The monoamine oxidase and catechol-O-methyltransferase activities in the dog liver

The activity of monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) was investigated in the liver of anesthetized dogs. The MAO activity was determined by means of the method described by Wurtman and Axelrod (1963, Biochem. Pharmacol. 12: 1439-1440) using tryptamine as a substrate. The activity of COMT was assayed with the method of Axelrod (1962, Methods in Enzymology, Vol. 5. Academic Press, New York) in the presence of epinephrine bitartrate as a substrate. The average values for the activity of MAO and COMT were found to be 57.7 nmol . min-1 . g-1 of liver and 101.8 nmol . min-1 . g-1 of liver, respectively. No significant difference in the enzyme activity was observed among various samples obtained from distinct zones (peripheral and central) and lobes at different time of sample collections. The results suggest the homogeneous distribution of MAO and COMT in the dog liver. The striking difference in the COMT activity in the dog liver as compared with that reported by others for the cat liver is probably due to a species difference between these two animals.