NADP-dependent isocitrate dehydrogenase from the mussel Mytilus edulis L. 2. Kinetic studies.

The kinetic mechanism for NADP-dependent isocitrate dehydrogenase from the digestive gland of the mussel Mytilus edulis has been investigated. Initial-rate studies and substrate-analogue and product-inhibition patterns are consistent with a rapid-equilibrium random-ordered reaction mechanism, both with respect to substrate addition and product release. Product inhibition by NADPH is non-competitive versus D-isocitrate concentrations. 2-Oxoglutarate inhibition is competitive versus D-isocitrate at all NADP concentrations. The inhibition by 2-oxoglutarate versus NADP is either non-competitive or uncompetitive. Tricarballylic acid (a substrate analogue of D-isocitric acid) gives competitive inhibition versus D-isocitrate at all NADP levels and mixed inhibition versus NADP, at sub-saturating levels of D-isocitrate.