Folic acid does not inactivate xanthine oxidase

Area of Science:

• Biochemistry
• Enzymology

Background:

• Folic acid was reported as a potent inactivator of xanthine oxidase.
• This study investigated the mechanism behind this reported enzyme inactivation.

Purpose of the Study:

• To elucidate the mechanism by which folic acid appears to inactivate xanthine oxidase.
• To identify the specific compound responsible for the observed inhibition.

Main Methods:

• Enzyme kinetics studies of xanthine oxidase with commercial and purified folic acid.
• Quantification of pterin aldehyde as a potential inhibitor.
• Determination of association constants for enzyme-inhibitor interactions.

Main Results:

• Commercial folic acid exhibited time-dependent, progressive inhibition of xanthine oxidase.
• This inhibition followed slow second-order kinetics, suggesting a contaminant.
• Partially purified folic acid showed reduced inhibitory effect.
• Pterin aldehyde, a folic acid photolytic product, was identified as a potent progressive inhibitor.
• Adjusted association constants for folic acid correlated with pterin aldehyde content.

Conclusions:

• The apparent inactivation of xanthine oxidase by folic acid is attributed to the presence of contaminating pterin aldehyde.
• Pterin aldehyde binds slowly to xanthine oxidase, causing progressive inhibition.