A DUAL COFACTOR-SPECIFIC ISOCITRATE DEHYDROGENASE FROM PYTHIUM-ULTIMUM

Isocitrate dehydrogenase is considered to be one of the key regulatory enzymes in the conversion of glucose into fatty acids by oleaginous m icroorganisms. A dual coenzyme-specific isocitrate dehydrogenase (EC 1 .1.1.41) (IDH) was isolated from the primitive fungus Pythium ultimum and purified by 211-fold by sequential ion-exchange, affinity, and gel filtration chromatographies. Specific activity of the partially purif ied enzyme was 76.2 mu mol/(min . mg protein) with NAD(+) and 40% less active with NADP(+). Optimum pH for activity was 8.5-9.5. K-m values for threo-D-isocitrate and NAD(+) were 0.031 and 0.55 mM, respectively . The estimated molecular mass of the IDH was 96 kDa under nondenaturi ng conditions and 48 kDa under denaturing conditions, suggesting that the enzyme is composed of two subunits of the same size. The enzyme wa s relatively stable up to 55 degrees C, but no activity was detected a fter exposure to 65 degrees C for 15 min. Mg2+ or Mn2+ were required f or activity.