Age-related impairment of mitochondrial matrix aconitase and ATP-stimulated protease in rat liver and heart.

Mitochondrial matrix proteins are sensitive to oxidative inactivation, and oxidized proteins are known to accumulate during ageing. The Lon protease is believed to play an important role in the degradation of oxidized matrix proteins such as oxidized aconitase. We reported previously that an age-related accumulation of altered proteins occurs in the liver matrix of rats and that the ATP-stimulated proteolytic activity, referred as to Lon-like protease activity, decreases considerably in 27 month-old rats, whereas no concomitant changes in the levels of Lon protein expression occur in the liver. Here, we report that this decline is associated with a decrease in the activity of aconitase, an essential Krebs' cycle enzyme. Contrary to what we observed in the liver, the ATP-stimulated protease activity was found to remain constant in the heart mitochondrial matrix during ageing, and the levels of expression of the Lon protease increased in the older animals in comparison with the younger ones. Although the ATP-stimulated protease activity remained practically the same in older animals as in younger ones, a decrease in the level of aconitase activity was still observed. Altogether, these results indicate that matrix proteins, such as the critical enzymes aconitase and Lon protease, are inactivated with ageing and that the effects of ageing vary from one organ to another.