Regulation of NAD(+)-dependent isocitrate dehydrogenase in the citrate producing yeast Yarrowia lipolytica.

The mechanism of the increased accumulation (overproduction) of citric acids in the yeast Yarrowia lipolytica while growing in the presence of glucose under nitrogen deficiency was investigated. The limitation of the yeast growth by the source of nitrogen decreases the total content of nucleotides and increases the ratios of ATP/AMP and NADH/NAD+. NAD+-Dependent isocitrate dehydrogenase, an enzyme of the tricarboxylic acid cycle playing a key role in the regulation of biosynthesis of citric and isocitric acids, was isolated from Y. lipolytica. The molecular weights of the native enzyme and its subunits were found to be 412 and 52 kD, respectively. It is concluded that the enzyme is a homo-oligomer consisting of eight subunits. Investigation of the effect of some intermediates of the tricarboxylic acid cycle on the activity of this enzyme suggests that the enhanced excretion of citric acids can be caused by the inhibition of NAD+-dependent isocitrate dehydrogenase due to the decrease in the content of AMP and increase in the NADH/NAD+ ratio in the cells of Y. lipolytica under depletion of nitrogen.