We did some binding studies with 3H-pargyline, the only radiolabeled MAOI we were able to obtain from a commercial source. Initial results suggested that it was labeling MAO. Since pargyline is an irreversible MAOI and MQB is reversible, we tried pretreating the membranes with pargyline to block MAO and then proceeding with the binding assay. Initial results suggested that this approach might work. Unfortunately, 3H-pargyline was discontinued and we were not able to complete the experiments.
Ideally, I would like to use this approach with a radioligand with higher affinity for the MQB site, perhaps Ro 41-1049. Roche radiolabeled the compound a number of years ago and used it to map the distribution of MAO-A in the CNS but it has never been available commercially. I believe that if we pretreat the membranes with clorgyline to block MAO-A, we should be able to label the MQB site. However, these experiments will have to to wait until I can afford a custom-synthesized radioligand.