Antioxidant properties of bile salt micelles evaluated with different chemiluminescent assays: A possible physiological role

The antioxidant activity of a representative series of free, glycine- and t aurine-conjugated bile acids was evaluated by two different chemiluminescen t assays: (a) the enhanced chemiluminescence system based on horseradish pe roxidase and luminol/oxidant/enhancer reagent, and (b) the hypoxanthine/xan thine oxidase/Fe2+-EDTA/luminol system. Bile acids were studied at final co ncentrations ranging from 1 to 28 mmol/L. All of the bile acids studied inh ibited the steady-state chemiluminescent reaction and the extent of inhibit ion depended upon the structure of the bile acids, whereas the duration was related to bile acid concentration. The mechanism of the light inhibition is probably due to trapping of oxygen free radicals generated in the chemil uminescent reactions, within bile acid micelles. The free radicals trapped into micelles reduced the formation of luminol radicals and consequently th e light output; when the micelles were saturated, the oxygen free radicals in solution again produced luminol radicals. The micelle interaction with r eactive oxygen species could be a physiological mechanism of defence agains t the toxicity of those species in the intestinal content. On the other han d, alterations in bile acid organ distribution, concentration and compositi on leads to a membrane damage caused by their detergent-like properties, wh ich could be associated to oxygen free radical production. (C) 1998 John Wi ley & Sons, Ltd.