Some observations upon a new inhibitor of monoamine oxidase in brain tissue

The inhibition of monoamine oxidase (MAO) in vitro and in vivo by N-methyl-N-propargyl-3(2,4-dichlorophenoxy) propylamine hydrochloride (M&B 9302) is described. The kinetics of the MAO-M&B 9302 reaction show a unique abnormality as compared with known inhibitors. The plot of percentage inhibition against concentration of M&B 9302 does not show a simple sigmoid curve, but reveals a pair of sigmoid curves joined by a horizontal section where the inhibition is invariant. The hypothesis that in the enzyme prepared, the MAO is a binary system of enzymes each of which has a detectably different sensitivity to this particular inhibitor, is put forward and discussed. Evidence after dialysis supports this hypothesis.