The biological action of cobalt and other metals II. The mechanism of the respiratory inhibition produced by cobalt in mammalian tissues

Abstract 1. 1. The intraperitoneal injection of 1.25 mg Co 2+ as cobalt chloride into rats caused a 25% depression in respiration of the liver within 21 h. 5 mg Co 2+ similarly injected produced this degree of respiratory inhibition within 15 min. 2. 2. When increasing amounts of cobalt were added to suspensions of isolated tissues the respiration progressively decreased, and then became essentially independent of the cobalt concentration. 3. 3. The oxidative activities of homogenates of liver and diaphragm were much more sensitive to inhibition by low concentrations of cobalt than the intact tissues; again, however, part of the respiration appeared insensitive to Co 2+ . 4. 4. Homogenates of thigh muscle showed consistent oxidation of succinate, but not of other intermediates in the tricarboxylic acid cycle. The rates of oxidation of keto acids declined rapidly; this could be prevented by the addition of DPN or cobalt chloride to the medium, although cobalt also caused an initial inhibition of such oxidations. 5. 5. Cobalt caused by a very slight inhibition of glycolysis by liver homogenates, and an initial stimulation of glycolysis by muscle homogenates. 6. 6. Of the oxidations of tricarboxylic acid-cycle intermediates by rat-liver mitochondria, those of keto acids were most inhibited by cobalt. In addition, the accumulation of keto acids in suspensions of mitochondria with citrate as substrate was increased by the addition of 5 μg Co 2+ /ml. 7. 7. Mitochondria of thigh muscle showed very low rates of oxidation of malate, fumarate and citrate. Keto acids were oxidized effectively; the oxidation of these substrates was readily inhibited by Co 2+ . 8. 8. In the presence of concentrations of cobalt and arsenite that gave the same degree of respiratory inhibition, the rates of accumulation of keto acids from the metabolism of malate by mitochondria were similar. 9. 9. The presence of a metabolizable substrate increased the uptake of isotopically labelled cobalt by liver mitochondria by at least 14%; the cobalt taken up under these conditions was removed less readily. 10. 10. It is suggested that the action of cobalt, like that of arsenite, is due to complex formation between the ion and the dithiol form of lipoic acid, a coenzyme of keto acid dehydrogenation.