INHIBITION OF MICROSOMAL LIPID-PEROXIDATION AND MONOOXYGENASE ACTIVITIES BY EUGENOL

Previously we reported that eugenol (4-allyl-2-methoxyphenol) inhibits non-enzymatic peroxidation in liver mitochondria (E. Nagababu and N. Lakshmaiah, 1992, Biochemical Pharmacology. 43,2393-2400). In the pres ent study, we examined the effect of eugenol on microsomal mixed funct ion oxidase mediated peroxidation using Fe+3-ADP-NADPH, carbon tetrach loride (CCL4)-NADPH and cumene hydroperoxide (CumOOH) systems. In the presence of eugenol the formation of thiobarbituric acid reactive subs tances (TBARS) was decreased in all the systems (IC50 values: 14 muM f or Fe+3-ADP-NADPH, 4.0 muM for CCl4-NADPH and 15muM for CumOOH). Oxyge n uptake was also inhibited to a similar extent with Fe+3-ADP-NADPH an d CumOOH systems. A comparative evaluation with other antioxidants sho wed that in Fe+3-ADP-NADPH and CumOOH systems, the antioxidant efficac y was in the order: butylated hydroxytoluene (BHT) > eugenol > alpha-t ocopherol, while in CCl4-NADPH system the order was alpha-tocopherol > BHT > eugenol. Time course of inhibition by eugenol indicated interfe rence in initiation as well as propagation of peroxidation. Eugenol di d not inhibit cytochrome P-450 reductase activity but it inhibited P-4 50 - linked monooxygenase activites such as aminopyrine-N-demethylase, N-nitrosodimethylamine demethylase, benzo(a)pyrene hydroxylase and et hoxyresorurin-O-deethylase to different extents. However, CumOOH suppo rted monooxygenases (aminopyrine-N-demethylase and benzo(a)pyrene hydr oxylase) required much higher concentrations of eugenol for inhibition . The concentration of eugenol required to inhibit monooxygenase activ ities was more than that required to inhibit peroxidation in all the s ystems. Eugenol elicited type 1 changes in the spectrum of microsomal cytochrome P-450. These results suggest that the inhibitory effect of eugenol on lipid peroxidation is predominantly due to its free radical quenching ability. Eugenol significantly protected against the degrad ation of cytochrome P-450 during lipid peroxidation with all the syste ms tested. These findings suggest that eugenol has the potential to be used as a therapeutic antioxidant. Further evaluation may throw more light on this aspect.