Tauropine dehydrogenase from the sandwormArabella iricolor (Polychaeta: Errantia): Purification and characterization

This is the first report of the purification of tauropine dehydrogenase (NAD: tauropine oxidoreductase) from a polychaete worm. In the sandwormArabella iricolor Montagu (Polychaet: Errantia), two forms of TaDH were detected: major component (pl = 7.5) and minor one (pI = 6.4). The major TaDH component was purified to homogeneity by means of (NH₄)₂SO₄ precipitation, anion-exchange, affinity, chromatofocusing and hydrophobic chromatography, and characterized. From the molecular mass of 43.7 kDa obtained by rapid gel-filtration and that of 43.5 kDa by SDS-PAGE, the sandworm enzyme appeared to be a monomeric protein. Maximum rates of reduction of pyruvate and oxidation of tauropine were observed at pH 7.0 and 8.5, respectively. Pyruvate and taurine were preferred substrate for the enzyme. Apparent Km values determined using constant co-substrate concentrations were: 35.7 mM, 0.34 mM, and 0.036 mM for taurine, pyruvate and NADH, respectively, in the tauropine synthesizing reaction; and 4.8 mM and 0.051 mM for tauropine and NAD⁺, respectively, in the tauropine oxidizing reaction. The tauropine synthesizing reaction was subject to substrate inhibition by pyruvate: maximum rate was observed at 2.5–3.0 mM (inhibitory range of pyruvate concentration producing half-maximal rate was 26.8 mM).