MAJOR CHANGES IN COMPLEX-I ACTIVITY IN MITOCHONDRIA FROM AGED RATS MAY NOT BE DETECTED BY DIRECT ASSAY OF NADH-COENZYME-Q REDUCTASE

We have investigated the respiratory activities and the concentrations of respiratory chain components of mitochondria isolated from the liv ers and hearts of two groups of rats aged 6 and 24 months respectively . In comparison with the adult controls (6 months), in aged rats there was a decline in total aerobic NADH oxidation in both tissues; only m inor (non-significant) changes, however, were found in NADH:coenzyme Q reductase and cytochrome oxidase activities, and there was no change in ubiquinol-cytochrome c reductase activity. The coenzyme Q levels we re slightly decreased in mitochondria from both organs of aged rats. T he lowered NADH oxidase activity is not due to the slight decrease obs erved in the coenzyme Q levels, but is the result of decreased Complex I activity. Since the assay of NADH:coenzyme Q reductase requires qui none analogues, none of which can evoke its maximal turnover [Estornel l, Fato, Pallotti and Lenaz (1993) FEBS Lett. 332, 127-131], its activ ity has been calculated indirectly by taking advantage of the relation ship that exists between NADH oxidation and ubiquinol oxidation throug h the coenzyme Q pool. The results; expressed in this way, show a dras tic loss of activity of Complex I in both the heart and the liver of a ged animals in comparison with adult controls.