Monoamine oxidase activity in peripheral organs and adrenergic tissues of the rat

Abstract

Monoamine oxidase activity in vitro was studied in the heart, kidneys, submaxillary glands, and superior cervical ganglia of control rats by the method of Lovenberg and co-workers ( J. Pharmacol. 1962). Comparison of monoamine oxidase activity in the absence and presence of exogenous aldehyde dehydrogenase revealed nearly saturating amounts of aldehyde dehydrogenase in the liver, submaxillary glands, lungs, spleen, and stomach (fundus, body, pyloric antrum). In the absence of exogenous aldehyde dehydrogenase, the uterus, ventricles, right atrium, left atrium, and proximal and distal small intestine showed activities of 24, 46, 58, 49, 65, and 68 per cent activity respectively. The activity in the superior cervical ganglia, stellate ganglia, thoracic chains, and retinas was 8.5, 11.0, 27, and 38 per cent. In the presence of exogenous aldehyde dehydrogenase, the greatest monoamine oxidase activity, based on the protein content, was noted in the superior cervical ganglia. On the wet weight basis, ganglionic monoamine oxidase activity was greater than that of all other tissues investigated except the liver. In the absence of exogenous aldehyde dehydrogenase, the reaction velocity of monoamine oxidase in ventricles, atria and pooled adrenergic tissue was linear for 30 min at the optimum substrate concentration. In the presence of exogenous aldehyde dehydrogenase, the monoamine oxidase reaction rates of the ventricles, atria, and adrenergic tissue were linear for up to 40 min at optimum substrate concentrations. Substrate inhibition became apparent at the highest substrate concentration used (11.67 mole/ml), and unsaturation of the enzyme was noted at the lowest substrate concentration (0.233 mole/ml). In homogenates of the ventricles, atria, and pooled adrenergic tissue, monoamine oxidase activity was inhibited by relatively small increases in enzyme concentration (tissue homogenate). This inhibition was not overcome by a tenfold increase in substrate concentration nor by the doubling of NAD and exogenous aldehyde dehydrogenase. Centrifugation at 50,000 g for 25 min localized the inhibitory component(s) in the pellet. The supernatant was devoid of monoamine oxidase activity.