Grape (Vitis vinifera L.) Malate Dehydrogenase. II. Characterization of the Major Mitochondrial and Cytosolic Isoforms and their Role in Ripening

Some structural and functional properties of the major cytosolic and mitochondrial L-malate: NAD+oxidoreductase or malate dehydrogenase (MDH, E.C.1.1.1.37) isoforms from grape berries were compared in order to understand their respective roles in malate metabolism during ripening. Apparent k m s for oxaloacetate (OA) and malate in the two directions of catalysis were not significantly different. Affinities for NADH and NAD were 0.03 and 0.62 m M , respectively, for the cytosolic isoform and 0.13 and 1.05 m M for the mitochondrial isoform. Whatever the isoform, a better affinity for the OA/NADH couple appeared in accordance with the known thermodynamic sense of MDH reaction (OA reduction). Analysis with inhibitors, ethanol, and p-chloromercuribenzoic acid, did not indicate a significant difference between the two isoforms. However, their kinetic behavior towards substrate analogs were quite different. More important differentiation appeared between the two major isoforms regarding their pH-dependence and especially their thermodependence. The biphasic Arrhenius plot obtained for the mitochondrial isoform indicated that it was more easily activated above 32°C than cytosolic protein. These findings, associated with the negative correlation between total MDH activity and malate concentration during ripening, tend to support the idea that mitochondrial MDH could participate to the malate catabolizing processes in grape in response to summer growth conditions, especially to thermic stress.