Studies of a halophilic NADH dehydrogenase. II. Kinetic properties of the enzyme in relation to salt activation.

1. An NADH dehydrogenase, obtained from an extremely halophilic bacterium, was activated by various salts when enzyme activity was measured as the observed velocity, whereas the maximum velocity was unaffected by either the salt concentration or the nature of the salt. 2. Two ion effects were observed; a quantitative cation effect, reflected in changes in the apparent Michaelis constant for 2,6-dichlorophenolindophenol, and a qualitative anion effect, reflected in the apparent Michaelis and dissociation constants for NADH. 3. The data suggest that cations act by neutralizing electrostatic charges surrounding the 2,6-dichlorophenolindophenol-binding site, whereas the anions affect the conformation of the enzyme by altering the accessibility of the NADH-binding site to the bulk solvent. 4. Thus, the apparent activation of this enzyme, obtained from an extremely halophilic bacterium, is a reflection of measuring enzyme activity at non-saturating substrate concentrations.