Isozymes V2: Physiological Function, 3: 380 (2012)

Malate Dehydrogenase Isozymes in Plants: Preparation, Properties, and Biological Significance

Irwin P. Ting, Irene Fuhr, Russell Curry, and William C. Zschoche

Summary and Conclusions

The available data indicate that vascular green plants have at least three different, site specific proteins catalyzing the NADH dependent reversible reduction of oxaloacetate to malate. Each is localized in a different subcellular compartment, viz., microbodies, mitochondria, and the cytosol. These organelle specific isozymes participate in different metabolic functions or sequences: the glyoxylate cycle and the photorespiration, glycolate pathways for the microbody-MDH, the citric acid cycle for the mitochondrial-MDH, and the nonautotrophic CO2 fixation pathway for the cytosol-MDH. There is evidence that redox shuttles are important roles for the isozymes as well. The latter statements should not necessarily be taken to mean that other roles or functions are excluded. Since these organelle specific isozymes are coded for by different genetic loci, multiple forms may occur within each organelle.

Because no dramatic substrate level regulation of the various MDH isozymes is known, we propose that metabolic regulation of the same reaction in the different metabolic pathways is largely by spatial compartmentation of organelle specific isozymes. Hence an important consequence of organelle specific isozymes is metabolic regulation by compartmentation.

Although the NADP-malate dehydrogenase does not fit into the set of isozymes with the NAD-malate dehydrogenases, physiologically the four must be considered together.

Perhaps, as good an example of organelle specific isozymes comes from the work of Anderson and Advani (1970). They have shown that three of the enzymes which are common to both the photosynthetic reductive pentose cycle, localized in chloroplasts, and to the glycolytic sequence are, in fact, not the same, but are isozymes. Schnarrenberger et al (1973) demonstrated the same for glucose-6-phosphate and phosphogluconate dehydrogenase. Here, compartmentation and different metabolic roles are clearly indicated. A variety of other organelle specific isozymes are known from higher plants including malate enzyme (Mukerji and Ting, 1968), and NADP-isocitrate dehydrogenase (Tolbert and Yamazaki, 1969).