Studies on monoamine oxidase. (Report 37) Effects of oxygen concentration on rat liver and brain monoamine oxidase

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Abstract
MAO [monoamine oxidase] activity in rat brain mitochondria with tyramine as substrate at 100% O2 concentration was 3 times that at 20%. When serotonin served as substrate, difference in activities between the 2 O2 concentrations was not significant. Similar results were obtained when rat liver MAO was used as the enzyme source. At 100% O2 concentration, pargyline showed most potent inhibition of MAO activity in liver mitochondria with tyramine as substrate, but inhibitions caused by pheniprazine and harmaline were unremarkable. At 100% O2 concentration, harmaline showed most potent inhibition of MAO activity in the liver when serotonin served as substrate, while inhibitions of MAO activity by pargyline and pheniprazine were weak. At 20% O2 concentration, harmaline showed most potent inhibition of MAO activity in brain when serotonin was used as substrate. This inhibitions were studied using Lineweaver-Burk plots. Pargyline revealed a noncompetitive inhibition to MAO activity in liver and brain with tyramine and serotonin as substrate, harmaline a competitive inhibition to MAO activity in liver and brain with tyramine as substrate, while noncompetitive inhibition to MAO activity in liver and brain was evident when serotonin was used as substrate.