Purification and properties of an NADP+-specific isocitrate dehydrogenase from Rhizobium meliloti

An NADP+-specific isocitrate dehydrogenase has been purified and characterized from Rhizobium meliloti. The enzyme showed Mn++ or Mg++ requirement. The apparent Km values were 2.00 x 10(-5) M and 1.51 x 10(-5) M for DL-isocitrate and NADP+, respectively. The enzyme was inhibited by ATP, to a lesser extent by ADP and AMP. alpha-Ketoglutarate also inhibited the enzyme activity. Oxalacetate and glyoxylate together inhibited the enzyme activity. The inhibition was competitive. Studies with thiol inhibitors suggested that the enzyme contained a sulfhydryl group at or near the active site. The enzyme has an approximate molecular weight of 60,000. Fluorescence studies suggested that the enzyme contained tryptophan.