ENZYME INHIBITION BY DERIVATIVES OF PHENOTHIAZINE: VII. INHIBITION OF GLUCOSE-6-PHOSPHATE DEHYDROGENASE ACTIVITY OF RABBIT ERYTHROCYTES

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Abstract
The effects of various phenothiazine derivatives were tested, by a NADP+-coupled spectrophotometric assay, on the glucose-6-phosphate dehydrogenase activity of hemolysates of rabbit erythrocytes. Phenothiazine, phenothiazine sulfoxide, thionol, promazine, chlorpromazine, and methylene blue were relatively weak inhibitors. Powerful inhibitors (concentration, for 50% inhibition, I50, in parentheses) were phenothiazone (90 μM) and New Methylene Blue N (75 μM). These two compounds caused a slow direct oxidation of NADPH; however, they appeared to exert an action on the enzyme as well. The enzyme was not inhibited by 2,6-dichlorophenol–indophenol, which is employed in a dye-reduction assay.The enzyme system was readily inactivated by ethanol (I5o = 1.4 M). Prior addition of NADP+afforded some protection.