Abstract
Effects of MnCl2 on MAO activity in dog brain, liver, serum and human placenta were studied radiometrically. At concentrations of 10-3 and 10-2 M, MnCl2 inhibited the oxidation of tyramine, 5-HT and benzylamine by MAO in dog brain, liver and serum. However MAO in human placenta with tyramine as substrate was activated by MnCl2 at concentration of 10-4 M while MAO in human placenta with 5-HT as substrate was inhibited by MnCl2 at concentration of 10-2 M. The activation and the inhibition mechanism of MAO in human placenta by MnCl2 were investigated by the method of Lineweaver-Burk plots using tyramine and 5-HT as substrate, and it was found that the inhibition of MAO by MnCl2 using 5-HT as substrate was noncompetitive one and the activation of MAO by MnCl2 using tyramine as substrate was competitive. On the other hand, it was also found that the activation of MAO by MnCl2 using tyramine as substrate was reversible, and the inhibition of MAO by MnCl2 using 5-HT as substrate was irreversible. These results suggest that there are at least two different forms of MAO in human placenta.
