Succinate-cytochrome c reductase activity in rat liver decreased to about 60% of the control value after a single injection of cobalt or in a steady state of intoxication, but the activity in the spleen was unaltered. 2. Incorporation of radioactive glycine and 5-aminolevulinate into heme of the liver was markedly inhibited by cobalt treatment. 3. 5-Aminolevulinate synthase [EC 2.3.1.37] activity in the liver decreased to 40% of the control value 4 hr after cobalt injection, and completely recovered 20 hr later. Phenylhydrazine-induced 5-aminolevulinate synthase activity in the spleen was not decreased by cobalt injection. 4. Porphobilinogen synthase [EC 4.2.1.24] activity in the liver decreased and reached its minimum value (42% of the control) 12 hr after cobalt injection. On the other hand, the activity in the spleen showed a marked increase 24 hr after coblat injection. 5. Ferrochelatase [EC 4.99.1.1] activity in the liver was essentially unaltered by cobalt treatment, while the activity in the spleen was elevated dramatically after 24 hr. 6. Concentrations of cobalt after a single injection were about 0.3 mM and 0.03 mM in the liver and spleen, respectively. 7. Inhibitions of 5-aminolevulinate synthase and porphobilinogen synthase activities by cobalt in vitro were not as marked as expected from in vivo experiments.
