Partial purification and comparison of the kinetic properties of ovine liver Echinococcus granulosus hydatid cyst fluid malate dehydrogenase and the cytoplasmic enzyme from the host liver

Purification, properties, and kinetic studies of cytoplasmic malate dehydrogenase from Taenia solium cysticerci

Malate dehydrogenase (L: -malate: NAD oxidoreductase, EC 1.1.1.37) from the cytoplasm of Taenia solium cysticerci (cMDHTs) was purified 48-fold through a four-step procedure involving salt fractionation, ionic exchange, and dye affinity chromatography. cMDHTs had a native M (r) of 64,000, while the corresponding value per subunit, obtained under denaturing conditions, was 32,000. The enzyme is partially positive, with an isoelectric point of 8.7, and had a specific activity of 2,615 U mg(-1) in the reduction of oxaloacetate. The second to the 21st amino acids from cMDHTs N-terminal group were P G P L R V L I T G A A G Q I A Y N L S. This sequence is 100% identical to that of Echinococcus granulosus. Basic kinetic parameters were determined for this enzyme. The optimum pH for enzyme reaction was at 7.6 for oxaloacetate reduction and at 9.6 for malate oxidation. K (m) values for oxaloacetate, malate, NAD, and NADH were 2.4, 215, 50, and 48 microM, respectively. V (max) values for the substrates and cosubstrates as described above were 1,490, 87.8, 104, and 1,714 micromol min(-1) mg(-1). Several NAD analogs, structurally altered in either the pyridine or purine moiety, were observed to function as coenzymes in the reaction catalyzed by the purified malate dehydrogenase. cMDHTs activity was uncompetitive inhibited by arsenate for both the forward (Ki = 8.2 mM) and reverse (Ki = 77 mM) reactions. The mechanism of the cMDHTs reactivity was investigated kinetically by the product inhibition approach. The results of this study are qualitatively consistent with an Ordered Bi Bi reaction mechanism, in which only the coenzymes can react with the free enzyme.

Purification and characterization of the malate dehydrogenase from Streptomyces aureofaciens

The malate dehydrogenase (MDH) from Streptomyces aureofaciens was purified to homogeneity and its physical and biochemical properties were studied. Its amino-terminal sequence perfectly matched the amino-terminal sequence of the MDH from Streptomyces atratus whose biochemical characteristics have never been determined. The molecular mass of the native enzyme, estimated by size-exclusion chromatography, was 70 kDa. The protein was a homodimer, with a 38-kDa subunit molecular mass. It showed a strong specificity for NADH and was much more efficient for the reduction of oxaloacetate than for the oxidation of malate, with a pH optimum of 8. Unlike MDHs from other sources, it was not inhibited by excess oxaloacetate. This first complete functional characterization of an MDH from Streptomyces shows that the enzyme is very similar in many respects to other bacterial MDHs with the notable exception of a lack of inhibition by excess substrate.