The characteristics of glutathione reductase from a number of maize cultivars with contrasting thermotolerance have been investigated. The apparent Km (Michaelis constant) for oxidised glutathione (GSSG) was measured between 10 and 45°C at constant pH. The enzyme from highland cultivars adapted to cool environments had a slightly lower apparent Km for GSSG than that from lowland tropical cultivars at low assay temperatures. Similarly the enzyme from lowland tropical cultivars had a lower apparent Km for GSSG at high assay temperatures. However these effects were small and regression lines plotted through the data were not significantly different in slope or intercept. There was a strong correlation (r = 0·939) between apparent Km and Vmax (maximum initial velocity) as assay temperature was varied. The interpretation of apparent Km/temperature relationships is discussed with hypothetical examples of the effects of temperature on enzyme activity/substrate concentration plots. It is demonstrated that an increase in apparent Km at higher assay temperatures need not necessarily reflect any temperature-dependent impairment of enzyme function. The apparent Km for GSSG of glutathione reductase from maize increased over four-fold when the temperature was raised from 10 to 45°C, but it is concluded that in vivo rates of reaction are likely to be increased rather than decreased by this same change in temperature. Glutathione reductase would thus appear to be equally well adapted to function at all these temperatures. This suggests that the potential of enzyme thermal kinetics to predict thermotolerance may be limited.
Turner, L.B.; Pollock, C.J.; Edmeades, G.O. Thermal Kinetics of Glutathione Reductase and their Relation to Thermotolerance in Diverse Cultivars of Maize. Annals of Botany (1994) 74 (3) 245-250. [DOI: 10.1006/anbo.1994.1115]