Some properties of the NADP-specific malic enzyme from the moderate halophile Vibrio costicola

NADP-specific malic enzyme (EC 1.1.1.40) has been purified about 160-fold from the moderate halophile Vibrio costicola. The enzyme has a molecular weight of about 120,000. The purified enzyme was unstable in dilute solutions but could be stabilised by NaCl or glycerol. NH4Cl or KCI caused maximal activation at 0.1M, but higher concentrations were inhibitory. NaCl did not activate and was instead a mixed-type inhibitor towards NH4Cl or KCI. The salt concentration affected the kinetic parameters of the reaction. The apparent Km for L-malate reached a minimal value at about 0.1 M salt; the value for NADP, on the other hand, increased continuosly with the Co2+ or Mg2+. NADH was a mixed-type inhibitor towards both substrates, whereas oxaloacetate was strictly competitive towards L-malate and non-competitive towards NADP. The inhibition kinetics were sigmoidal for NADH and hyperbolic for oxaloacetate. The malic enzyme form V. costicola was similar to those of a marine Pseudomonas and Halobacterium cutirubrum in kinetic and regulatory properties but showed a response to salts intermediate between those of the latter enzymes.