HETEROGENEITY OF MICROSOMAL MEMBRANE FLUIDITY - EVALUATION USING INTRINSIC TRYPTOPHAN ENERGY-TRANSFER TO PYRENE PROBES

Membrane fluidity measurements based on excimer formation of pyrene an d pyrene derivatives as a measure of lateral diffusion yield a decreas ed fluidity in the presence of proteins [1-3]. It was the aim of our s tudy to investigate whether the reduced excimer formation is due to a rigidifying effect of proteins on the whole membrane or if the fluorop hore mobility is mainly hindered in the immediate protein environment. Resonance energy transfer in microsomal membranes between intrinsic t ryptophan residues and pyrene were used to study the excimer formation rate in the vicinity of proteins. The excimer-to-monomer fluorescence ratio at excitation via resonance energy transfer is lower than that observed for the direct excitation. The results suggest that, because of a reduced fluidity in the neighbourhood of proteins, pyrene and pyr ene fatty acids do not diffuse homogeneously in the membrane plane. A fluidity gradient exists from the membrane proteins to the bulk lipid.