Decrease in Ca-ATPase activity in aged synaptosomal membranes is not associated with changes in fatty acyl chain dynamics

We have examined lipid peroxidation (LPO) and fatty acid acyl chain dynamic s in synaptosomal membranes isolated from aged rat (Fischer 344 x Brown Nor way F1 hybrids) brains, correlating these results with measurements of enzy matic activity of the synaptic plasma membrane Ca2+-ATPase (PMCA). Calcium- dependent ATPase activity in these membranes exhibits progressive decreases with a maximal loss of activity with age of approximately 35%. The sensiti vity of this membrane-bound ion transporter to the lipid composition of the surrounding membrane, as well as the high abundance of oxidatively sensiti ve polyunsaturated fatty acyl chains in synaptosomal membranes, suggests th at this age-related loss in catalytic turnover may result from LPO-mediated protein modification and/or changes in the physical structure of the bilay er. However, high-performance liquid chromatography analysis of 2,4-dinitro phenylhydrazone derivatives reveals no significant age-related increases in the content of reactive aldehydes (malondialdehyde, formaldehyde, acetalde hyde or acetone) which comprise breakdown products of lipid peroxidation. E lectron paramagnetic resonance measurements employing 5- and 12-stearic aci d spin labels with the nitroxide reporter groups at two depths in the bilay er were used to assess the fatty acyl chain dynamics (fluidity) of synaptos omal membranes. The resulting spectra demonstrate anisotropic lipid dynamic s of two populations of lipids, i.e. lipids in direct association with memb rane proteins (boundary lipids) and bulk lipids that do not directly associ ate with proteins. The nanosecond dynamics of both lipid populations is una ltered with age indicating that any compositional changes occurring with ag e are insufficient to result in alterations in bilayer fluidity relevant to PMCA activity. Thus, the observed age-related decline in PMCA activity may be explained by direct modification of membrane protein. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.