Activity of the succinic dehydrogenase-cytochrome system in different tissue preparations

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Abstract
A study of the succinic dehydrogenase-cytochrome system in various types of tissue extract, with special reference to the effect of colloidal structure on enzyme activity was made. Heart-muscle prepns. showed optimum activity in 0.15[image] phosphate; at lower phosphate concns. addition of proteins or certain metallic ions raised the activity to the optimum level. In this respect insoluble denatured proteins were the most effective. The activity of these prepns. fell considerably after freezing, but was restored by addition of cytochrome c. The pronounced loss of activity that accompanied vacuum drying was fully reversed only by adding both cytochrome c and either proteins or certain metal salts. Kidney prepns. were feebly active unless cytochrome was added. They were virtually inactive after vacuum drying, but were restored to full activity by addition of both cytochrome c and protein, although each added separately had very little effect. The activity of these prepns. fell as the buffer concn. was increased; there was no optimum concn. The activation by metals was obtained in phosphate buffer, where gelatinous precipitates were formed, but not in glyoxaline buffer. It was considered that the catalysts in the colloidal particles, as in the intact cells, were more or less rigidly held together within a framework which ensured their mutual accessibility and a consequent high catalytic activity. Any change in colloidal structure would therefore modify the overall activity without necessarily removing or destroying any individual catalyst. The inhibitory effects of freezing, drying, extremes of phosphate concn. were intimately bound up with the formation from the enzyme material of flocculent precipitates and a consequent decrease in interavailability. Activation, however, arose when flocculent precipitates of foreign materials (proteins, gelatinous phosphates) were formed in the medium. Such material provides new surfaces upon which reorientation took place. Since the pigeon prepns. already consisted of flocculent material they did not undergo activation. It was clearly demonstrated that added cytochrome c acted in these expts. solely as an oxido-reduction catalyst and not as a protein affecting the colloidal structure. Two types of factor (activating or inhibiting) were distinguished. They might have functioned either by reacting specifically with a catalyst in the system or by affecting the colloidal structure of the enzyme material. Activation of the latter type has often been misinterpreted as evidence for additional carriers or "links" in the succinic dehydrogenase-cytochrome system.