Platelet membrane fluidity individuals at risk for Alzheimer's disease: a comparison of results from fluorescence spectroscopy and electron spin resonance spectroscopy

Rationale: Previous fluorescence studies employing 1,6-diphenyl-1,3,5-hexat riene (DPH) have revealed an increase in the fluidity of platelet membranes from individuals with Alzheimer's disease (AD) and their first-degree rela tives. This biophysical alteration has been reported to be relatively speci fic for the hydrocarbon core of platelet membranes, where DPH preferentiall y localizes; this effect is not reflected by the fluorescent reporter triet hylamino-DPH, which labels membranes at the lipid-aqueous interface. Object ive: The goal of this study was to explore the validity and reproducibility of these findings using an independent biophysical technique, electron spi n resonance (ESR) spectroscopy. Methods: Platelet membranes prepared from f irst-degree relatives of patients with AD were labeled with DPH, or the spi n-labeled fatty acid, probes 5-doxylstearate (5-DS) and 12-doxylstearate (1 2-DS). These spin labeled probes provide an index of structural order at th e respective depths of their nitroxide moieties in the membrane. The result ing preparations were examined by fluorescence and ESR spectroscopy Results : Increased platelet membrane fluidity (PMF), as determined by the fluoresc ence anisotropy of DPH, was associated with only a modest reduction in the order parameter derived for 5-DS labeled membranes. In contrast, the mean o rder parameters derived from the paired samples labeled with 12-DS differed substantially from each other, and revealed decreased order (increased flu idity) in the hydrocarbon 12-C region where DPH preferentially localizes. C onclusions: These results provide an independent validation of the biophysi cal alterations of platelet membranes that are manifested by a subgroup of patients with AD and their first-degree relatives.