DECREASED CONFORMATIONAL STABILITY OF THE SARCOPLASMIC-RETICULUM CA-ATPASE IN AGED SKELETAL-MUSCLE

Sarcoplasmic reticulum (SR) membranes purified from young adult (4-6 m onths) and aged (26-28 months) Fischer 344 male rat skeletal muscle we re compared with respect to the functional and structural properties o f the Ca-ATPase and its associated lipids. While we find no age-relate d alterations in (1) expression levels of Ca-ATPase protein, and (2) c alcium transport and ATPase activities, the Ca-ATPase isolated from ag ed muscle exhibits more rapid inactivation during mild (37 degrees C) heat treatment relative to that from young muscle. Saturation-transfer EPR measurements of maleimide spin-labeled Ca-ATPase and parallel mea surements of fatty acyl chain dynamics demonstrate that, accompanying heat inactivation, the Ca-ATPase from aged skeletal muscle more readil y undergoes self-association to form inactive oligomeric species witho ut initial age-related differences in association state of the protein . Neither age nor heat inactivation results in differences in acyl cha in dynamics of the bilayer including those lipids at the lipid-protein interface. Initial rates of tryptic digestion associated with the Ca- ATPase in SR isolated from aged muscle are 16(+/- 2)% higher relative to that from young muscle, indicating more solvent exposure of a porti on of the cytoplasmic domain. During heat inactivation these structura l differences are amplified as a result of immediate and rapid further unfolding of the Ca-ATPase isolated from aged muscle relative to the delayed unfolding of the Ca-ATPase isolated from young muscle. Thus ag e-related alterations in the solvent exposure of cytoplasmic peptides of the Ca-ATPase are likely to be critical to the loss of conformation al and functional stability. (C) 1997 Elsevier Science B.V.