The energy dependent redox responses of heme and copper in cytochrome oxidase in rat brain in situ

Energy-dependent redox state of heme a + a3 and copper of cytochrome oxidase in perfused rat brain in situ

Using the blood-free perfused rat brain, we examined the redox behavior of cytochrome oxidase of two chromophores, heme a + a3 and copper. When perfusate inflow was stopped to induce global ischemia, the reduction of heme a + a3 was triphasic, with a rapid phase, a slow phase, and a second rapid phase. In contrast, the reduction of copper was monophasic after the rapid phase of heme a + a3. The triphasic reduction of heme a + a3 was diminished by energy-depleting treatments, such as addition of an uncoupler. The time course of the reduction of copper was not affected by the energy depletion. During global ischemia the decrease in creatine phosphate nearly paralleled the reduction of heme a + a3, whereas ATP remained at the control level until approximately 60% of heme a + a3 was reduced in the rapid phase. In the slow phase, ATP started to decrease with the reduction of copper. The redox behavior of copper was similar to the slow phase of the reduction of heme a + a3 because of the higher oxygen affinity of copper than of heme a + a3. Therefore, the rapid phase of the reduction of heme a + a3 can be used as an alarm before a decrease in ATP, whereas the reduction of copper indicates a decrease in ATP under severe hypoxia. Thus the copper signal in noninvasive near-infrared spectroscopy is a useful parameter for the clinical setting.

Optical characterization of heme a + a3 and copper of cytochrome oxidase in blood-free perfused rat brain

For the precise examination of the optical characteristics of cerebral tissue, we prepared hemoglobin-free perfused rat heads, from which trace amounts of blood were completely removed. In this preparation at 30 degrees C, the redox responses of the cytochrome oxidase components, heme a + a3 and copper, were followed spectrophotometrically in visible and near-infrared regions, and were correlated with the changes in neural activity as monitored by electroencephalography (EEG). During the aerobic-anaerobic transition, there was clear dissociation of the time courses of the reduction of heme a + a3 and copper; the reduction of heme a + a3 preceded the reduction of copper. The EEG activity decreased earlier than the reduction of heme a + a3. Pentylenetetrazole administration in normoxia caused the partial reduction of heme a + a3 but not of copper. The redox behaviors of cytochrome oxidase components in the brain were identical to those observed in isolated mitochondria. The usefulness of brain preparation for bridging the in vivo and in vitro studies is documented where various circulatory parameters could be controlled artificially.

Redox behavior of cytochrome oxidase in the rat brain measured by near-infrared spectroscopy

Using near-infrared spectroscopy, we developed a new approach for measuring the redox state of cytochrome oxidase in the brain under normal blood-circulation conditions. Our algorithm does not require the absorption coefficient of cytochrome oxidase, which differs from study to study. We employed this method for evaluation of effects of changes in oxygen delivery on cerebral oxygenation in rats. When fractional inspired oxygen was decreased in a stepwise manner from 100 to <10%, at which point the concentration of oxygenated hemoglobin ([HbO2]) decreased by approximately 60%, cytochrome oxidase started to be reduced. Increases in arterial PO2 under hyperoxic conditions caused an increase in [HbO2], whereas further oxidation of cytochrome oxidase was not observed. The dissociation of the responses of hemogloblin and cytochrome oxidase was also clearly observed after the injection of epinephrine under severely hypoxic conditions; that is, cytochrome oxidase was reoxidized with increasing blood pressure, whereas hemoglobin oxygenation was not changed. These data indicated that oxygen-dependent redox changes in cytochrome oxidase occur only when oxygen delivery is extremely impaired. This is consistent with the in vitro data of our previous study.