NADP⁺- dependent isocritrate dehydrogenase from human kidney mitochondria

Abstract

NADP-IDH was purified from the mitochondria of human kidney to homogeneity to about 1953-fold and a yield of 19.0%. The enzyme is a dimer with a molecular mass of about 220 to 240 kDa and dependent on Mn⁺² or Mg⁺² but other divalent cations do activate it at various degrees. Antibodies that were raised in the rabbit against the enzyme completely inhibited the activity of the enzyme, even when the enzyme could be protected partially by pre-incubating it with isocitrate or ADP. It can be protected fully by preincubating the enzyme with a mixture of isocitrate and ADP in the presence of Mg⁺² or Mn⁺². The K_m values of the protected enzyme had the same K_m values for the substrates as those for the purified enzyme. Mononucleotides phosphatases have no effect on NAD-IDH, but all trinucleotides phosphatases especially ATP inhibit the enzyme. The inhibition by ATP (or NADH) cannot be counteracted by ADP in the presence of isocitrate, so ADP cannot enhance NAD-IDH activity nor reverse inhibition by ATP (or NADH), while isocitrate will bind to the enzyme and prevent it from interacting with ADP. The activity of NADP-IDH in mitochondria is probably controlled in a complex way by NADPH, ATP and divalent ions.

Key words: NADP+-IDH, isocitrate, P cibacron blue, mitochondria, antibody interactions.