activity of monoamine oxidases (MAOs) by itself and through an active metabolite, isopropylhydrazine. The formation of isopropylhydrazine from iproniazid has been observed without MAOs present. Both iproniazid and isopropylhydrazine react near the active site of MAOs. The reaction is a progressive first-order reaction with a high activation energy. In the presence of oxygen it is an irreversible reaction, as dehydrogenation of iproniazid at the active site of the enzyme takes place. This dehydrogenation resembles the first step of amine oxidation. After dehydrogenation iproniazid further reacts with the enzyme. Inhibition of MAOs by iproniazid is competitive and sensitive to changes in pH and temperature, similar to oxidation of the monoamine substrate. Inhibition cannot be reversed by addition of the substrate. Iproniazid is able to displace non-hydrazine inhibitors, but not other hydrazine inhibitors from the active site of the enzyme. To increase the inhibition of monoamine oxidase, cyanide can be used. The reaction however remains oxygen-dependent. MAO inhibition can be decreased by addition of glutathione, suggesting non enzymatic conjugation of either iproniazid or isopropylhydrazine with glutathione. Metabolism and toxicity Iproniazid is metabolized in the body. Iproniazid is converted to isopropyl hydrazine and isonicotinic acid in an initial hydrolysis reaction. Isopropyl hydrazine can either be released in the blood or it can be metabolically activated by microsomal CYP450 enzymes. This oxidation of isopropyl hydrazine is a toxification reaction that eventually can lead to the formation of an alkylating agent: the isopropyl radical. Hepatic necrosis was found in rats with doses as low as 10 mg/kg. Isopropyl radical The presence of the isopropyl radical was indicated by another observed product of the metabolism of iproniazid: the gas propane. Alkylating agents have the capability to bind to chemical groups such as amino, phosphate hydroxyl, imidazole and sulfhydryl groups. The formed isopropyl radical is able to form S-isopropyl conjugates in vitro. This diminishes covalent binding to other proteins, however it was only observed in vitro. In vivo, hepatotoxic doses of isopropyl hydrazine, the precursor of the isopropyl radical, did not deplete sulfhydryl-group containing compounds. Liver necrosis The isopropyl radical formed as a result of the metabolism of iproniazid, is able to covalently bind to proteins and other macromolecules in the liver. These interactions are the reason for the hepatotoxicity of iproniazid. Covalent binding results in liver necrosis by presumably changing protein function leading to organelle stress and acute toxicity. However, the exact mechanism of how the