Regulation of the Nicotinamide Adenine Dinucleotide-specific Isocitrate Dehydrogenase from a Higher Plant

The NAD+-specific isocitrate dehydrogenase from pea stems is shown to respond to the mole fraction of NADH when total NAD (NAD+ plus NADH) is kept constant. Fifty per cent inhibition occurs when the mole fraction of NADH is 0.1. In this region the enzyme is extremely sensitive to NADH concentration and it is suggested that this acts in a similar manner to energy charge in other systems. In an attempt to simulate conditions in vivo, the response of the enzyme to constant ratios of citrate to isocitrate was studied and it was found that even under these conditions the enzyme showed sigmoid kinetics.