INVOLVEMENT OF XANTHINE-OXIDASE IN OXIDATIVE STRESS AND IRON RELEASE DURING HYPERTHERMIC RAT-LIVER PERFUSION

The hepatotoxic effects of hyperthermia have been proposed to be relat ed to lipid peroxidation as a consequence of oxidative stress. This ca n result from exposure of the cell to "radical oxygen" species such as the superoxide and hydrogen peroxide generated by the activity of the oxidase form (type O) of xanthine oxidase (XO), which is converted to that form by perfusion of the liver at hyperthermic temperatures. The se radical species are not reactive enough in themselves to cause cell damage but require the presence of a catalyst such as low molecular w eight chelated iron. In these studies, ferritin was shown to be a sour ce of iron for the oxidative stress of hyperthermia. (a) Iron was rele ased from ferritin in vitro by the activity of rat liver XO. The rate of iron release from ferritin in this incubation system was a function of the amount of type O XO present and the temperature. Inclusion of allopurinol or superoxide dismutase in the incubation resulted in sign ificantly lower rates of iron release. (b) Livers from Sprague-Dawley rats were perfused at 42.5-degrees and 37-degrees-C for 1 h. During th e recirculating perfusion, loss of iron from the liver into the perfus ate was significantly greater (P < 0.05) at 42.5-degrees-C than at 37- degrees-C. Also, there was a pronounced increase in the lactate dehydr ogenase and aspartate aminotransferase enzymes in the perfusate during perfusion at 42.5-degrees-C. Furthermore, intrahepatic levels of low molecular weight chelated iron were significantly (P < 0.05) increased following perfusion at 42.5-degrees-C. All these responses were abrog ated by the inclusion of allopurinol in the perfusate. (c) Oxidative s tress, assessed by the efflux of glutathione and oxided glutathione fr om the liver at 42.5-degrees and 37-degrees-C, was significantly (P < 0.05) increased at the hyperthermic temperature. This oxidative stress was inhibited by iron chelation and allopurinol. These results demons trate that there is a causal relationship between the generation of su peroxide by type O XO produced by hyperthermic perfusion and mobilizat ion of iron from ferritin to form a pool of low molecular weight chela ted iron. This iron pool in combination with active oxygen species lea ds to oxidative stress and lipid peroxidation.