抄録
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-iosynthetic 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 0.25-0.35mM and the inhibitory concentration giving half-maximal rate was 3.2mM. The tauropine-catabolic reaction was inhibited by the substrate tauropine: the optimal concentration was 2.5-5.0mM. Apparent Km values determined using constant cosubstrate concentrations were 37.0mM for taurine, 0.068mM for pyruvate, and 0.036mM for NADH in the tauropine-biosynthetic reaction; and 0.39mM for tauropine and 0.16mM for NAD+ in the tauropine-catabolic reaction.
