Influence of the temperature on the MDH activity of cytosol and submitochondrial fractions of rat liver in different buffers.

The temperature optimum and the temperature stability of malate dehydrogenase (MDH) in cytosol and submitochondrial fractions (matrix and inner membranes) in the buffers: Tris-HCl, potassium-phosphate and HEPEC-NaOH at pH 7.5, are determined. The temperature optimum of MDH (EC 1.1.1.37) in cytosol does not depend on the type of the buffer used, unlike the optimum in mitochondrial preparations. The mitochondrial enzyme is less resistant to temperature compared to the cytosol preparation. The effect of all three buffers used on the native and on the temperature-changed enzyme in the preparations in preserved the same. However, the presence of the buffer during the temperature action on the MDH-activity in the preparations changes the temperature stability of the enzyme. Changes are assumed in the conformation of the mitochondrial enzyme under the effect of phosphate ions, leading to activation of the native enzyme and to a rise in its temperature stability.