Properties and regulation of leaf nicotinamide–adenine dinucleotide phosphate–malate dehydrogenase and ‘malic’ enzyme in plants with the C₄-dicarboxylic acid pathway of photosynthesis

1. NADP–malate dehydrogenase and `malic' enzyme in maize leaf extracts were separated from NAD–malate dehydrogenase and their properties were examined. 2. The NADP–malate dehydrogenase was nicotinamide nucleotide-specific but otherwise catalysed a reaction comparable with that with the NAD-specific enzyme. By contrast with the latter enzyme, a thiol was absolutely essential for maintaining the activity of the NADP–malate dehydrogenase, and the initial velocity in the direction of malate formation, relative to the reverse direction, was faster. 3. For the `malic' enzyme reaction the K_m for malate was dependent on pH and the pH optimum varied with the malate concentration. At their respective optimum concentrations the maximum velocity for this enzyme was higher with Mg²⁺ than with Mn²⁺. 4. The NADP–malate dehydrogenase in green leaves was rapidly inactivated in the dark and was reactivated when plants were illuminated. Reactivation of the enzyme extracted from darkened leaves was achieved simply by adding a thiol compound. 5. The activity of both enzymes was low in etiolated leaves of maize plants grown in the dark but increased 10–20-fold, together with chlorophyll, when leaves were illuminated. 6. The activity of these enzymes in different species with the C₄-dicarboxylic acid pathway was compared and their possible role in photosynthesis was considered.