Inhibition of methemoglobin and metmyoglobin reduction by cobalt

Because cobalt compounds tend to form stable complexes, there has been continued interest in the use of the salts and chelates of cobalt in cyanide poisoning, and continued uncertainty about the precise nature of their protective effects. We have found that cobalt ions inhibit the enzymatic reduction of both methemoglobin and metmyoglobin. Virtually total inhibition of methemoglobin and metmyoglobin reductase activity occurred with the addition of 2.5 mM cobalt acetate to the assay system. Both enzymes were inhibited by lower levels of cobalt in a dose-dependent manner. The similarity in susceptibility of cobalt inhibition is further evidence that the enzymes which reduce methemoglobin and metmyoglobin are functionally comparable. The inhibition of methemoglobin reductase activity may be, in part, responsible for the therapeutic effectiveness of cobalt salts and chelates in cyanide poisoning.