Abstract
Studies on multiplicity of mitochondria MAO in rabbit and rat liver were carried out, in vitro and in vivo, by investigating differences in the substrate specificity and inhibitor sensitivity to various MAO inhibitors, such as harmine, harmaline and clorgyline (type A MAO inhibitors) and deprenyl and pargyline (type B MAO inhibitors) . The results obtained with these two preparations were compared. The rabbit liver MAO showed the greatest rate of oxidation of β-phenylethylamine (PEA), a specific substrate for type B MAO, but showed the weakest rate of oxidation of 5-hydroxytryptemine (5-HT), a specific substrate for type A MAO. On the other hand, the rat liver MAO greatly oxidized tyramine, a substrate for both types of MAO, and it oxidized both 5-HT and PEA at a relatively smaller rate. The type A MAO inhibitors, clorgyline and harmine, caused a weak inhibition for rabbit liver MAO activity with PEA and benzylamine as substrates, but clorgyline strongly inhibited activity towards 5-HT, whereas deprenyl slightly inhibited the activity. In rat liver, type A MAO inhibitors strongly inhibited activity towards 5-HT, but it was these found to be weak for activities towards PEA and benzylamine. Inhibition curves of activity towards tyramine, by these type A MAO inhibitors, were found to be all double-sigmoidal. With deprenyl as inhibitor, almost similar results were obtained, but benzylamine oxidizing activity was the most sensitive to this inhibitor. After pre-treatment with harmaline, inhibitory action of pargyline to oxidation of 5-HT was remarkably reduced, but with other substrates, this pre-treatment did not show any remarkable protection against inhibitory effects of paryyline. This non-protective effect of the pre-treatment was also observed for rabbit liver MAO with these substrates.
These results suggest that mitochondria of rabbit and rat liver would contain both types of MAO. but the former predominantly contains type B MAO.
