Kinetics of Inactivation of NADP⁺-Linked Isocitrate Dehydrogenase from Germinating Mung Bean (Vigna radiata)

Metal chelating agent EDTA inhibits the activity of mung-bean NADP⁺-linked isocitrate dehydrogenase (ICDH) in a competitive manner. The activity of the Apo-enzyme was restored by divalent metal ions with the order of effectiveness found to be Mn²⁺> Mg²⁺ > Zn²⁺ > Co²⁺ > Cu²⁺. There appeared to be a single type of metal binding site that was saturated either with 0.5 mM of Mn²⁺ or with 2.5 mM of Mg²⁺. ADP, ATP and NADPH inhibit the enzyme in competitive manner. On titration with 5, 5-dithiobis (2nitrobenzoate), i.e. DTNB, the mung bean isocitrate dehydrogenase showed 4.0 reactive –SH groups per molecule. The denatured ICDH enzyme of mung bean possess 8.1–SH groups per molecule. The blocking of this group with –SH reagents, lead to the inactivation of mung bean ICDH enzyme. Time-dependent inactivation of ICDH with iodoacetamide and Nethylmaleimide (NEM) revealed decay in the activity in a single exponential manner.