Interactions among phospholipids of guinea pig rough microsomes, effect of fat deficiency

The fatty acid composition and the steady-state fluorescence anisotropy (rs) of 1,6-diphenyl-1,3,5-hexatriene (DPH) were determined for each of the five major phospholipid (PL) classes present in the liver rough microsomes (RM) of guinea pigs fed with control and fat-deprived diets. In order to obtain information about PL-PL interactions and their contribution to the overall membrane fluidity the experimental rs of phospholipid mixtures were compared to the molar weighed average values of the individual phospholipid rs values. The PL ratios in the mixtures were the same to those found in the RM membranes. Binary mixtures of phosphatidylcholine (PC) with phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) and sphingomyelin (SM) show higher values of rs than those estimated from the individual component parameters indicating a 'rigidizing' interaction. The rigidizing effect of PE was also observed when this phospholipid was sonicated with mixtures of PC with PS and PI. However, no rigidizing effect of PE was observed in the PC bilayers when SM was included in the composition suggesting that PE-SM interactions prevent 'rigidizing' effects of PE. Besides, in spite that PC-PI and PC-PS mixtures have 'rigidizing' interactions, the incorporation of PI and PS to PC-PS and PC-PI mixtures, respectively, have a 'fluidizing' effect. In consequence, phospholipid polar head groups interact in RM membranes modifying the molecular packing and/or the rotational diffusion of acyl chains. The complexity and variety of constituent phospholipids could prevent major changes in the fluidity. The comparison of results obtained with PL mixtures of control and fat-deficient animals indicate that a change in the number of double bonds does not evoke a significant difference between either the 'rigidizing' of 'fluidizing' effects. However, there is a general tendency indicating that phospholipids with higher double bond index evoke lower 'rigidizing' and 'fluidizing' interactions. Moreover, PL of animals fed a fat-deficient diet have less fluidity than those of control animals.