HTHQ (1-O-HEXYL-2,3,5-TRIMETHYLHYDROQUINONE), AN ANTI-LIPID-PEROXIDATIVE COMPOUND - ITS CHEMICAL AND BIOCHEMICAL CHARACTERIZATIONS

Recently, it has become apparent that reactive oxygen species (ROS) pl ay many important roles in biological systems. For example, relationsh ips between many diseases, such as cancer, cardiac infarction and arte riosclerosis, and ROS have been found. It is also well known that anti -oxidative agents scavenge ROS in biological systems, which in turn pr events ROS-related diseases. In our previous efforts to develop effect ive anti-oxidative compounds, we found that 1-O-hexyl-2,3,5-trimethylh ydroquinone (HTHQ), which is a hydroquinone monoalkyl ether, is a pote nt anti-oxidative agent. Here, the scavenging activities of HTHQ again st ROS, such as superoxide anion radicals, hydroxyl radicals, t-butyl peroxyl radicals and singlet oxygens, were examined by the ESR (electr on spin resonance)-spin trapping method. Among ROS, HTHQ scavenged t-b utyl peroxyl radicals most effectively (IC50 = 0 31 +/- 0 04 mM:), sho wing approximately twice the activity of a well-known lipophilic anti- oxidant, D,L-alpha-tocopherol (IC50 = 0.67 +/- 0.06 mM), as measured b y IC50 values defined as the 50% inhibition concentration of the gener ated ROS. In addition, a relatively stable ESR spectrum of free radica ls due to HTHQ was observed during the reaction of HTHQ and t-butyl pe roxyl radicals, indicating a direct reaction of HTHQ and t-butyl perox yl radicals. The free radicals due to HTHQ were more stable than those derived from D,L-alpha-tocopherol under the same conditions examined. On the basis of these results, we evaluated anti-lipid-peroxidative a ctivity of HTHQ in three systems involving micelles, liposomes and rat liver microsomes. HTHQ exhibited a similar anti-oxidative activity to that of D,L-alpha-tocopherol against lipid peroxidation in linolate m icelles initiated by addition of Fe2+. On the other hand, HTHQ exhibit ed approximately 4.8-fold higher anti-lipid-peroxidation activity than that of D,L-alpha-tocopherol against the peroxidation in phosphatidyl choline liposomes initiated by addition of Fe2+. Furthermore, HTHQ sca venged the lipid peroxides at a rate approximately 150 times higher th an that of D,L-alpha-tocopherol against Fe3+-ADP-induced lipid peroxid ation in rat liver microsomes, indicating that the anti-lipid-peroxida tion activity of HTHQ might be substantially elevated in biological sy stems in comparison with that of D,L-alpha-tocopherol. Based on these results, we suggest that HTHQ reacts directly with peroxyl radicals, s uch as t-butyl peroxyl radicals and peroxides of linolate micelles, li posomes and microsomes, by scavenging them to form stable free radical s. The resulting free radicals are presumed to be reduced by several r educing mechanisms in biological systems similarly to those of D,L-alp ha-tocopherol, and then the lipid-peroxidation reactions will be termi nated. In conclusion, HTHQ was :found to be a potent anti-lipid-peroxi dative compound and its antioxidation activity to be extremely elevate d in biological systems, such as that of liver microsomes via the gene ration of stable free radicals. We propose that HTHQ is a potent anti- oxidative agent for use in future treatments for lipid-peroxide releva nt diseases. (C) 1998 Elsevier Science B.V. All rights reserved.