Biochemical studies in marine species-I. NADP⁺-dependent isocitrate dehydrogenase from turbot liver (Scophthalmus maximus L.)

1. 1. An NADP⁺-dependent isocitrate dehydrogenase was extracted from turbot liver. The enzyme required divalent cations (Mg²⁺ or Mn²⁺) for its activity but was inhibited by high salt concentrations. 2. 2. The enzyme had an optimum activity in the pH range between 7.5 and 9.0. The enzymic activity increased with temperature (in the range of 5 to 68°C) with an E_a of 23.5 kJ/mol and a Q₁₀ of 1.34. 3. 3. The apparent K_m values for the substrates were 6.5 µM for NADP⁺, 56 µM for Mg²⁺, 87 µM for Mn²⁺ and 4.2 and 73.5 µM for the complexes Mg-isocitrate and Mn-isocitrate, respectively. The physiological significance of these results is discussed. 4. 4. The enzyme was inhibited by citrate and adenine nucleotides. The degree of inhibition depended on the relation between the concentrations and that of magnesium. A possible regulating mechanism is proposed. © 1989.