Evidence for a clorgyline-resistant monoamine metabolizing activity in the rat heart

When benzylamine was used as substrate, a component of the total monoamine oxidase (MAO) activity in the rat heart was found to be resistant to inhibition by clorgyline. The proportion of the total activity represented by this component, decreased as the rat grew. It was also inhibited by both semicarbazide and isoniazid but not by potassium cyanide. Inhibitor studies with MAO in subcellular fractions showed that this component was more concentrated in the microsomal and soluble fractions. However, it could not be concluded that the activity was entirely a soluble enzyme. Determination of quasi-Michaelis constants ("Km") for total benzylamine oxidizing activity revealed a high ("Km" of approximately 10(-5)M) and low ("Km" of approximately 5 X 10(-4)M) affinity component. The high affinity component was inhibited by semicarbazide and the low affinity component by clorgyline. In the presence of 10(-3)M clorgyline, the high affinity component showed substrate inhibition at higher substrate concentrations. The possibility is discussed that the clorgyline-resistant activity is due to an amine-oxidizing activity distinct from mitochondrial MAO.