Purification and properties of tauropine dehydrogenase from a red alga, Rhodoglossum japonicum

Tauropine dehydrogenase which is a member of 'opine' dehydrogenases and catalyzes the reductive condensation of taurine with pyruvate was purified from a red alga, Rhodoglossum japonicum using a combination of ammonium sulfate fractionation, gel filtration, affinity, and ion exchange chromatography. The molecular mass of this enzyme, obtained by HPLC using TSK SW2000G in its native form and SDS-PAGE in its denatured form, was 39000 and 42000, respectively. This means tauropine dehydrogenase has monomeric structure like other opine dehydrogenases. The relative activities for amino acids as substrate were 100 for taurine, 17 for valine and 12 for homotaurine. The apparent Km values for taurine, pyruvate and NADH were 15.0 mM, 0.80 mM and 0.04 mM, and for tauropine and NAD⁺ were 30 mM and 0.12 mM, respectively. Diurnal change of tauropine content was observed in R. japonicum, tauropine increased in the daytime and decreased in the nighttime.