Studies of Hepatic, Brain Monoamine Oxidase and Brain Serotonin in Rats

ABSTRACT

The effects of 7-ethyl-8-methylf1avin (7-Et) and 7-methyl-8-ethyl-flavin (8-Et) on rat hepatic monoamine oxidase (MAO), brain MAO activity and 5-hydroxytryptamine (5-HT or serotonin) in rat brain were investigated. In the study of hepatic MAO activity, kynur-amine a nonphysiological substrate for both A and B type MAO, was used for a spectro-photometric method, and [14C]-labeled amines were also used for a radiometric procedure for camparison with MAO activity determined by the spectrophotometric method. The rate of change in MAO activity of hepatic mitochondria from rats receiving Rb-def and 7-Et and 8-Et flavin showed the activity was severely reduced during 8 weeks. Rapid reduction of enzyme activity (50% in def-group, 35% in 7-Et group and 8% 8-Et flavin group) was observed at the end of 2 weeks. The enzyme activity lasted with slow decre-ment of enzyme level from 4 weeks to the end of 8 weeks as low as 16% in def, 18% in 7-Et and 3% in 8-Et flavin group. The trend of decrement of MAO activity when kynura-mine was used as a substrate appears to be similar with the small variation of MAO activity when [14C]-labelled tyramine, dopamine, serotonin and tryptamine respectively were used as substrate. The rate of decay of brain mitochondrial MAO activity in rats receiving each respective f1avin was not rapid and severely depressed as the MAO activity we have found in liver mitochondrial MAO of rats during the 8 week experimental time, but a similar tendency of decay of MAO in each group was observed. The potent inhibitory effect of 8-Et on brain MAO was confirmed by the study of the simultaneous measure-ment of MAO activity in each experimental group. when the reduction Of brain MAO activity in rats receiving 8-Et after 6 weeks was approximately 80% of normal and in the same rats the concentration of brain 5-HT showed a 60% increment of that of the normal mts. During the experimental period there is no absolute parallelism between the MAO inhibition and 5-HT increase. However when the reduction of MAO activity reached 80% of normal value, the concentration of 5-HT increased dramatically as much as 60% of normal value. The results so far suggest clearly that 8-Et produces a much more potent inhibitory effect on the hepatic MAO a s well as brain MAO in rats. Therefore our present and previous results suggest that 7-Et and 8-Et flavin should bind itself to hepatic, brain MAO apoenzyme in the condition of total absence of riboflavin in these animals, and the holenzyme is catalytically inactive.