When the body fights oxidative damage, it calls up a reservist enzyme that protects cells, known as Lon protease – but only if those cells are relatively young.  Kelvin J. A. Davies, from University of Southern California (USC; California, USA), and colleagues found that when oxidative agents attack the mitochondria of young cells, the cells respond by increasing the levels of Lon – to break up and remove damaged proteins.  However, as the cells age, they lose the ability to mobilize large numbers of Lon.  The finding may help explain why humans lose energy with age.   The team submits that: “Loss of Lon stress inducibility is part of a pattern of diminishing stress adaptability that predisposes cells to senescence.”