Flip-flop rates of individual molecular species of phosphatidylcholine in the human red cell membrane.

Trace amounts of four different, well-defined species of phosphatidyl[N-methyl-14C]choline ([14C]PC), differing in their fatty acyl constituents, were introduced exclusively into the outer membrane leaflet of the intact erythrocyte by using a PC-specific phospholipid transfer protein. The rate of transbilayer equilibration of these probe molecules was calculated from the time-dependent decay in specific radioactivity of the PC pool in the outer monolayer, which was discriminated from that in the inner leaflet by treating the intact cells with phospholipase A2 in the presence of sphingomyelinase C. At 37 degrees C, 1,2-dipalmitoyl-, 1,2-dioleoyl-, 1-palmitoyl-2-linoleoyl- and 1-palmitoyl-2-arachidonoyl-PC revealed halftime values for the rate of their transbilayer equilibration of 26.3 +/- 4.4, 14.4 +/- 3.5, 2.9 +/- 1.7 and 9.7 +/- 1.6 h, respectively.