Changes in Properties of Malate Dehydrogenase Isolated fromSolanum nigrumL. Seeds Imbibed at High Temperatures

Exposure of Solanum nigrum L. seeds for 48 h to high temperature (50°C) during imbibition, lowered considerably the activity of malate dehydrogenase in the mitochondria and in the cytosol. The affinity of both isozymes for their substrates and co-enzymes was strongly depressed by this treatment. Optimal pH and optimal temperature for maximal specific activity were also changed. Enzyme from treated seeds, although much less active, was more heat tolerant. Separation on a DEAE cellulose column showed two distinct peaks of activity for cytosolic malate dehydrogenase from both treated and control seeds. Gel filtration through a G-75 Sephadex column showed only one peak of activity in both cases. Gel electrophoresis of s-MDH from control seeds showed two bands of enzyme activity for each of the peaks eluting from the DEAE column. Only one activity band could be detected for enzyme from heat treated seeds and this band corresponded to one of the activity bands found in the control. From analysis of the data it appears that while part of the enzyme was denatured, due to exposure of the seeds to 50°C, a conformational change in the other part of the enzyme might have occurred which resulted in a change of enzyme properties and activity. However, on the basis of the reported results an induction of synthesis of a new ‘heat tolerant’ enzyme, due to the treatment, cannot be ruled out.