The Effect of Atabrine and Promazine Derivatives on the Activity of the Old Yellow Enzyme.

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Abstract
Atabrine, chlorpromazine, and other promazine derivatives inhibited the activity in the aerobic Zwischenferment-OYE system, but only when preincubated with the apoprotein of the OYE. The inhibition was independent of the FMN concentration. Fluorimetric studies of the apoprotein-FMN association reaction showed that compounds which inhibited the enzyme activity slowed down the reaction rate immediately after being added to the apoprotein, but did not lower the binding capacity for FMN. The mechanism of reaction between inhibitors and enzyme is discussed. Chlorpromazine potentiated the aerobic OYE activity when added to the apoprotein at the same time as or after the FMN. Other promazine derivatives had the same ability, whereas the corresponding sulphoxides were ineffective. The anaerobic OYE activity was inhibited but not potentiated by chlorpromazine. The potentiating effect could not be explained by the experimental results, although these allowed certain conclusions. The dissociation constants of complexes, formed between FMN and some promazines, were determined by spectrophotometric measurements.