INTERACTION OF LYSOPHOSPHOLIPID TAURODEOXYCHOLATE SUBMICELLAR AGGREGATES WITH PHOSPHOLIPID-BILAYERS

The equilibrium partitioning and the rate of transfer of monoacylphosphatidylethanolamines (lysoPEs) between phospholipid bilayers and lysoPE/taurodeoxycholate submicellar aggregates (SMAs) were examined with a series of environment-sensitive fluorescent-labeled N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-1-monoacylphosphatidylethanolamine (N-NBD-lysoPE) probes of differing acyl chain length. Our previous work has demonstrated the formation of SMAs between bile salts and lysophospholipids [Shoemaker & Nichols (1990) Biochemistry 29, 5837-58421. The experiments in the current work demonstrate that SMAs can coexist with phospholipid vesicles and can function as shuttle carriers for the transfer of lysophospholipids between membranes. The formation of submicellar aggregates of N-NBD-lysoPE and taurodeoxycholate (TDC) in equilibrium with 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles was determined from the increase in fluorescence generated upon addition of TDC to POPC vesicles containing 3 mol % N-NBD-lysoPE and 3 mol % N-(lissamine rhodamine B sulfonyl)dioleoylphosphatidylethanolamine (N-Rh-PE) as a nonextractable fluorescence energy-transfer quencher. The fraction of lysolipid extracted increased as a function of decreasing acyl chain length of the N-NBD-lysoPE molecule. The half-time for equilibration was independent of acyl chain length and averaged 44 ms at 10-degrees-C. The delivery of N-NBD-lysoPE from preformed N-NBD-lysoPE/TDC SMAs into POPC vesicles containing the energy-transfer quencher N-Rh-PE was measured by the rate of fluorescence decline. The initial rate of insertion increased with decreasing acyl chain length of the N-NBD-lysoPE molecule and as a function of vesicle concentration. The measured rates of N-NBD-lysoPE/TDC SMA interaction with vesicles indicate that the dynamic interaction of SMAs with membranes can facilitate intermembrane lysolipid transfer in the presence of nonlytic concentrations of TDC.