in nature and are biodegradable. They are utilized by a range of bacteria for growth, or otherwise biotransformed, returning them to the global mineral cycle. Taurine (2-aminoethane sulfonate) is present in both terrestrial environments (e.g. mammals in eyes, brain, heart and conjugated to bile acids), and marine systems (e.g. osmolytes in algae and other invertebrates, and in sulfide detoxification in chemoautotrophic bacteria-invertebrate symbioses). Taurine desulfonation has been demonstrated in a variety of bacteria, but the complete details of taurine metabolism in anoxic systems have not been fully elucidated.

Two anaerobic bacteria were isolated by enrichment culture using taurine: Clostridium pasteurianum (sole sulfur source) and Desulforhopalus singaporensis (carbon, energy, and nitrogen source). Cell extracts from taurine-grown organisms showed the presence of aminotransferase (Tpa) activity that utilized only taurine as an amino donor and pyruvate as an amino acceptor, resulting in the products sulfoacetaldehyde and L-alanine. Where taurine was utilized only as a sole sulfur source (C. pasteurianum), 0.24 µmol/mg protein of alanine was measured from the aminotransferase reaction. As expected, when taurine was used as a carbon, energy, and nitrogen source (D. singaporensis), there was an increase (four fold) in the amount of alanine measured in the aminotransferase reaction, 1.06 µmol/mg protein. This enzyme activity was not detected in extracts of cells grown without taurine. Extracts of taurine-grown cells were examined for presence of another enzyme activity (sulfoacetaldehyde sulfolyase) likely significant in taurine metabolism. Detection of this activity in extracts was dependent on Tpa to first produce sulfoacetaldehyde from taurine; i.e., sulfolyase did not desulfonate taurine itself but acted only on its deamination product. The inducible nature of Tpa, its substrate specificity, and its presence in a variety of bacterial systems suggests a significant role of the enzyme in taurine biotransformation by anaerobic bacteria.