Cd2+ activation of L-threonine dehydrogenase from Escherichia coli K-12.

Homogeneous preparations of L-threonine dehydrogenase (L-threonine:NAD+ oxidoreductase, EC 1.1.1.103) from Escherichia coli K-12, after having been dialyzed against buffers containing Chelex-100 resin, have a basal level of activity of 10-20 units/mg. Added Cd2+ stimulates dehydrogenase activity approx. 10-fold; this activation is concentration-dependent and is saturable with an activation Kd = 0.9 microM. Full activation by Cd2+ is obtained in the absence of added thiols. The pH-activity profile of the Cd2+-activated enzyme conforms to a theoretical curve for one-proton ionization with a pKa = 7.85. Mn2+, the only other activating metal ion, competes with Cd2+ for the same binding site. Km values for L-threonine and NAD+ as well as the Vmax for 'demetallized', Cd2+-activated, and Mn2+-activated threonine dehydrogenase were determined and compared.