Tissue fractionation studies. 14. The activation of latent dehydrogenases in mitochondria from rat liver

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Abstract
A comparison was made between the activation of a latent mitochondrial enzyme glutamic dehydrogenase, and that of a lysosomal enzyme, acid phosphatase, by freezing and thawing, hypotonicity, the neutral detergent Triton X-100 and blender treatment. Malic and D-[beta]-hydroxybutyric dehydrogenases also occurred in mitochondria in a latent form. The activation of these enzymes was compared with that of glutamic dehydrogenase. In graded activation experiments the activation curves for the 3 dehydrogenases followed each other closely, but they were distinct from the curves for acid phosphatase. A marked irreversible osmotic activation occurred at sucrose concentrations below 0.025[image]. At higher sucrose concentrations only a small degree of reversible activation of glutamic dehydrogenase could be observed. Activation led to a liberation of glutamic and malic dehydrogenases into solution, whereas [beta]-hydroxybutyric dehydrogenase remained bound to the particle residue. Activation is interpreted as being due to an increased permeability of the mitochondrial membrane. It is suggested that 2 types of activation may be distinguished, depending on whether the increase is sufficiently large to allow enzyme molecules to leak out, or only to allow substrate molecules to penetrate. The former kind is irreversible whereas the latter can possibly be reversed.