Fluidity of sarcoplasmic reticulum membranes investigated with dipyrenylpropane, an intramolecular excimer probe.

Intramolecular excimer formation with 1,3-di(1-pyrenyl)propane was used to probe the fluidity of sarcoplasmic reticulum (SR) membranes where the probe could be incorporated (in a probe/lipid molar ratio of 1/2000 or less) without inducing any detectable damage. The temperature was varied between -1 and 40 degrees C. The fluidity of the native SR membrane changes from 52 cP at 40 degrees C to 325 cP at 5 degrees C. The native SR membranes are less fluid than liposomes prepared from total membrane lipids. The fluidity of membranes reconstituted from the SR ATPase and a total lipid extract of the SR depends on the lipid/protein molar ratio and sharply decreases when this ratio becomes lower than 44, for all temperatures studied. This is in accord with literature reports describing the occurrence of approximately 30 lipid molecules around the Ca-ATPase molecule in the SR membrane. Arrhenius plots of the excimer to monomer fluorescence intensity ratio in native and reconstituted SR membranes display a break at about 20 degrees C which is not observed when the lipid/protein ratio is lower than 44. This break is interpreted as being a characteristic of the lipid portion of the membrane.