Does luminol chemiluminescence detect free radical scavengers?

Abstract

Thiol compounds have been reported to abolish hypoxanthine/xanthine oxidase induced luminol chemiluminescence and this effect has been attributed to scavenging of superoxide (O2‐)/(H2O2) produced from hypoxanthine/xanthine oxidase. Yet other workers have reported that thiol compounds have shown little, if any, reactivity towards O2‐/H2O2. The aim of this study was to examine the discrepancy between these two sets of findings further. Captopril (a thiol angiotensin‐converting enzyme (ACE) inhibitor) and MPG (a simple thiol) were observed to abolish hypoxanthine/xanthine oxidase induced chemiluminescence. The reactivity of captopril and MPG towards O2‐/H2O2 was then determined by measurement of thiol oxidation in captopril and MPG after their incubation with hypoxanthine/xanthine oxidase. Incubation (at 10 min, 37 degrees C) with 4 mM hypoxanthine/0.03 u ml‐1 xanthine oxidase resulted in 7% and 20% thiol oxidation in captopril and MPG (at 1 mM) respectively. Captopril and MPG, therefore, appeared to be ineffective scavengers of oxidants produced by hypoxanthine/xanthine oxidase. Captopril and MPG also did not affect urate production or oxygen consumption by xanthine oxidase which indicated that captopril and MPG quench luminol chemiluminescence by a mechanism that excludes the inhibition of xanthine oxidase. Hypoxanthine/xanthine oxidase induced luminol chemiluminescence may, therefore, be an unsuitable method for measuring free radical scavenging activity by drugs. Copyright © 1995, Wiley Blackwell. All rights reserved