DECREASED PLASMA-MEMBRANE CALCIUM-TRANSPORT ACTIVITY IN AGING BRAIN

We have assessed the functional properties of both calmodulin (CaM) an d the plasma membrane Ca2+-ATPase in brains of young, middle aged, and old Fisher 344 rats. Under optimal conditions of saturating Ca2+ and ATP, the CaM-activated Ca2+-ATPase activity was decreased with increas ing age, particularly when CaM isolated from the brains of aged rats w as used to stimulate the enzyme. In the case of CaM, structural modifi cations within the primary sequence of the protein from aged brains we re identified. We found that during normal biological aging approximat ely 6 methionine residues were modified to their corresonding sulfoxid e per CaM, and no other amino acids were modified. Some aspects of the age-related decline in the effectiveness of CaM as an activator of Ca 2+-ATPase could be simulated using a range of reactive oxygen species (including hydrogen peroxide and oxoperoxynitrite) and, in the latter case, the extent of oxidative modification of specific methionine resi dues was directly related to their surface accessibility. The pattern of oxidative modification of the methionines in the aged CaM was less straightforward, though both in vitro oxidation of CaM and aging withi n the brain markedly decreased the functional properties of this impor tant Ca2+-regulating protein.