Monoamine oxidase activity in the liver of rats given diethylnitrosamine

The effect of diethylnitrosamine (DEN), a well known experimental carcinogen, MAO-A and MAO-B activity of rat liver was investigated. The oxidations of both 5-HT (MAO-A) and beta-PEA (MAO-B) were inhibited by DEN in vitro. DEN is reversible inhibitor, after enzyme-inhibitor preincubation. MAO-B activity in rat liver was decreased by continuous administration of 50 ppm DEN for several definite periods to rats. The almost 2-fold increase in the ratio of MAO-A/MAO-B was observed in rat liver mitochondria and microsomes. When 5-HT was used as substrate, there was no significant difference of Km and Vmax values between mitochondrial preparations from livers of control and DEN fed rats. However, when the mitochondria from livers of DEN fed rats were used as enzyme preparation, Vmax decreased to about 47% of the control with beta-PEA as substrate. DEN might change the proportions of the multiple forms of MAO activity in tumor cells.