The effects of ZnSO4 on types A and B monoamine oxidase (MAO) isozymes in monkey brain mitochondria were investigated, in vitro. Type A MAO activity in monkey brain decreased to about 50% with 1 muM ZnSO4 using serotonin as a substrate, and this inhibition was proportional to the concentration of ZnSO4, ZnSO4 had no effect, however, on type B MAO activity in monkey brain using Pphenylethylamine as a substrate. The inhibition by ZnSO4 of type A MAO activity was competitive and reversible. ZnSO4 did not inhibit either type A or type B MAO activity in rat brain mitochondria. Almost similar results were also obtained when ZnCl2 was used, ill vitro. These results indicate that the inhibiting action of zinc ion differs depending on animal species and organ. Type A MAO in monkey brain mitochondria was highly sensitive to zinc ion. while type B activity was less sensitive. (C) 2002 Elsevier Science Inc. All rights reserved.