The stability of monoamine oxidase of rat liver mitochondria to heating has been studied, using kynuramine as substrate in a spectrophotometric assay. After suspensions of mitochondria were heated for 50 minutes at 50°, about 30% of the enzymatic activity is lost; at 53° for the same time, 50% is lost. At 60° most of the activity is lost in the first 10 minutes. Activity as a function of pH was studied; resulting curves had a shoulder around pH 6.5 and a peak at pH 7.4 in phosphate buffer (or at 8.1 in borate buffer). The corresponding curves for heated suspensions of mitochondria (50°, 30 minutes) showed a relatively greater decrease in activity at pH 7.0 than at pH 6.5. This resulted in the disappearance of the shoulder and the appearance of a new peak at pH 6.5. Tranylcypromine and iproniazid inhibited the mitochondrial enzyme more strongly at pH 8.1 (borate buffer) than at the lower pH's, but pargyline inhibited more at pH 6.5. Chelating agents such as 8-hydroxyquinoline and thenoyltrifluoroacetone inhibited the enzyme more strongly at the lower pH's. Extracts of livers of riboflavin-deficient rats showed less activity than those from vitamin-supplemented animals. Substrates and certain inhibitors of monoamine oxidase protected the enzyme from inactivation during heating, but amines that are not substrates provided no such protection. Unsuccessful attempts were made to identify different types of monoamine oxidase activity in the mitochondria fractionated on sucrose density gradients.
Studies on Monoamine Oxidase (Report 13) Isolation of Two Different Types of Monoamine Oxidase from Beef Liver Mitochondria