Thermal acclimation and in vitro stabilization of malate dehydrogenase from Machaeranthera gracilis

Abstract

Thermal acclimation of NAD+-malate dehydrogenase (MDH) was examined in leaves of two chromosomal races of Machaeranthera gracilis, an annual composite. Plants of both the parental foothills race and the derived desert race, produced a more thermostable enzyme when grown at 35:25 °C than when grown at 25:15 °C. Increased enzyme thermostability during acclimation may potentially be augmented by thermal protection of malate dehydrogenase (MDH) by cellular metabolites, including most significantly the coenzyme nicotinamide adenine dinucleotide (reduced from) (NADH). The Arrhenius activation energy for MDH from plants of both races did not show adjustments induced by growth at higher temperatures. Growth at 35:25 °C, resulted in MDH with a relatively more constant coenzyme binding affinity (assayed from 25 to 24 °C), as determined from substrate dissociation constants. Substrate Michaelis constants for MDH were largely independent of plant growth temperatures. Responses by MDH during acclimation by both races of M. gracilis are unique to those reported from other less thermotolerant plants, but do not explicitly account for habitat preferences between the two genetic races.