Purification and Characterization of Tauropine Dehydrogenase from the Marine Sponge Halichondria japonica Kadota (Demospongia)

Tauropine dehydrogenase (tauropine: NAD oxidoreductase) was purified to homogeneity from the sponge Halichondria japonica Kadota (colony). Relative molecular masses of this enzyme in its native form and in its denatured form were 36,500 and 37,000, respectively, indicating a monomeric structure. The maximum rate in the tauropine-biosynthetic reaction was observed at pH 6.8, and that in the tauropine-catabolic reaction at pH 9.0. Pyruvate and taurine were the preferred substrates. The enzyme showed significant activity for oxalacetate as a substitute for pyruvate but much lower activities for other keto acids and amino acids. The tauropine-biosynthetic reaction was strongly inhibited by the substrate pyruvate. The optimal concentration of pyruvate was 0.25-0.35 HIM and the inhibitory concentration giving half-maximal rate was 3.2 mM. The tauropine-catabolic reaction was inhibited by the substrate tauropine: the optimal concentration was 2.5-5.0 mM. Apparent K_m values determined using constant cosubstrate concentrations were 37.0 mM for taurine, 0.068 mM for pyruvate, and 0.036 mM for NADH in the tauropine-biosynthetic reaction; and 0.39 mM for tauropine and 0.16 mM for NAD⁺ in the tauropine-catabolic reaction.