Haloperidol reductase in human and guinea pig livers

Area of Science:

• Biochemistry
• Pharmacology
• Enzymology

Background:

• Haloperidol reductase activity has been investigated in various species.
• Understanding the localization and characteristics of drug-metabolizing enzymes is crucial for pharmacokinetics.
• Previous studies have suggested potential roles for reductases in drug metabolism.

Purpose of the Study:

• To develop a novel gas chromatography (GC) assay for quantifying haloperidol reductase activity.
• To determine the tissue distribution and cofactor dependency of haloperidol reductase in guinea pig and human liver.
• To characterize the substrate specificity and inhibitory profile of haloperidol reductase, comparing it to known ketone reductases.

Main Methods:

• Development of a specific gas chromatography (GC) assay method.
• Fractionation of liver tissues (cytosol and microsomes) from guinea pigs and humans.
• Enzyme activity assays using NADPH as a cofactor.
• Inhibition studies with known ketone reductase substrates (menadione, daunorubicin, ethacrynic acid).

Main Results:

• Haloperidol reductase activity was detected in both guinea pig liver cytosol and microsomes.
• In human liver, haloperidol reductase activity was exclusively found in the cytosol.
• The enzyme activity was dependent on Nicotinamide adenine dinucleotide phosphate (NADPH) in both species.
• Human haloperidol reductase was significantly inhibited by known ketone reductase substrates, suggesting shared characteristics.

Conclusions:

• A reliable GC assay for haloperidol reductase was successfully developed.
• Haloperidol reductase exhibits distinct species-specific tissue distribution between guinea pigs and humans.
• The enzyme's dependency on NADPH and inhibition by ketone reductase substrates indicate it is a type of ketone reductase.