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Thermal Dependence of Glutathione Reductase; Thermal Limitations on Antioxidant Protection in Plants

Plant Stress and Water Conserv. Unit, USDA-ARS, Route 3, Lubbock TX 79401

^* Corresponding author (jmahan{at}lubbock.ars.ag.gov).

Though plants have endogenous systems to prevent and/or repair thermal damage, thermal stresses often limit agronomic yields. The enzyme glutathione reductase plays a role in the detoxification of oxidants that often result from exposure of plants to high or low temperatures. Since thermal limitations on the function of glutathione reductase could limit the ability of a plant to withstand thermal stress, it is important to understand its thermal dependence. Previously, the thermal dependence of maximal velocity and apparent K_M have been used to describe the thermal dependence of enzymes and define an optimal thermal range for their function. While both maximal velocity and apparent K_M are factors in the thermal response of the velocity of an enzyme reaction, they are not necessarily indicative of the thermal dependence of velocity under physiological conditions. In this study, a computer program was used to develop estimates of the thermal dependence of reaction velocity at a given temperature and substrate concentration. Optimal thermal ranges for the glutathione reductascs from spinach (Spinacia oleracea L.), corn (Zea mays L.), and cucumber (Cucurais sativum L. cv. Ashley) were predicted from the thermal dependence of maximal velocity, predicted velocity, V_max/K_M, and apparent K_M. These results suggest that the predicted thermal dependence of velocity may be a useful indicator of the optimal thermal range of plant enzymes.

Received for publication December 6, 1993.

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