Effects of Various Solutes on the Thermostability of Isocitrate Dehydrogenase and Malate Dehydrogenase of Isolated Plant Mitochondria.

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Abstract
Proline and betaine increased the thermostability of NAD-isocitrate dehydrogenase (EC 1.1.1.41) and of NAD-malate dehydrogenase (EC 1.1.1.37) when mitochondria isolated from pea leaves (Pisum sativum L.), wheat leaves (Triticum aestivum L.) and cauliflower buds (Brassica oleracea L.) were heated. Potassium chloride and choline chloride also increased the thermostability of isocitrate dehydrogenase in the three species, but their effects on malate dehydrogenase varied. Protection was found with both intact and disrupted mitochondria, indicating that it was not dependent on an osmotic effect. Proline and KCl also prolonged membrane integrity, as measured by impermeability to NAD+, during heating of pea leaf and cauliflower bud mitochondria. Phenylalanine reduced the thermostability of isocitrate dehydrogenase, indicating that protection is not a general solute effect.