Changes in rat liver and brain monoamine oxidase activity after acute treatment with some heavy metal salts

In in vivo and in vitro experiments the effects of some heavy metal salts (Cu, Co, Cd, Pb, Ni, Zn, Hg, As, Bi and Sn) on rat liver and brain mitochondrial monoamine oxidase (MAO) activity was studied using three different substrates (tyramine, 5-hydroxytryptamine (5-HT) and beta-phenylethylamine (2-PEA). It was established that some of the metals (Cu, Cd, Bi) inhibited MAO activity both in vivo and in vitro experiments, others like Ni, Zn, As and Sn inhibited it only in vivo while Hg exerted inhibitory action only in vitro. The in vivo experiments showed significantly higher sensitivity of brain MAO as compared with liver MAO to the inhibitory action of metals. The same higher susceptibility was shown by liver and brain MAO-A form. It was concluded that the inhibitory effects of some heavy metal salts on MAO activity were not directly connected with their action on enzyme thiol groups but more probably with changes in the enzyme membrane surroundings in the different organs.