The inhibition of MAO in vivo by pargyline in mice has been studied to evaluate the usefulness of n-octylamine · HC1[1-11C], a substrate of MAO that is rapidly metabolized to 11CO2, as a probe of MAO activity in vivo. In pargyline-pretreated mice, the rate of decline in tissue levels of [11C]n-octylamine after intravenous administration was decreased to an extent that varied with the particular tissue and the time after pargyline pretreatment. Likewise, pargyline-treated mice showed a reduced excretion of 11CO2 after [11C]n-octylainine administration but not after octanoie acid[1-11C] or octanol[1-11C] injection. The duration of inhibition of 11CO2 excretion after pargyline treatment paralleled reasonably well the inhibition of MAO measured directly in intestine and liver, but the inhibition of MAO in the lung, kidney and brain persisted longer. Direct measurement of mitochondrial MAO activity using [14C]octylamine showed the following: intestine > liver > lung > brain 》 kidney. At 94 hr after pargyline administration, 11CO2 excretion was close to the control value yet the MAO activity of brain, lung and kidney was less than half of the control value. Thus, the rate-determining step in the overall rate of 11CO2 excretion appears not to be the MAO-catalyzed step but the reactions occurring after deamination.
