Azoreductase activity by purified rabbit liver aldehyde oxidase

Area of Science:

• Biochemistry
• Enzymology
• Drug Metabolism

Background:

• Hepatic microsomal cytochrome P450 enzymes catalyze azoreduction of lipophilic dyes.
• Cytosolic molybdoflavoenzymes, such as aldehyde oxidase, are also involved in xenobiotic metabolism.

Purpose of the Study:

• To investigate the azoreduction of dimethylaminoazobenzene (DAB) analogs by mammalian liver aldehyde oxidase.
• To compare the substrate specificity and kinetics of aldehyde oxidase with cytochrome P450 for azo dye reduction.
• To elucidate the influence of pH, electron donors, and inhibitors on aldehyde oxidase-mediated azoreduction.

Main Methods:

• Purification of rabbit liver aldehyde oxidase.
• Assay of azoreduction activity using various azo dyes as substrates.
• Kinetic analysis (Vmax, Km) at different pH values and with different electron donors (2-hydropyrimidine, N1-methylnicotinamide, propionaldehyde, butyraldehyde).
• Inhibition studies using menadione and SKF 525-A.

Main Results:

• Aldehyde oxidase efficiently reduced water-soluble, charged azo dyes but not lipophilic ones.
• 2-hydropyrimidine was the optimal electron donor for most substrates.
• Maximal azoreductase activity was observed at specific pH values for different dyes, correlating with substrate ionization.
• No direct relationship was found between Vmax and Km.
• Menadione and SKF 525-A differentially inhibited azoreduction, suggesting distinct mechanisms.

Conclusions:

• Mammalian liver aldehyde oxidase plays a significant role in the reduction of water-soluble azo dyes.
• The rate of azoreduction is influenced by substrate ionization, which is pH-dependent.
• Aldehyde oxidase exhibits distinct substrate preferences and kinetic properties compared to cytochrome P450.
• Inhibitor sensitivity suggests complex regulatory mechanisms for aldehyde oxidase activity.