MALIC DEHYDROGENASES IN CORN ROOT TIPS

Three malic dehydrogenase isozymes isolated from 2.5-day-old corn root tips were separated by anion-exchange column chromatography. By pH-dependent properties and electrophoretic mobility, the two dominant isozymes were shown to correspond to a soluble-malic dehydrogenase and a mitochondrial-malic dehydrogenase. The soluble form differed from the mitochondrial form by showing greater oxaloacetate substrate inhibition at low pH and less at high pH. The soluble enzyme was more active than the mitochondrial enzyme with the nucleotide analog, thionicotinamideadenine dinucleotide. Oxaloacetate Michaelis constants for both isozymes increased with pH with a definite change in slope at about pH 8.0. At high pH, the Michaelis constant for the soluble isozyme was larger than that for the mitochondrial isozyme. Gel filtration indicated that the molecular weights of the two major isozymes were approximately equal and equal to pig heart mitochondrial-malic dehydrogenase. A second mitochondrial isozyme appeared to be twice as large as the other two. In its properties, the second mitochondrial form appeared to correspond to the major mitochondrial isozyme. It is considered very significant that the properties of corn root soluble- and mitochondrial-malic dehydrogenase are similar to avian and mammalian forms. © 1968.