Thermal stability and kinetic properties of NADP+-malate dehydrogenase isomorphs in two populations of the C4 weed species Echinohloa crus-galli (Barnyard grass) from sites of contrasting climates

The thermal stability and kinetic properties of purified NADP⁺-malate dehydrogenase (NADP⁺-MDH; EC 1.1.1.82) isomorphs were analyzed from plants of two populations of Barnyard grass from contrasting thermal environments. Plants from Québec (QUE) and Mississippi (MISS) were acclimated under controlled conditions at 26/20°C and 14/8°C (day/night). While the enzyme from QUE showed one isomorph, 3 isomorphs were detected in all plants from MISS, suggesting the presence of gene duplication and fixed heterozygosity for the expression of this dimeric enzyme. This findig raises the possibility that the enhanced acclimatory potential of NADP⁺-MDH from MISS plants, as found from previous studies with the partially purified and unfractioned enzyme, may result from differential kinetic properties of isomorphs which would allow for the proper modulation of catalysis over a wide temperature range. The thermal stability of the QUE isomorph was significantly higher than that of any of the MISS isomorphs. The apparent activation energy of the QUE isomorph was within the range of values found for the 3 MISS isomorphs which were similar to each other. The Michaelis-Menten constants (K_m) for oxalacetic acid were not significantly different among isomorphs or between thermoperiods, but K_m (NADP⁺) values for the QUE isomorph were significantly higher than those of two of the MISS isomorphs over the 15–25°C assay range V_max/K_m ratios for OAA and NADP⁺ were not significantly different among isomorphs or between thermoperiods. Our data indicate that, under highly purified conditions, the single NADP⁺-MDH isomorph of QUE plants possesses good acclimatory potential for maintaining catalytic efficiency under a wide range of temperature conditions. In vitro thermal and kinetic data do not support the hypothesis that the the multiple NADP⁺-MDH isomorphs found in MISS plants may have been selected to optimize the thermal and catalytic efficiency of NADP⁺-MDH under warm temperature conditions.