Mitochondrial Monoamine Oxidase

Inhibitions of the relatively highly purified flavoenzyme, bovine kidney mitochondrial monoamine oxidase, previously described in Paper I (Erwin, V. G., and Hellerman, L., J. Biol. Gem., 242, 4230 (1967)), by several agents of current interest have been investigated. The inhibitors include Nmethyl-N-(Z-propynyl)benzylamine (pargyline), (+)-fransphenylcyclopropylamine (d form of tranylcypromine), as well as various substituted hydrazines including l-hydrazinophthalazine (hydralazine) and ,&phenylethylhydrazine (phenelzine). With the exception of hydralazine which produced a reversible, substrate-competitive inhibition (K1, 2 X 10m5M), the inhibitory hydrazine derivatives investigated caused irreversible inhibitions, differing primarily in the rate of attack on the enzyme; aromatic hydrazines, e.g. phenylhydrazine, were more aggressive than aliphatic hydrazines, e.g. methylhydrazine, members of either class being more active than hydrazides. When graded amounts of arylhydrazine, tranylcypromine, or pargyline were allowed to react with the enzyme, the activity of the enzyme was observed to decrease as a linear function of the amount of inhibitor added; complete inhibition was attained at a ratio of 1 mole of inhibitor per lo5 g of protein. It is proposed that these results represent titration at the catalytic site of monoamine oxidase. (+)-Tranylcypromine is known to act as a substratecompetitive inhibitor of monoamine oxidase; for our preparation, Kr was determined as 5.8 x low8 M and inhibition was seen to be only partially reversed by dialysis in the presence of substrate. Inhibition by pargyline was not reversible and studies performed with 14C-pargyline (labeled at C-7) indicated that