ENHANCED CYTOCHROME-OXIDASE ACTIVITY AND MODIFICATION OF LIPIDS IN HEART-MITOCHONDRIA FROM HYPERTHYROID RATS

In order to further investigate the mechanism regulating the control o f mitochondrial respiration by thyroid hormones, the effect of the hyp erthyroidism on the kinetic characteristics of cytocrome c oxidase in rat heart mitochondria was studied. Mitochondrial preparations from bo th control and hyperthyroid rats had equivalent K, values for cytochro me c, while the maximal activity of cytochrome oxidase was significant ly increased (by around 30%) in mitochondrial preparation from hyperth yroid rats. This enhanced activity of cytochrome oxidase was associate d to a parallel increase in mitochondrial State 3 respiration. The hor mone treatment resulted in a decrease in the flux control coefficient of the oxidase. The enhanced activity of cytochrome oxidase in hyperth yroid rats does not appear to be dependent on an increase in the mass of this enzyme complex in that the heme are, content was equivalent in both hyperthyroid and control preparations. The Arrhenius plot charac teristics differ for cytochrome oxidase activity in mitochondria from hyperthyroid rats as compared with control rats in that the breakpoint of the biphasic plot is shifted to a lower temperature. Cardiolipin c ontent was significantly increased in mitochondrial preparations from hyperthyroid rats, while there were no significant alterations in the fatty acid composition of cardiolipin of control and hyperthyroid prep arations. The results support the conclusion that the enhanced cytochr ome oxidase activity in heart mitochondrial preparations from hyperthy roid rats is due to a specific increase in the content of cardiolipin.